Technical Memorandum 83848 X-RAY ASTRONOMY 1980s Edited by Stephen S. Holt November 1981 IN THE Proceedings of a Workshop held at the Goddard Space Flight Center October 5,6 and 7,1981. Nat10nar Aeronauttcs and Space Administration Greenbelt. Maryland 20771 Oodd#dSpscsflightC4UlttM https://ntrs.nasa.gov/search.jsp?R=19820018179 2020-03-21T07:36:09+00:00Z X-RAY ASTRONOMY IN THE 1980's Edited by STEPHEN S. HOLT November 1981 Proceedings o f a Yorkshop held a t the Goddard Space F l ight Center October 5 , 6 and 7 , 1981. iii PRECEDING PAGE BLANK NOT FILMED PREFACE A Workshop on X-ray Astrononly and Spectroscopy was he ld a t the Goddard S.pace F1 i g h t Center wi October 5 through 7, 1981, under the sponsorship o f GSFC and NASA. The purpose of t h i s workshop was t o evaluate the cur ren t s ta tus of X-ray astronomy fo l low ing a decade o f very rewarding explorat ion i n which i t matured t o the po in t o f having comparable importance t o UV, op t i ca l , I R and rad io astronomy fo r addressing the important questions concerning the observational universe. Theorists were asked t o present the status of our understanding of a wide range o f problems i n astrophysics, and t o explore what key Observational parameters i n X-rays are needed t o shed new l i g h t on the cur ren t ly unanswered questions. l a t e s t developments i n instrument design c a p a b i l i t i e s and the types of missions on which these instruments might be u t i l i z e d . Experimental ists were asked t o present the Af te r the theore t ica l requirements and instrument c a p a b i l i t i e s had been presented during the f i r s t two days o f the Workshop, the f i n a l day o f the meeting was opened up f o r an in tens ive discussion o f the types o f missions t h a t should be recommended f o r f u r the r study. of the ant ic ipated Explorer program i n astrophysics, inc lud ing EUVE, ROSAT and XTE, a general consensus was reached on several important issues, namely: I n add i t ion t o a reendorsement 1. The importance o f a continuing program of balloon-borne research as a cost -ef fect ive means by which studies a t energies i n excess o f 20 keV may be pe r f o m d . i v 2. The unique s c i e n t i f i c opportuni ty presented by the Space Transportat ion System (STS) t o develop low-cost experiments which are beyond the scope o f balloon-borne capab i l i t ies , bu t which may not requi re the long exposures of a f ree- f l yer platform. Trad i t iona l Spacelab instruments, as wel l as EOP and IPS-mounted experiments, are important elements i n a cont inuing balanced program of research. 3. The highest p r i o r i t y moderate cost f ree- f l yer mission i s one which should be devoted t o high throughput ( ra the r than reso lu t ion) i n the areas o f spectroscopy, imaging and timing. HTM (High - - Throughput - Mission). A study group should be formed t o r e f i n e the mission beyond the conceptual precepts discussed a t the Workshop. We have ten ta t i ve l y ca l l ed i t It was widely f e l t among the par t i c ipants t h a t the above recomnendations are essential t o maintaining the v igor o f X-ray astronomy, and hence astrophysical research, during the coming decade. A more de ta i led discussion o f thes- recommendations i s presented i n the Summary sect ion o f t h i s report. Oraanizi na Comni t t e e G.P. Garmire S.S. Ho l t R. McCray S. Rappaport ti. Tananhaum TABLE OF CONTENTS Preface Agenda V Summary J.L. Linsky Coronae o f Non-degenerate Single and Binary Stars: A Survey o f Our Present Understanding and Problems Ripe fo r Solution 0.0. Lamb Continuum and Line Spectra o f Degenerate Dwarf X-Ray Sources F.K. Lamb Neutron S t a r Binaries, Pulsars, and Burst Sources J.M. Shull X-Ray Spectroscopic Observations and Model 1 ing o f Supernova Remnants L.L. Cowie In te rs te l l a r * Intracluster and Supercluster Gas A.C. Fabian X-Ray Emission from Galaxies and the Universe A.P. LI ghtmn X-Rays from Quasars and Active Galaxies J .L. Cul halie Y. Tanaka A European X-Ray Astrophysics Mission X-Ray Astronomy Prqram i n Japan P. Gorenstein and 0. Schwartz The LAMAR: f o r a Moderate Cost Mission A High Throuqkput X-Ray Astrononiy F a c i l i t y R.C. Catura, W.A. Brown and E.G. Joki A Wolter Type I LAMAR G.R. Riegler, R.A. Stern, K. Liewer, F. Vescelus, J.A. Nousek and G.P. Garmire Performance o f Single Photon-Counting X-Ray Charge Coupled Devices C.J. Hailey, W.H.-M. Ku and M.H. Vartanian An Imaging Gas Sc in t i l l a t i on Proportional Counter fo r the Detection o f Subki 1 oel ectron-vcl t X-rays iii 1 3 13 37 77 107 119 133 143 183 219 227 27 1 289 30 1 v i R. Petre A Wide-field Soft X-Ray Camera M.P. Ulmer X-Ray Imaging Above 3 keV R.J. Proctor Temporal Aperture Modulation J.E. Grindlay and S.S. Murray Hard X-Ray Imaging from Explorer G.S. Vaiana The Ste l la r Coronal X-Ray Explorer: STCOEX D. McCamnon A Diffuse Soft X-Ray Spectrometer W.C. Cash, Jr. Requirements fo r High Qua l i t y X-Ray Spectroscopy i n an Explorer C1 ass M i ssion C.R. Canitares Future Prospects fo r High Resolution X-Ray Spectrometers P . J . Serl emi tsos Broad-Band X-Ray Telescope (BBXRT) P. Meszaros Some Observational Tests o f X-Ray Pulsar Emission Models G. Wunner, H. Ruder, H. Herold and J. Trumper Observabil ity o f Atomic Line Features i n Strong Magnetic Fields R.K. h i t h e r A New Method fo r Focusing and Imaging X-Rays and Gamma-Rays with D i f f rac t i on Crystals R.E. Griffi ths High Resolution Iron-Li ne Spectroscopy M. Cantin, L. Koch-Miramond, B. Mougin and R. Rocchia Laboratory Studies on a Spherically Curved Bragg Spectrometer fo r Cosmic X-Ray Spectroscopy H.W. Schnopper and P.O. Taylor Cy1 indr ica l Crystal Imaging spectrometer ( C C I S ) f o r Cosmic X-Ray spectroscopy J.L. Matteson Spectroscopy i n the 10 keV t o 10 MeV Range 319 325 335 349 371 379 403 42 5 44 1 46 1 47 1 47 7 497 52 3 527 549 v i i D . J . Helfand, G.A. Chanan, R. Novick, C.J. MacCallum and M. Leventhal Spectroscopy from 2 t o 200 keV R.E. Rothschild Hard X-Ray Astrophysics L i s t o f Part ic ipants Index o f Contributors 56 7 599 62 1 62 5 ORIGINAL PAGE IS OF POOR QUALITY A G E N D A 1 . Monday October 5, 1981 S. Ho l t F. Mart in J. Linsky D. Lamb F. Lamb M. Shull L. Cowie A. Fabian A. Lightman Y. Tanaka L. Culhane Tuesday October 6, 1981 P. Gorenstein R. Catura G. Riegler W. Ku R. Petre M. U l m e r R. Novick R. Proctor 3 . Grindlay G. Vaiana D. NcCamn W. Cash C. Canizares P. Serl emi tsos P. Meszaros G. Wunner R. Smither R. G r i f f i t h s G. Ricker J . Matteson D. Helfand R. Rothschi ld Wednesday October 7, 1981 G. Garmire S. Ho l t S. Rappaport H. Tananbaum R. McCray Chairman, S. Ho l t We1 come Introductory Remarks Main Sequence Stars Degenerate Dwarfs and Cataclysmics Neutron S t a r Binar ies and Bursters Chai man, S . Rappaport Supernovae and Supernova Remants Clusters and the I n t e r s t e l l a r Medium Gal ax i es and Cosmology Active Galactic Nuclei The Japanese X-Ray Astronomy Program The European X-Ray Astronomy Program Chairman H. Tananijaum Large Area Modular Array o f Ref lectors (LAMAR) LWAR with Wolter I Optics Imaging Gas S c i n t i l l a t i o n Proport ional Counter (GSPC. Charge Coup1 ed Device (CCD) Development Wide F i e l d Sof t X-Ray Telescope X-Ray Imaging above 3 keV Large Focussing Polarimeter Hard X-Ray Imaging i 1 Hard X-Ray Imaging i Chairman G. Garmire Stel 1 a r Coronal Explorer S o f t X-Ray Spectroscopy o f Di f fuse Sources Objective Re l f lec t ion Grating Spectrograph Dispersive Sof t X-Ray Spectroscopy Broad Band X-Ray Tel escope (BBXRT) X-Ray Emission from Pulsars Emission Lines i n Strong Magnetic Fields Focussing Bragg X- and Gamna-Rays Spher ica l ly Curved Imaging Spectrometer (SCIS) Fard X-Ray Detector Telescope (HXDT) 10 keV - 10 MeV Spectroscopy w i t h Germanium 5 - 500 keV Spectroscopy w i t h Germanium Hard X-Ray S c i n t i l l a t o r Arrays Chairman, R. McCray Discussion and Summary Presentations by the Members o f the Organizing Comnittee PAECEDiNG PAGE B W K NOT SUMMARY I. BACKGROUND FOR THE WORKSHOP Opportunities f o r conducting X-ray astronomy observations i n the immediate future appear t o be qu i te l imi ted. spectroscopic capabi l i t ies o f past and ant ic ipated missions i s given i n Table 1. A summary of the sens i t i v i t y and TABLE I. APPROXIMATE MISSION^ CAPABILITIES Mission 2Launch 3101 . s 4Range UHURU 1969 0 2-10 HEAO-1 1977 -1 .l-3 2-60 20-1 000 HEAO-2 1978 EXOSAT 1982 ROSAT 1987 -3 .l-3 .5-4 -2 .04-2 1-50 2-80 -3 . l - 3 E 5 - 7 1 7 50 1 2 1 7 7 1 3 2 7 5 100 15 2 5 10 3 200 200 1000 200 .2 200 15 700 150 300 3 4 XTE AXAF 1988 1990 -1 -5 2-60 7 7 5000 . l-8 1 1000 2 .5-8 7 50 100 'Representative X-ray astronomy f ree f lyers. Past (e.g. Ar ie l -5, SAS-C, OSO-8) and f u t u r e (Astro-B, Astro-C) missions w i t h c a p a b i l i t i e s comparable t o those indicated f o r missions i n a s i m i l a r time frame are omitted for the sake o f c l a r i t y . *Actual o r estimated launch date 3S i s the advert ised source detect ion s e n s i t i v i t y , i n u n i t s o f o f the Crab nebula i n t e n s i t y 'Effect ive energy range (keV 1 5Typical photon energy i n range 6Resolving power (E/AE) a t E 7Net area avai lab le f o r R a t E - - O f the U.S. X-ray astronmy missions, only the X-ray Timing Explorer i s This mission i s envisioned current ly i n l i n e f o r f l i g h t i n the l a t e 1980's. t o have wide p a r t i c i p a t i o n i n the form o f guest invest igator observations. The Extreme U l t r a v i o l e t Explorer, operating a t energies lower than the X-ray band, i s a s ing le i n s t i t u t i o n mission and w i l l have only l i m i t e d outside a c t i v i t y . which requires congressional approval before i t can be i n i t i a t e d . current c l imate o f budget reductions i n Washington, i t i s d i f f i c u l t t o be l ieve t h a t t h i s major new program w i l l be s tar ted before the mid 1980's, w i t h a launch a t the end o f the decade a t the very e a r l i e s t . The Advanced X-ray Astrophysics F a c i l i t y i s a major U.S. program I n the 5 Foreign missions which appear t o be coming along w i t h a high p r o b a b i l i t y f o r f l i g h t i n t h i s decade are: EXOSAT, ROSAT, Astro B and Astro C. another mission, X-80, i s i n the formative stages and w i l l possibly f l y by the end o f t h i s decade as wel l . substant ia l p a r t i c i p a t i o n by U.S. invest igators, a1 though the other missions may be open t o guest observers on a l im i ted basis f o r some por t ion o f the useful l i f e o f the various s a t e l l i t e s . Only the ROSAT mission i s expected t o have any From the above discussion i t i s obvious t h a t the opportuni t ies f o r U.S. invest igators t o p a r t i c i p a t e i n a f l i g h t program are very l im i ted . Any f u t u r e mission t h a t i s recommended w i l l represent a ra re opportunity. This being the case, the proposed mission must maximize the amount of science t h a t can be accomplished; i t must s i g n i f i c a n t l y surpass and avoid overlap w i t h current o r envisaged mission capab i l i t ies , and i t should appeal t o the s c i e n t i f i c p r i o r i t i e s o f a wide sector o f the astronomical community. proposed program should be moderate i n scope, cost ing less than a major observatory, b u t i n the general range of current explorers such as IRAS, COB€, or XTE. F i n a l l y , the 6 11. AN OVERVIEW OF THE WORKSHOP PRESENTATIONS The f i r s t day o f the Workshop was devoted t o a sc ieb i t i f i c summary o f the current status o f astrophysics covering a broad spectrum o f topics. Each reviewer das asked t o examine h i s top ic c a r e f u l l y and suggest c r i t i c a l X-ray observational data which woul d provide substant ia l advancements toward a be t te r understanding o f the given subject. 3 These topics and recommendations are presented i n the body of the report. The fol lowing tab le summari. es the suggested observationzl requirements f o r a s i g n i f i c a n t advance i n each area. TARLE I 1 OUESTION OBSERVATIONAL REQUIREMENTS OBSERVATION 'R' REOUIRED Stel 1 ar Sources Temps;.atures Low reso lu t ion spectroscopy Gwmetry Moderate reso lu t ion spectroscopy A c t i v i t y Monitor s o f t X-ray and UV f l u x Densit ies Moderate resol u t i on spectroscopy Winds? Moderate reso lu t ion spectroscopy Differences between high resolut ion spectroscopy young Et o l d s tars Do hot s ta rs have coronae? Moderate reso lu t ion spectroscopy 10 100 200 200 2 500 200 7 Imaging needed t o separate doubles AWO. 5 ' DeQenerate k a r f s and Cataclvuic Variables E m i ssion mechanism Weutron Stars Disk fed o r wind fed accret ion Cyclotron scat ter ing and high magnetic f i e l d e f f e c t s SuDernova Remnants non-Sedov e f f e c t s (temperature, pressure s t ructure 1 reverse shock spectroscopy abundances i n d i f f e r e n t broadband spectroscopy 0.1-100 keV temporal monitoring UVISoft X-ray spectroscopy and t iming rap id temporal var ia t ions and hard spectra imaging spectroscopy imagi ng spec Lroscopy 3-10 50 10 100 100 10-100 par ts of remnapt 8 Interstellar Media aml Cluster of Qlaxies Is there a galact 'c halo? arc minute reso lu t ion w i t h large area ISM tenperaturc sbsorpti on spectra In tergal ac t i c wedi m Gal axies high s e n s i t i v i t y f o r arc minute reso lu t ion d i f f use mi s s i on arc minute resolut ion spectra Galactic coronae X-ray background o r i g i n f luctuat ions deep source c-unts Active &lactic Nuclei Power source Je t s t ructure Radi a t i an Processes large spectral range, high s e n s i t i v i t y 1 keV-10 MeV temporal behavior, spectra high reso lu t ion - arc seconds high s e n s i t i v i t y IC 103 5 5 The second day o f the Workshop was devoted t o a discussion o f instrunentat ion and mission concepts. These are presented i n the body or' the Proceedi ngs. A general assessment o f the techno1 ogical advancements made since the i n s t r m e n t select ion for the HEAO-1 and E ins te in Observatory reveals tha t s ign i f i can t improvements can be made i n detector e f f i c iency , energy resolut ion, background reduction and send t i v i t y over a broad energy range. Mass production techrvqiies f o r moderate arlgular reso lu t ion (- 1' 1 telescopes i 1 i 9 have advanced t o the stage rtrere i t appears feas ib le t o construct an array o f telescopes w i th an area an order o f magnitude greater than AXAF a t a moderate cost. Advances i n spectrometer designs show tha t a one o r two order o f ragni tude improvement i n s e n s i t i v i t y can be achievzd over the E ins te in Ohserva tory capabi 1 i t y w i t h comparabl e resol v i ng power. keV) s?vetJl new concepts were presented t o provide detectors with be t te r angular res--:ution and l w e r backgrounds. A t higher enerqy ( >10 The overa l l impression gained from the many presentations was t h a t the technology i s ava i l able t o make order-of-magni tude improvements over previous and cur ren t ly planned missions i n almost every dimension o f parameter space. 13 111. RECOYMENPATIONS OF THE WORKSHOP The f i n a l day o f the meeting was devoted t o a discussion of the strategy t o carry out the programs required t o achieve the s c i e n t i f i c aims ou t l ined i n the f i r s t day's discussions and w i t h i n the context of the instrumental c a p a b i l i t i e s presented on the second day. The group unanimously reendorsed the s c i e n t i f i c mer i ts o f the X-ray Timing Explorer and the ROSAT missions. These programs should be car r ied out as soon as possible. The next moderate cost f ree- f l yer mission that should be i n i t i a t e d , fo l lowing the XTE and ROSAT, should be d high throughput mission optimized f o r energies below 10 keV with a c o l l e c t i n g area subs tan t ia l l y greater than AXAF, and which combines the c a p a b i l i t i e s o f imaging (-1 ' 1 , spectroscopy (R - lO-lOO), and t iming ( m sec). Such a mission received v i r t u a l l y unc.rimous support. by NASA tcr defirle t h i s mission and t o provide i n i t i a l cost estimates, since i t was recognized tha t such a program must be wel l defined ' f a moderate cost was t o be credible. It was recommended tha t a study group be formed Experiments which are designed f o r f l i g h t OII the Space Shut t le cons t i tu te an e f f e c t i v e u t i l i z a t i o n o f t h i s resource. These experiments can provide fundamentally new data on important astrophysical problems and are complementary t o the ongoing program of moderate cost f r e e - f l y e r missions. Important spec i f i c s c i e n t i f i c questions can be addressed by these experiments which k i l l f a l l outside the scope o f the larger, long durat ion missions. These experiments could b e n e f i t from the LOP o r GAS i n many cases, as we l l as the IPS. More opportuni t ies f o r such special ized observations using the STS should be vigorously pursued. F lna l l y , balloon-borne expelrments were i d e n t i f i e d as a r e l a t i v e l y 11 inexpensive means f o r obtaining answers t o key s c i e n t i f i c questions i n the energy range above 20 keV. experiments f o r the STS o r f o r s a t e l l i t e s . ranging from neutron s ta rs and black holes t o the nuclei o f ac t i ve galaxies represent an area i n which important s c i e n t i f i c questions remain unanswered, and support f o r balloon-borne exper , :mts represents a very cost e f f e c t i v e approach t o advancing t h i s area of astrophysics These experiments can cost f a r less than The high energy spectra of cb jects I n summary, tne Workshop endorsed a pragmatic approach t o near-term X-ray astronomical research. F i r s t , the cont inuat ion o f a vigorous bal oon-borne program i s a necessary complement t o the GRO a t energies t 20 keV Second, opportuni t ies f o r special ized experiments af forded by the SIS can be a unique inexpensive resource t h a t shoul d be f u l l y exploited. i s s i m i l a r l y modest i n i t s cost implica:ions, insofar as i t singles out one area o f inves t iqa t ion which c l e a r l y has p r i o r i t y over a l l others i n the consideration of candidates f o r a moderate cost f r e e - f l y e r mission. Purposely s a c r i f i c i n g reso lu t ion f o r throughput i n a cost-constrained program, the mission may ap t ly be dubbed "HTM", f o r - High - Throughput - Mission. The t h i r d recommendation PRECEDING PAGE BLANK Mol FILMED CORONAE OF NONDEGENERATE SINGLE AND BINARY STARS: A SURVEY OF OUR PRESENT UNDERSTANDING AND PROBLEMS RIPE FOR SOLUTION J e f f r e y L. Linsky J o i n t I n s t i t u t e f o r Laboratory Astrophysics Nat ional Bureau of Standards and Un ive r s i ty of Colorado Boulder, Colorado 80309 1 ABSTRACT E i n s t e i n has discovered X-ray emission from stars loca ted i n nea r ly every p o r t i o n of the HR diagram, and, as a consequence, has completely changed our understanding of s te l lar coronze. Despi te t h i s great accom- plishment or perhaps because of i t , we now recognize t h a t t h e r e are many important unanswered ques t ions t h a t r e q u i r e the c a p a b i l i t i e s of the next gene ra t ion of X-ray in s t rumen ta t ion . In t h i s survey I review what E i n s t e i n has t o l d us about the coronae of stars i n d i f f e r e n t p o r t i o n s of the HR dia- gram, and how the c h a r a c t e r i s t i c s of such coronae compare with what we now know about the s o l a r corona. For each type of s ta r , I then l i s t some impor- t a n t unanswered ques t ions and the gene r i c type of X-ray instrument r equ i r ed t o answer these quest ions. Th i s survey c l e a r l y po in t s out t he c r i t i c a l need f o r a s e n s i t i v e X-ray instrument with both moderate s p e c t r a l r e s o l u t i o n (E/AE = 100-300) and imaging (E/AE - 3) c a p a b i l i t y t h a t can monitor s e l e c t e d t a r g e t s fo r long pe r iods of t i m e . There is a l s o a need f o r high s p e c t r a l r e s o l u t i o n (E/AE = 103-104), provided s e n s i t i v i t i e s can be improved g r e a t l y over E i n s t e i n , and near simultaneous u l t r a v i o l e t spectroscopy. I. SOME INTRODUCTORY COMMENTS While looking through the l i t e r a t u r e i n p repa ra t ion f o r t h i s t a l k , I was s t r u c k by how l i t t l e we knew about s te l lar coronae as l a t e as 1979. P r i o r t o the launch of E i n s t e i n , the l i t e r a t u r e on t h i s t o p i c cons i s t ed of t h e o r e t i c a l papers , most of which are now recognized t o be based on f a l s e premises, X-ray f l u x upper l i m i t s t h a t were not very i n t e r e s t i n g , and a few t a n t a l i z i n g obse rva t ions by rocke t s and the ANS, SAS-3, and IiEAO-1 satel- l i tes of nearby stars t h a t d i d not adequately s,*mple t he HR diagram. Re- v i e w s of t h i s t o p i c p r i o r t o E i n s t e l n (e.g. Linsky 1977; Vaiana and Rosner 1978; Ulmschneider 1979; Mewe 1979) have thus become very dated with t h e rapid onslaught of E i n s t e i n obse rva t ions . ~~ 'S taff Member, Quantum Physics Divis ion, Nat ional Bureau of Standards. 14 P r i o r to E i n s t e i n the f e w de t ec t ed X-ray sources among normal stars cons i s t ed of RS CVn-type b inary systems (e.g. Walter et al. 19801, s e v e r a l nearby la te - type dwarf stars l i k e u Cen tG2V + K l V ) , q Boo (GO IV), and < Boo A (C8 V ) , s e v e r a l M dwarf s t a r s de t ec t ed dur ing l a r g e f la res , and t h e puzz l ing d e t e c t i o n s of Vega ( A 0 V ) and S i r i u s ( A l V + WD). C l e a r l y , w e needed an instrument t h a t could observe sources s e v e r a l o r d e r s of magnitude f a i n t e r than t h e HEAO-1 sky survey. To achieve t h i s ga in i n s e n s i t i v i t y i n sof t X-rays, such an inst rument would r e q u i r e a poin ted t e l e scope wi th imaging o p t i c s and two-dimensional d e t e c t o r s . "he launch of E i n s t e i n ( c f . Giaconni et a l . 1979) i n 1978 f i l l e d t h i s need. Among i ts many accomplishments, E i n s t e i n has l i k e l y had i ts g r e a t e s t impact on s t e l l a r astronomy because i t rep laced a p i c t u r e of s t e l l a r coronae based on a handful of d e t e c t i o n s , which were biased by the h igh s e n s i t i v i t y l i m i t of p rev ious surveys , wi th a p i c t u r e based on d e t e c t i o n s of X-rays from nea r ly every type of star ( c f . Vaiana e t a l . 1981). In p a r t i c u l a r , E i n s t e i n d e t e c t e d X-rays from stars of s p e c t r a l c l a s s e s 0, B, and A, con t r a ry t o most prev ious p r e d i c t i o n s , and frnm s ta rs of a l l ages and s t a g e s of evo lu t ion . The only reyion of t he HR diagram from which no s te l la r X-rays have ye t been d e t e c t e d is t h e upper r i g h t hand corner i nc lud ing t h e K-M s u p e r g i a n t s and g i a n t s (Ayres et a l . 1981). Another s a l i e n t f e a t u r e of t h e Vaiana e t a l . (1981) survey is t h a t t h e r e is a f a c t o r of t h r e e hundrzd spread i n t h e X-ray l u m i n o s i t i e s of la te - type stars of t h e same s p e c t r a l type and luminosi ty c lass , Implying t h a t e f f e c t i v e temperature and g r a v i t y are not the main parameters determining t h e p r o p e r t i e s of s te l la r coronae. As w e s h a l l see, magnetic f i e l d s and s t e l l a r r o t a t i o n p lay important r o l e s . While E i n s t e i n has t o l d us a g rea t d e a l about t h e types of s tars t h a t have hot coronae and the range i n Lx for each spec t r a l - luminos i ty class, i t has t o l d us verv l i t t l e about t he importanc phys ica l p r o p e r t i e s of these coronae. For example, t h e IPC d a t a have so t a r permi t ted only a few crude measurements of coronal tempera tures , f o r example, t h e M5.5e V f l a r e s ta r Proxima Cen t su r i (Haisch et a l . 1980). and temperature estimates us ing SSS d a t a are a v a i l a b l e only f o r n 1 UMa (GO V) (Swank 1981) and a few RS CVn-type systems and Algol (Swank et a l . 1981). To my knowledge the OGS d a t a have been used t o estimate a coronal temperature f o r Capel la (G6III + F 9 I I I ) (Mewe et a l . 1980), but no o t h e r nondegenerate stars or systems. E i n s t e f n lacked the s e n s i t i v i t y at high s p e c t r a l r e s o l u t i o n t o measure coronal d e n s i t i e s . f l o w v e l o c i t i e s , and t o t a l energy ou tpu t f o r any nondegenerate stars. Thus we can only specu la t e concerning t h e geometry, hea t ing rates, wind a c c e l e r a t i o n nechanisms, and the mechanisms r e spons ib l e f o r coronal v a r i a b i l i t y and dynamic phenomena such as f l a r e s . 11. CRITICAL X-RAY MEASUREMENTS NEEDED TO UhUUERSTAND STELLAR CORONAE S ince the purpose of t h i s Workshop is t o p lan for f u t u r e X-ray as t rono- my miss ions , I would l i k e t o o u t l i n e what types of measurements a r e needed t o answer the important problems of s te l la r coronae t h a t E i n s t e i n could not answer. In Table 1 I l is t t h e s p e c i f i c measurements d e s i r e d and t h e mini- mum s p e c t r a l r e s o l u t i o n (E/AE) needed t o make these measurements. I do not s p e c i f y t h e s igna l - to-noise needed, but c l e a r l v s p e c t r a wi th i n s u f f i c i e n t ORIGINAL PAGE IS OF POOR QUALff'Y Q, d m O L . d Y \ \ \ d 3 aJ \ \ \ \ e (I) C m c 4 i C 0 .d U m L, aJ d o, V U 4 V C 3 -d C c aJ m C 0 L c U L, cp aJ CI! -u C m U a? L, Y Q C 3 0 Q aJ 0 aJ Y z rb u C aJ B CJ L Y m (0 aJ z x m p: I x m U " u r( L U M 4 u L) d x L u m C Y x m p: I x \ rr - 0 C c c '. a ? \ w 3 1 ' Q, Q , u .-I 4 " a? a m C o x 4 L x u u w 1 0 , Y " 0 c L , u u u c m r -a. c L I Q E o , a J > u + m U L , m " m Tu! ( P c C C w ¶c W s¶ Q, 4 d m u C rl \ C 0 .d 3 m U .d \r 4 ml u Y E a, U c cp oc C 4 21 u Y m 0, I lo signal- to-noise cannot be used t o d e r i v e information r e q u i r i n g the f u l l spectral r e so lu t ion . The instruments l i s t e d are g e n e r i c , but by energy d i s t r i b u t i o n photometry I have i n mind an instrument with the s p e c t r a l r e s o l u t i o n of an IPC, and by low r e s o l u t i o n spectroscopy I have i n mind an instrument with the r e s o l u t i o n of the SSS a t t h e high energy po r t ion of i t s bandpass. Moderate r e s o l u t i o n spectrometers could have t r ansmiss ion or r e f l e c t i o n g r a t i n g s , e i t h e r mounted before o r af ter the o b j e c t i v e . I n gen- e r a l , one would use an instrument with the lowest poss ib l e s p e c t r a l resolu- t ion capable of making the necessary measurement as t h e throughput g e n e r a l l y dec reases with inc reas ing s p e c t r a l r e s o l u t i o n . The types of measurements each instrument is capable of performing are the following: ( 1 ) Broadband imaging instruments are best s u i t e d f o r i d e n t i f i c a t i o n of t a r g e t s , t i m i n g experiments , and monitoring of coronae f o r v a r i a b i l i t y due t o r o t a t i o n of a c t i v e rc,ions on and off the v i s i b l e d i s k , i n t r i n s i c changes i n t he a c t i v e r eg ions , and flares. ( 2 ) Energy d i s t r i b u t i o n photometry can give crude estimates of coronal temperatures and emission measures from hardness r a t i o s o r t he d i 3 t r i b u t i o n of f l u x measured in each energy b in , provided the d e t e c t o r is w e l l c a l i - b ra t ed , t h e o r e t i c a l estimates of the continuum and l i n e f l u x emit ted by an o p t i c a l l y t h i n plasma i n s t eady- s t a t e equ l l ib r lum a r e a c c u r a t e , and these assumptions a r e r e a l i s t i c . ( 3 ) With somewhat higher s p e c t r a l r e s o l u t i o n ( E / A E - 10-30). a capa- b i l i t y I refer t o as low r e s o l u t i o n spectroscopy, one can more r e l i a b l y es- tlmate coronal temperatures and emission measvies, provided the plasma is isothermal , o r begin t o determine a two-temperature f i t t o t he temperature d i s t r i b u t i o n as Swank et a l . (1981) have done f o r the PS CVn binary systems using the E i n s t e i n SSS. Holt et a l . (1979) have a l s o shown t h a t i t is pos- s i b l e t o estimate abundances of some elements l i k e Mg, Si, S, and Fe using such da ta . (4) Moderate r e s o l u t i o n spectroscopy ( E / A E = 100-300) has the capa- b i l i t y of measuring f l u u e s of i nd iv idua l s p e c t r a l l i n e s or c l o s e blends formed over a wide range of temperatures. The power of such d a t a has long been recognized by s o l a r astronomers, because s e l e c t e d pairs of l i n e s are o f t e n accu ra t e temperature and e l e c t r o n d e n s i t y d i a g n o s t i c s . Jordan and Brown (1981) and Dere and Mason (1981) have r e c e n t l y reviewed the l i t e r a t u r e concerning such diagnost i c s . ( 5 ) F i n a l l y , high r e s o l u t i o n spectroscopy ( E / A E - 103-10b) permits the measurement of l i n e widths , shapes, and Doppler s h i f t s , as well as the sepa- r a t i o n of i nd iv idua l l i n e s i n c l o s e blends. Such data permit t he measure- ment of f l o w v e l o c i t i e s , f o r example winds, i n s te l lar coronae i n t he way t h a t u l t r a v i o l e t s p e c t r a from Copernicus and IUE have permit ted measurements of outflow v e l o c i t i e s i n c o o l e r plasmas. - These measurements are the necessary p r e r e q u i s i t e s f o r answering spe- c i f i c q u e s t i o n s concerning s t e l l a r coron:? such as: 17 ( 1 ) What is t he geometry of t h e emitting plasma? Imaging experiments with high angular p r e c i s i o n can begin t o answer t h i s ques t ion by i d e n t i f y i n g t h e X-ray source with a s p e c i f i c s t a r , and by monitoring the X-ray f l u x varl- a b i l i t y f o r p e r i o d i c changes due to s te l lar r o t a t i o n and t o i n t r i n s i c source changes. Such d a t a con ta in crude information on t h e inhomogeneity of t h e e m i t t i n g s t r u c t u r e s i n a corona and t h e i r dimensions. Estimates of t he volume of t he emitting s t r u c t u r e s r e q u i r e measurements of the emission mea- s u r e and the e l e c t r o n d e n s i t y , and thus r equ i r e moderate or high r e s o l u t i o n spectroscopy. ( 2 ) What is the energy balance i n a s te l lar corona? The r a d i a t i v e com- ponent of t he e o t a l coronal l o s s e s can be determined d i r e c t l y from broadband instruments t h a t measure the t o t a l s o f t X-ray f l u x , provided one knows the s te l la r d i s t a n c e . However, when t h e coronal temperature is less than about 2 x lo6 K , i t is necessary t o estimate the extreme u l t r a v i o l e t emission from the emission measure and temperature. For many stars the coronal energy l o s s e s may not be pr imar i ly i n the form of X-radiation. Measurement of wind expansion l o s s e s r e q u i r e s a de t e rmina t ion of t he mass l o s s rate and co rona l temperature. Since the mass l o s s rate is one r e q u i r e s high r e s o l u t i o n spectroscopy t o ueasure the expansion v e l o c i t y ( v ) and mess d e n s i t y ( p ) , but the r a d i a l p o s i t i o n ( r ) corresponding t o these q u a n t i t i e s must be est imated t h e o r e t i c a l l y . I am s k e p t i c a l t h a t X-ray in- s t ruments with r e s o l u t i o n s of 103-104 and s u f f i c i e n t s e n s i t i v i t y t o s tudy many stars w i l l be f e a s i b l e soon, so t h a t for the f o r e s e e a b l e f u t u r e es t i - mates of coronal wind expansion l o s s e s may have t o be made on t h e basis of u l t r a v i o l e t P Cygni-type p r o f i l e s and microwave f l u x e s ( c f . reviews by C a s s i n e l l i 1979, and Conti 1981). Such d a t a may be adequate f o r 0 stars , but X-ray s p e c t r a are needed t o a..,clure the wind p r o p e r t i e s cf l a t e - type s' 'rs with hot coronae. The t h i r d important coronal l o s s mechanism is o f t e n thermal conduction t o t h e t r a n s i t i o n region, which appears as u l t r a v i o l e t emission l i n e f lux . Thus u l t r a v i o l e t s p e c t r a are needed in a d d i t i o n t o X- ray d a t a t o assess the t o t a l coronal energy hudget of many stars. ( 3 ) What a r e the important coronal hea t ing mechanisms i n d i f f e r e n t types of s t a r s ? Th i s d i f f i c u l t ques t ion has not y e t been answered even f o r t he Sun ( c f . reviews by Kuperus, Ionson, and Spicer 1981; Hollweg 1981; and Ulmschneider 1981 1. Necessary p r e r e q u i s i t e s f o r answering t h i s q u e s t i o n are estimates of the t o t a l input f l u x and i t s d i s s i p a t i o n l eng th t o g e t h e r with information on whether the e m i t t i n g volume is s p h e r i c a l l y symmetric or p r i - m a r i l y in c losed magnetic loops. As descr ibed below, coronal h e a t i n g i n the Sun is enhanced i n c losed magnetic loops; consequently, estimates of coronal f i e l d s t r e n g t h s and f i l l i n g f a c t o r s e i t h e r from e x t r a p o l a t i o n s of photo- s p h e r i c f i e l d s o r from microwave f l u x measurements would be u s e f u l . These types of d a t a , as necessary as they a re . are not s u f f i c i e n t . To c l e a r l y i d e n t i f y the dominant hea t ing mechanism i n s p e c i f i c t y p e s of s t a r s , we need r e l i a b l e t h e o r e t i c a l c a l c u l a t i o n s t h a t point out the unique s i g n a t u r e s of t h e d i f f e r e n t p o s s i b l e mechanisms i n X-ray and o the r da t a . 13 ( 4 ) What are the important wind a c c e l e r a t i o n mechanisms i n d i f f e r e n t t y p e s of stars with hot coronae? Th i s is a l s o a d i f f i c u l t ques t ion because d i f f e r e n t mechanisms have been proposed ( r a d i a t i o n pressure ; momentum depo- s i t i o n by a c m s t i c waves and by magnetohydrodynamic waves), and t h e unique s i g n a t u r e s of these d i f f e r e n t mechanisms have not yet been worked ou t . C a s s i n e l l i (1919). Hearn (1981) , Castor (19811, and Linsky (1981) have re- c e n t l y reviewed these mechanisms and the types of s t a r s fo r which each may be important . C l e a r l y i t is necessary to determine e m p i r i c a l l y mass loss r a t e s , t e rmina l v e l o c i t i e s , and temperatures fo r d i f f e r e n t types of s t a r s , and e s p e c i a l l y how these q u a n t i t i e s depend on s t e l l a r luminos i ty , g r a v i t y , e f f e c t i v e tempera ture , and age. As prev ious ly noted, t h i s is d i f f i c u l t because one gene ra l ly needs high r e s o l o t i o n X-ray s p e c t r a ; however, f o r t h e 0 stars u l t r a v i o l e t s p e c t r a and microwave f luxes may provide much of t h e necessary d a t a t o d i sc r imina te among d i f f e r c n t mechanisms. 111. WHAT WE KNOW ABOUT THE SOLAR CORONA THAT WILL LIKELY BE OF RELEVANCE TO OTHER STARS Before proceeding t o a d i scuss ion of t he s t e l l a r d a t a and the unan- swered ques t ions they raise, it is important t o summarize what has been learned recently about t he one s t e l l a r corona t h a t can be s tud ied wi th t h e necessary s p a t i a l r e s o l u t i o n . Z do not wish t o imply t h a t a l l s t e l l a r coronae a r e s i m i l a r t o the Sun or even t h a t the s o l a r corona e x h i b i t s a l l t he phys ica l p rocesses t h a t occur i n s t e l l a r coronae. However, t he Sun probably does provide u s e f u l pro to types of phenomena and mechanisms t h a t we shogld search f o r on s ta rs , but t h e r e a r e some types of s tars l i k e t he 0 s t a r s t h a t have coronae t h a t may be q u a l i t a t i v e l y d i f f e r e n t from the s o l a r example. Recent reviews of phenomena and s t r u c t u r e s seen i n X-ray obser- va t ions of the Sun inc lude Withbroe and Koyes (19771, Vaiana and Rosner ( 1 9 7 8 ) . and Webb (1981). a ) Geometry of the Solar Corona ( 1 ) Perhaps t h e most b a s i c s ta tement t h a t can be made about t he s o l a r corona as imaged i n s o f t X-rays is t h a t it is not i n any way homogeneous o r s p h e r i c a l l y svmmetric. On the ContrLry, i t is highly s t r u c t u r e d . Thus any t h e o r e t i c a l model of a s t e l l a r corona t h a t assumes s p h e r i c a l symmetry l acks e s s e n t i a l physics . ( 2 ) The bas i c s t r u c t u r e i n the s o l a r COroila is t he c losed magnetic f l u x tube. There a r e th ree l i n e s of ev idence t h a t support t h i s s ta tement . F i r s t , s o f t X-ray images of the corona c l e a r l y show s t r u c t u r e s t h a t appear t o be i n - d i v i d u a l loops or a rcades of loops. Second, p o t e n t i a l ( i . e . fo rce - f r ee ) ex- t r a p o l a t i o n s of the observed photospher ic magnetic f i e l d s i n t o the corona are near ly co inc ident with the observed X-ray loops ( c f . P o l e t t o e t a l . 19751, but the match is not p r e c i s e , implying t h a t t he re a r e e l e c t r i c c u r r e n t s i n the corona. Thus the s o f t X-ray emission o u t l i n e s t h e three-dimensional magnetic f i e l d s t r u c t u r e of t he corona. Th i rd , the s o l a r corona is a low 0 plasma ( i . e . the thermal energy is much less than the magnetic energy) , s o t h a t t he plasma should be confined by c losed loop s t r u c t u r e s . 19 ( 3 ) The s o l a r corona has t h r e e types of reg ions . Act ive r eg ions con- sist of b r igh t loops t h a t g e n e r a l l y have a t least one f o o t p o i n t i n a spo t penumbra or umbra. These loops have s t r o n g magnetic f i e l d s and connect areas of oppos i t e magnetic p o l a r i t y . Qu ie t reg ions a l s o con ta in c losed loop s t r u c t u r e s , but t he f i e l d s are weak and t h e r e are no sunspots . F i n a l l y , coronal ho le s are regions of very weak X-ray emission and have f i e l d l i n e s t h a t are open t o space. (4) I n a c t i v e and q u i e t reg ions e s s e n t i a l l y a l l of t h e observed X-ray emission is from the c losed loops. Thus the c losed loops are t h e solar corona, and meaningful coronal models must inc lude a loop geometry and in- co rpora t e magnetic fo rces . - (5) Empir ical models of coronal s t r u c t u r e s c l e a r l y show t h a t d i f f e r - ences i n phys i ca l cond i t ions ( tempera tures , d e n s i t i e s , mass and energy f l u x ) from point t o poin t across the Sun appear t o be i n t ima te ly r e l a t e d to the conf igu ra t ion and s t r e n g t h of t h e magnetic f i e l d s , i n p a r t i c u l a r whether t h e f i e l d s are open or closed. ( 6 ) The s t r u c t u r i n g of t h e solar atmosphere i n t o loops dominates the whole atmosphere from t h e chromosphere, through the t r a n s i t i o n reg ion , and out some d i s t a n c e i n t o the corona. ( 7 ) Coronal ho les are the o r i g i n s of high speed wind streams (wi th ve- l o c i t i e s up t o 800 km s - l ) and perhaps most of t h e mass loss from t h e Sun. The l a t t e r po in t is d i f f i c u l t t o v e r i f y as coronal ho le s t y p i c a l l y l i e at high l a t i t u d e s and wind measurements are made mostly i n t h e e c l i p t i c . Z i rke r (1981) has r ecen t ly reviewed the p r o p e r t i e s of coronal ho les . (8) There is s t r o n g evidence t h a t magnetic f i e l d s c o n t r o l t h e energy balance i n loops by channel ing the flow of mass and energy as well as pre- sumably p lay ing a major r o l e i n t h e hea t ing rate (see Holweg 1981 f o r a recent review of t h i s t o p i c ) . Withbroe acd Noves (1977) have e s t ima ted the coronal energy budget f o r a c t i v e reg ions ( r e p r e s e n t a t i v e of loops wi th s t r o n g f i e l d s ) , qu ie t reg ions ( r e p r e s e n t a t i v e of loops with weak f i e l d s ) , and coronal ho le s (open f i e l d r eg ions ) . I t is i n t e r e s t i n g t h a t a l though t h e t o t a l coronal l o s s rates (presumably equal t o t h e hea t ing r a t e s ) are 30 times l a r g e r i n a c t i v e than i n q u i e t r eg ions , both reg ions a r e cooled p r i - m a r i l y by conduction down t o the t r a n s i t i o n region and X-radiat ion. For both reg ions t h e wind losses are very small, presumably beca. .se c losed f i e l d l i n e s i n loops prevent the escape of plasma t o space. The t o t a l l o s s rates f o r coronal ho le s and q u i e t reg ions a r e s i m i l a r , but ho les a r e cooled p r i - mar i ly by t h e s o l a r wind flux i n s t ead of conduction down to t h e t r a n s i t i o n region and X-radiation. Ala0 ho le s have lower tempera tures , p r e s s u r e s , and temperature g r a d i e n t s than quf e t and active regions. ( 9 ) Thus t h e s o l a r corona i n r e a l i t y c o n s i s t s of many loops wi th d i f - f e r e n t phys ica l p r o p e r t i e s t h a t coex i s t due t u t he thermal i s o l a t i o n pro- vided by c losed magnetic t i e l d l i n e s . 20 b) Proper t ies of Solar Coronal Loops (1) Individual X-ray images of the solar corona deceptively suggeet tha t the loops are static; but on the contrary, the Solar corona is always changing. The brightness of individual loops changes with time due to the f i l l i n g and draining of f l u x tubzs wlth changes i n the heat ing rate and res t ruc tur ing of magnetic f i e l d l i n e s on a time scale of hours. Many loop s t ruc tures , ca l led empherical regions, appear and fade within a day. so-called br ight points are compact, high density loop s t ruc tu res tha t rep- resent newly emerging magnetic flux. day, hut even the large ac t ive region loops l i v e for only a few days. Thus theore t ica l loop models t ha t assume steady-state conditione and hydros ta t ic equilibrium are gross oversimplif icat ions. The These typ ica l ly survive less than a (2) When f i r s t seen i n X-rays, loops a r e generally small, hot , dense, and bright. As they evolve, the loops generally expand, decrease i n plasma temperature and densi ty , and thus i n X-ray brightness. Golub - et al. (1980) presented evidence tha t the energy densi ty of loops (UT = nkT) is Inversely proportional t o the loop length (L) and age (t). UT - f!l7, where QT is the magnetic f lux , and tha t the gas pressure P - 6 , where B is the average longi tudisa l f i e ld . They a l s o argued t h a t (3) Observed loop lengths, L - 10e-lO1O cm, are much smaller than the so la r radius, (Re = 7 x lOl0 cm) and the coronal pressure scale height (-0.23 Rg for T = 2.5 x lo6 K). Although loop s i zes form a continuous dis- t r i bu t ion , there a re many mre small than large loops. Loop temperatures generally l i e i n the small range, T = 2-3 x lo6 K, and d e n s i t i e s l i e i n the much la rger range, P - 0.03 - 3 dynes c m There is considerable evidence tha t individual f l ux tubes are isothermal, both i n the longi tudinal and r ad ia l d i r ec t ions , and tha t they are i sobar ic , consis tent with t h e i r heights being smaller than a pressure scale height. There is no obvious reason why loops cannot be larger than a stellar radius or coronal pressure sca le height i n s t a r s with d i f f e ren t g rav i t i e s and magnetic f i e l d configurat ions, but the so l a r data suggest tha t at l e a s t dwarf stars with br ight X-ray emission probably have small, dense loop s t ruc tu res . = 108-1010 cnr3. Thus loop pressures l i e i n the range, ne_2 (4) There a re several empirical sca l ing laws for the proper t ies of so la r coronal loops (cf . Withbroe 1981). 1.4 x lo3 (PL)'I3, where TmX is the maximum loop temperature, proposed by Rosner, Tucker and Valana (1978). tha t the empirical scatter about t h i s sca l ing l a w IS very la rge , and the assumptions of hydrostat ic equilibrium, absence of flows along the loop or conductive heat flow a t the bottom, and constant loop cross sec t ion made by Rosner - et al. may not be val id . Recently, Ser io - et al. (1981) have gener- a l ieed the Rosner - et al. sca l ing law t o include loops l a rge r than a preseure scale height, and P r i e s t (1981) has reviewed the theory of loop flows and ins t a b i l i t ies. One widely quoted law is Tpax = Since sol r loops have only a small range i n temperature, t h i s law implies t ha t P - L- r . Webb (1981) has pointed out 21 (5) A t present we should view recent loop models ae moderately succes8- f u l as they can i n t e r r e l a t e plasma parawtercl roithla a loop i n a mamet consis tent with observations. Such models sha-tld also ba thermally s t a b l e provided that the maximum temperature occurs at the top of the loop. How- ever , the amjor problem with these models L that the loop proper t ies , i n pa r t i cu la r h, appear to depead only on the heat iq rate and not on the heat ing mechanism or even where i n the loop the heating occurs. are no unique s igna tures of the heating process. t i ons can help. (1981) a d Kuperus - et al. (1981). Thus there Perhaps s t e l l a r observa- For a recent overview of heating mchanisms see lbllwag (6) Final ly , we should ask why the range i n solar loop temperatures is so small. respond to changes i n the heating rate by evaporation or condeneation of material at t r a n s i t i o n region t e q e r a t u r e s (-105 K) at the loop footpoints. For example, increased heat ing anywhere in the loop leads to enhanced con- duct ive heating a t the footpoints t ha t evaporates t r a n s i t i o n region gas i n t o coronal (i.e. T > 10621C) gas. This process is s t a b l e because r ad ia t ive los- ses from the loop (-ne) can then balance the increased heat ing with l i t t l e change i n temperature . of coronal gas at the footpoints , decreased radiative losses, and energy balance with l i t t l e change i n loop temperature. Thue stellar coronal tem- peratures much i n excess of 3 x lo6 K may indica te a very d i f f e r e n t energy balance or geometry i n such coronae. I bel ieve t h a t t h i s small range indica tes that loops can e a s i l y Conversely, decreased heating leads to condensation IV. DWARF STARS OF SPECTRAL TYPES F, G , K AND M Given as background what we now know about the solar corons, I would l i k e to survey the HR diagram and ask two questions. F i r s t , what have we learned in general about the coronae of each group of stars from Einstein and other experiments? Second, what ore the important unanswered questions tha t could be answered by the next generation of X-ray experiments? would l i k e t o compare and cont ras t the solar and stellar da ta , I w i l l begin with the stars tha t are most solar- l ike and gradually move on to stars tha t have very d i f f e r e n t coronae or perhaps outer atmospheres t h a t should not even be ca l led coronae. a) What We Have Learned from Einstein Since I (1) Einstein has detected X-rays from almost every type of star with the exception of the cool supergiants, as w i l l be discussed below. comprehensive survey of Eins te in stellar observations, Vaiana - et al. (1981) detected e s s e n t i a l l y a l l nearby F-M dwarf stars tha t were observed f o r suf- f i c i e n t l y lon times. For many of these stars the IPC count rates exceeded 0.1 count8 S-', correspcnding to 2 x ergs C u r 2 8.' f o r the 0 . 2 5 4 keV band, and typ ica l l imi t ing s e n s i t i v i e s were 10-13 ergs car2 8-1 i n 2000 sec- ond exposures. The HEAO-1 A2 experiment a l l sky survey was unable to de tec t many F-M dwarfs because its s e n s i t i v i t y was only -6 x I n the f i r s t e rgs c r 2 8-l. (2) It is clear tha t the s o f t X-ray emission is from analogues of the solar corona ra ther than accret ion as i n the classical X-ray binary systems because both s ing le and binary stars are s o f t X-ray sources, and as w i l l be 22 deecribed later, the X-ray emission from the t i d a l l y synchronous binary sys- tems ( l ike the RS CVn systems) is similar to that of single stars wlth the same ro ta t iona l welocities. (3) Perhaps the u m t f a r reaching r e s u l t Ls that the standard stellar parameters of mass, effect iwe temperature, and gravi ty , which de tere ine where a star is located i n tlre KR diagram, are wt the mst inportant parameters determining the s o f t X-ray 1uPinosi t ies of F+l dwarf etare a d aost o the r typea of stars. There are two aain reasons f o r t h i s conclusion. F i r s t , the Vaiana et al. (1981) survey shaus t h a t fo r stars of the - effect iwe tem- perature and lumlnosity class there is a fac to r of 300 spread i n Lx, which could be even la rger because of thz s e n s i t i v i t y threshold of Einstetn. second, thee re811 value of L~ fo r the P, G , K, ~d M *rfs IS roughly 1028.5 ergs so’, independent of spec t r a l type even though L b ? decreases rap id ly toward the cooler stars. With appropriate hindsight we should have expected t h i s r e su l t because the solar X-ray brightness varies g rea t ly fram point to point across the solar surface, and it Is apparently control led by the local magnetic f i e l d s t reugth d geometry. p (4) The range in observed f o r +type dwarf stars is e n t i r e l y aonsis- t e n t with the range In seen in the Sun. For exaaple, the rean value of tm of the d i s t r ibu t ion for ear?y C dwarfs, whereas the value of & for the *le solar surface if covered with active regions ($ 1029*3 ergs s-1. would l ie near the top of t h i s d is t r ibu t ion . This latter coincidence could be Interpreted either tha t the br ightes t solar-type stars are covered en- t i r e l y by coronal s t ruc tu res similar i n brightness to solar plages ( ac t ive regions) or that they are p a r t i a l l y covered with superplages (regions with X-ray brightness mch l a rger than is seen i n solar active regions). Hyades data s t rongly support the existence of mperplages. fo- the quiet Sun is roughly ergs S-I, which lies close to the bot- The (5) There is convincing evidence tha t stellar ro t a t ion plays a critical ro le in determining the relative X-ray lumlnosity of stsrs with sidlar spec- tral type and luminosity class, although the funct ional form of t h i s r e l a t i o n .has not yet been decided conclusively. The Importance of ro t a t ion can be seen at once comparing three earl G dwarf stars - u1 UMa (00 V), a Cen A served by Vaiana et al. (h = spec t ra l ? s c (u sin i = 9 ka 8-l) . have low X-ray l d n o s i t i + s (L, - 1027*1 and IOa6.* ergs s-l, respect ively) and sam11 r o t a t i o d ve loc i t i e s (v s i n i - 2 (G2 V), and the quiet Sun (62 V). u T UMa i s the mst luminous G dwarf ob- and it is a rapid ro t a to r f o r its - By comparison a Cen A and the quie t Sun s-1). However, there is disagreemnt 88 to the dependence of L, 011 r o t a t i o n a l parameters for F-# dwarfs. and RS CWn systems, Ayres and Linsky (1980) suggested that L x / L b l increarres rapidly with v s i n 1. observations of ssngle F7-I45 stars t o sbow tha t L, - (v s i n i)l*ge*’p inde- pendent of L b l and 1wPinositp class. This r e s u l t is cons is ten t with the X- ray surface f l u x being proportional to 92, where o is the angular ro t a t iona l velocity. On the other hand, Walter and Bawper (1981) and Walter (1981) found tha t h / L b 1 - Q f a i r l y toe l l represents the Einst. in and HEAO-1 data Using HSAO-1 observations of a few s ing le stars Pa l l av ic in i - et el. (1981) used the ex i s t ing Eins te in - - for GI RS Cvn system and 13 r a p i d l y r o t a t i n g F8-C5 dwarfs s tars . Subse- quent ly , Walter (1982) proposed t h a t no s i n g l e power law can f i t t h e E i n s t e i n observa t ions of s i n R l e Fa-C2 dwarfs inc luding the Sun and Hyades stars, but i n s t e a d t w o power laws are needed of t h e form L , / L b l 5 @ w i t h a break near a rotational per iod of 12 days. I t is interesting t h a t Vaughan and Preston (1980) and Vaughan (1980) f i n d evidence chat t h e c h a r a c t e r of s te l la r dyna- mos changes when dwarf s t a r s slow down t o a rotat ional per iod of 12 days , which occurs at an age of about 1 x lo9 yr . ( 6 ) The age of la tc - type s tars an t h e main sequence is also an impor- t a n t parameter, perhaps i n a more fundamental s e n s e than r o t a t i o n . I n t h e 1960s t h e r e w e r e a number of important s t u d i e s of the r o t a t i o n of F and C. dwarf stars by Wilson, K r a f t , Skumanich, and o t h e r s , who concluded t h a t s te l la r r o t a t i o n a l v e l o c i t i e s decrease wi th age on the main sequence, pre- sumably as a consequence of angular mmentum loss In s te l la r winds. On t h e b a s i s of t h e s e d ta , Skumanich (1972) proposed t h a t the s t e l l a r e q u a t o r i a l v e l o c i t y , v - t l T 2 , where t is t h e age on t h e main sequence. Recent ly , r o t a t i o n a l v e l o c i t y measurements have become mre p r e c i s e as a r e s u l t of increased throughput f o r high r e s o l u t i o n spectroscopy, Four ie r techniques f o r analyzing l i n e p r o f i l e s (cf. Gray 19761, and programs ts monitor t h e r o t a t i o n a l modulatlcn of s t e l l a r a c t i v e reg ions by observing t h e Ca 11 f l u x (e.p. Vaughan et a l . 1981). Duncan (1981) and Soderblom (1981) showed t h a t these new d a t a confirm t h e Skumanich (1972) rotat ion-age c o r r e l a t i o n . As ve t t h e r e is no thorough survey of the r e l a t i o n of s te l la r X-ray luminosi ty or s u r f a c e f l u x wi th age. However, t h e S t e r n et a l . (1981) s tudy of t he Hyades c l u s t e r ( t - l o e o 6 ) stars wi th E i n s t e i n c l e a r l y i n d i c a t e s a d e f i n i t e c o r r e l a t i o n of b r i g h t X-ray emission wi th youth. Despi te t h e i r sen- s i t i v l t y t h r e s h o l d , which corresponds to a factor of 10 h igher than the q u i e t Sun valu,? of Lx, they d e t e c t e d 80% of t h e F and C dwarfs i n t h e c l u s t e r . Thei r b r i g h t e s t source is 71 Tau, an M) V star wi th v s i n i - 200 km s-l and L, 2 1030 e r g s s-l, which & - I O 3 ' e r g s s-'. X-rays as the Sun would he I f covered e n t i r e l v by plages , i t ms: c o n t a i n f l u x tubes which are beyond the range of b r i g h t n e s s , and t h u s presumably a l s o of d e n s i t y , t y p i c a l l y seen i n t h e solar corona. S t e r n et a l . (1981) have proposed, on the b a s i s of s c a l i n g laws, t h a t t h i s s ta r c o n t a i n s super- a c t i v e regions covering about 10% of t h e s t e l l a r sur face . and t h e i r b r i g h t e s t G s ta r is HD 27836 (C1 V ) for Since t h i s s t a r is about f i v e t i m e s as b r i g h t i n (7) By analogy wl th the solar corona, t he F-M dwarf starq, e s p e c i a l l y t h e young stars with b r i g h t X-ray emission, should have s t r o n g c losed magnet- IC f i e l d s t r u c t u r e s i n t h e i r coronae. This argument is no longer specula- t i o n , but has now been confirmed by two s e p a r a t e l i n e s of evidence. F i r s t , Robinson, Worden, and Harvey (1980) have measured photospheric magnetic f i e l d s i n two young dwarfs, C Boo A (C8 V) and 70 Oph A (KO V ) by merrsurlng l i n e s p l i t t l n g i n unpolar ized l i g h t . For B o o A, they found t h e f i e l d t o be 2,600 t 400 Gauss covering 30% of the sur face . Harcy (1980) has d e t e c t e d s a g n e t i c f ie lds i n a d d i t i o n a l s tars and is now s tudying v a r i a b i l i t y and to- t a t i o n a l m d u l a t i o n of s t e l l a r a c t i v e reg ions wi th l a r g e magnetic f i e l d s . I t is i n t e r e s t i n g t h a t the m J o r d i f f e r e n c e noted so f a r between the photo- s p h e r i c magnetic f i e l d s of these young stars compared to t h e Sun Is t h a t t h e strong f i e l d s cover a much l a r g e r f r a c t i o n of the s te l la r sur face . 24 The second t y p e of d i r e c t evidence is obse rva t ions of microwave emis- s i o n from stellar coronae. Gary and Linsky (1981) detected s t e a d y 6 cm emission from x1 O r i ((10 V ) ahid u\’ Cat (dI4lS.Se) w i th t h e VLA, which they i n t e r p r e t e d as gyroresonant emission from e l e c t r o n s s p i r a l l i n g along coronal magnetic f i e l d l i n e s . Th i s emission is c o n s i s t e n t w i t h coronal f i e l d s of roughly 300 Gauss, s l i g h t l y l a r g e r than tha f i e l d s t r e n g t h 8 i n solar coronal loops. Both stars are b r i g h t X-ray sourcee for t h e i r s p e c t r a l types. ( 8 ) There are as ye t very few w a s u r e m n t s of coronal temperatures f o r la te- type dwarfs. I n p r i n c i p l e , the E i n s t e i n IPC pu l se he igh t s p e c t r a can provide crude temperature estimates, but meaningful temperatures await t h e reprocessing of thcue d a t a . The E i n s t e i n SSS inutrument h a s proved to be u s e f u l i n e s t ima t ing coronal temperatureu f o r RS CVn systems, but 90 far t h e only coronal temperature f o r l a t e - type dwarf stars ob ta ined wi th t h e SSS is for the young (10 V star x1 LNa, f o r which !hank (1981) e s t ima ted T - 3-5 x lo6 K. Haisch et el. (1980) have 0180 es t ima ted T - 4 x loa K for Proxima Centauri ( d M k ) at qu ie scen t times us ing IPC pu l se he igh t spec t r a . b) S o r Important Unanswered Quest lot is 1. What is the range of coronal temperatures t h a t occurs i n magnetic loop s t r u c t u r e s ? I n p a r t i c u l a r , does the temperature increase wi th s te l lar X-ray luminosity or s u r f a c e f l u x , and does i t depend at all cm s p e c t r a l type among the cool dwarfs? T h i s q u e s t i o n can be answered wi th energy d i s t r i b u - t i o n photometry, perhaps with reprocessed E i n s t e i n IPC data, and low resolu- t i o n spectroscopy (see Table 1). 2. What f r a c t i o n of the coronal volume is f i l l e d with b r i g h t X-ray emitting loops for stars of d i f f e r e n t ages, s p e c t r a l types, and $? Since the volume of the e m i t t i n g region can be derived from the emission uea3ure and e l e c t r o n d e n s i t y , t h i s q u e s t i o n r e q u i r e s moderate r e s o l u t i o n s p e c t r a . 3. What are the evo lu t iona ry time scales of coronal loops and a c t i v e regions, and what are t h e p r o p e r t i e s of stellar magnetic c y c l e s and dynamos? Monitoring of stars with imaging experiments is important f o r s tudy ing t h e s e t h e scales, but such obse rva t ions should be accompanied by simultaneous measurement of the s te l lar t r a n s i t i o n regions witn u l t r a v i o l e t spectroscopy, photospheric magnetic f i e l d s with o p t i c a l spectroscopy, and microwave obser- va t ions with the VLA. 4. What are the d e n s i t i e s of coronal loops i n d i f f e r e n t t y p e s of s t a r s ? l a r g e L, are dense and small, or tenuous and large, or both. Moderate or high r e s o l u t ion spectroscopy is needed to measure densi ty-senoi t i v e l i n e r a t i o s i n o rde r t o answer t h i s quest ion. Of s p e c i f i c i n t e r e s t is the ques t ion of whether loops i n stars wi th 5 . What mechanism or lPechanisms heat coronae i n late-type dwarf s t a r s ? The obse rva t iona l d a t a needed t o answer t h i s d i f f i c u l t q u e s t i o n cannot be pred ic t ed e a s i l y due to our lack of knowledge as t o how even t h e solar co- rona is heated. balance i n dwarf star coronae changes with e f f e c t i v e temperature , we, and Lx, and whether the geometry of t h e s e coronae are s o l a r - l i k e as we p r e s e n t l y M y guess ir t h a t we need to know more about how the energy 25 bel ieve. An understandinq of the energy balance w i l l r e q u i r e energy d i s t r i - but ion photometry, high r e s o l u t i o n spectroscopy, and u l t r a v i o l e t spec t ro - scopy (see Table 1). Also, the s tudy of e c l i p e i n g systems wi th imaging experiments w i l l provide information on where t h e coronal cooling occur s f o r comparison with t h e o r e t i c a l models. 6 . Do a c t i v e dwarf stars (Lx >> Lx(S)) have hot winds and low -8s loss rates l i k e the Sun? Th i s q u e r t i o n might be unanswerable wi th fo re see - a b l e in s t rumen ta l developments, but it Is important to t r y t o answer t h i s ques t ion by sea rch ing for Doppler-shifted X-ray l i n e s with a high throughput high r e s o l u t i o n spec t roscop ic experiment. 7. What are the fundamental d i f f e r e n c e s between the coronae of young and o ld dwarf stars. A r t t h e d i f f e r e n c e s p r i m a r i l y i n t h e f r a c t i o n of t h e coronal volume f i l l e d wi th loops, t h e loop l e n g t h s , d e n s i t i e s , temperatures , or total coronal hea t ing r a t e s ? Since the measurement of coronal d e n s i t i e s is cr i t ica l to answering t h i s ques t ion , we need moderate or high resolution s p t c t ra . 8. What are t h e hottest dwarf stars with s o l a r - l i k e corona? A high throughput imaging experiment should be a b l e to answer t h i s quest ion. V. PRE-MAIN SEQUENCE STARS Pre-main sequence (preMS) stars, inc lud ing the T Taur i stars, the young stars i n c l u s t e r s such as Orion, and t h e r e c e n t l y i d e n t i f i e d post-T Tauri stars, probably have coronae t h a t are q u a l i t a t i v e l y similar t o t h e young F-H dwarfs t h a t r e c e n t l y becalae main sequence (nS) stars. However, as we s h a l l see, t h e X-ray emission from t h e p r e 6 stars can be absorbed by o v e r l y i n g c i r c u m s t e l l a r gas. a) What We Have Learned from E i n s t e i n 1. When de tec t ed as X-ray sou rces , the T Tauri and o t h e r preHS s ta rs are the most luminous among the l a t e - type stars t h a t are not known t o be close b i n a r i e s . Since t h e s e stars are d i s t a n t , t h e d e t e c t i o n t h r e s h o l d s are l a rge ; f o r example, Lx - IO3O e r g s 8-l for t h e Taurus-Aurigrre cloud and Nevertheless , 10 out of t h e 14 known T Tauri stars i n the Taurus-Aurigae cloud b r i g h t e r than aqr = 13 have now been d e t e c t e d as X-ray sources by Walter and Kuhi (19811, and t h e f a i n t e r T Tauri s ta rs were l i k e l y not de t ec t ed due t o the s e n s i t i v i t y l i m i t of E i n s t e i n . Feigelson and de Camp11 (1981) de tec t ed Dc Taur i at Lx - 8 x I O m ergs E-', and C a b !1980) d e t e c t e d Th 12 a t L, - 6 x lo3' e r g s 8-l in t h e Taurus-Aurigse cloud. q u i e t Sun. The most luminous preMS s tar de tec t ed so far is <;w O r 1 (L, - 5 x lO3I e r g s S-') observed by Fcigelson and de Camp11 (1981). - 2 x 1031 e r g s 6-l for t h e Orion cloud. These stars are 10' times more luminous than t h e 2. Given t h a t the de t ec t ed prcHS stars are so luminous in X-rays, an important ques t ion is why many of ches t s ta rs in the Taurus-Auriga and Orion clouds, p r imar i ly those with s t r o n g h emission and blue excess emission, are not d e t e c t e d as X-ray sources . Cahm (1980) argued t h a t i n t e r s t e l l a r abaorpt ion is not t h e mason for the mnny nondetect ions on t h e basis t h a t 26 RW Aurigae is a T Tauri star with extrelaely br ight o p t i c a l and u l t r a v i o l e t emission l i n e s , yet i t is not a detected X-ray source desp i t e its ly ing aut- s i d e the dark obscuring regions of the Taurus-Aurigae cloud, and the small i n t e r s t e l l a r gas column dens i ty i n its l i n e of sight. Instead he argued tha t t he preHS stars have hot coronae t h a t are surrounded by extensive cool c i rcumste l la r gas envelopes tha t can t o t a l l y absorb the X-ray emission in 80- cases. For example, he estimated o p t i c a l depth$ f o r the RW Aurigae envelope of ~ ( 0 . 6 keV) 1) 72 and ~ ( 2 k e V ) 2. Thus the e o f t X-ray emission would be t o t a l l y absorbed, but t h i s star might be a hard X-ray source. Walter and Kuhi (1981) supported C a b ' s (1980) model by f ind ing an inverse co r re l a t ion among the T Tauri etars of Ho equivalent widths and X-ray fluxes. They concluded tha t the T Tauri stars have small so lar - l ike coronae surrounded by extensive cool envelopes t h a t produce the Ha emission and X- ray absorptiori . 3. Feigelson and de Camp11 (1981) observed rapid v a r i a b i l i t y i n the X- ray f lux of the T Tauri star Dc Tau. They detected no f lux during the f i r s t 35 minutes of observation and then a rapid increase i n f lux on a time scale of 4 minutes. This rapid f l a r e - l ike v a r i a b i l i t y also suggests emission from a small region of Ugh dens i ty close t o the star, and l i k e the b r igh te s t X- ray f ea tu res on the Sun (the so-called br ight po in ts ) could be emission from small dense loops of newly emerging magnetic f lux j u s t above the photosphere. 4. In T Tauri s r a r s f o r which X-ray emission Is detected, k / L b l - lo-', which is similar to the r a t i o for the youngest F and G main sequence stars. f r m the corona is Parch loss than from the chromosphere and/or envelope, and the r ad ia t ive lo s ses are comparable t o the wind losses in T Tauri stars. I I / L ~ ~ 2 x 10% LCa I I / L ~ ~ - 5 x lo-', and LWind/Lbl - 5 X f o r e same stars. Thus the e a l s s l o n BY comparison ha/~bl - 10-3, 5 . The coronal temperatures for T Tauri stars are poorly kncwn, but Feigelson and de Camp11 (1981) estimated T 2 5 x LO6 K f o r DG Tau during its flare., and Walter and Kuhi (1981) estimated T 2 14 x lo6 K f o r AA Tau on t he bas i s of IPC pulse height spectra. Estimated T Tauri coronal temperatures are 1ikel.P a f fec ted by c i rcumste l la r gas absorption. 6 . Recently Feigelson and Kriss (1981) and Walter and Kuhl (1981) dis - covered f i v e X-ray sources tha t l ie 1-3 magnitudes above the main sequence yet have weak Ha emission, no u l t r a v i o l e t excess, and show no evidence f o r o p t i c a l v a r i a b i l i t y or winds. They therefore believe these stars to be in- termediate i n age between the T Taur l s and mein sequence stars. are lumlnous (L, - 1030 e rgs sol) , and presumably have coronae similar t o but more ac t ive than the stars tha t have recent ly a r r ived on the mein sequence. These stars 7. Three mechanisms have been proposed t o explain high temperature emiselon from preMS s t a r s . Ulrich (1976,1978) and Mundt (1980) studied accre t ion shocks, but t h e i r models typically predic t T < 3 x lo6 K, which appears t o be smaller than observed. Heating by shocks at the d n d - i n t e r s t e l l a r medium i n t e r f a c e w a s proposed b;l Kuhl (1964) and Ku and Chanan (1979). hut the observed an t i co r re l a t ion of Hu equivalent atidth and X-ray emission are hard t o explain by t h i s model. A t present, the most p l aus ib l e model appears t o be tha t of a small corona vurrounded by an extensive cool circumetellar gas envelope. da t e , then the coronae of preM stars are probably extreme examples of the so l a r corona with emission from small dense magnetic loop s t ruc tu res . I f t h i s model is va l id , as is suggested by t h e b) Some Important Unanswered Questions 1. What are the physical p roper t ies of the X-ray emi t t ing regions i n prePtS stars: t h e i r temperatures, d e n s i t i e s , and volumes? As previously dis- cuesed i n S I V , temperature measurements require e i t h e r energy d i s t r i b u t i o n photometry or low reso lu t ion spectroscopy, but the latter is probably needed fo r these stars because one must measure both the c i rcumste l la r absorption and coronal temperature together. While i n p r inc ip l e well-calibrated energy d i s t r i b u t i o n photometry could measure both parameters, experience with the Eins te in IPC points out the need f o r higher reso lu t ion data. Measurements of density and volume requi re at least moderate reso lu t ion spectroscopy. 2. What is the geometry of the emit t ing regions i n preMS s t a r s ? I Thus monitoring the X-ray emission from these stars f o r would estiamte tha t the ro t a t iona l periods f o r there stars probably l i e i n the range 3-8 days. t h i s time period with an imaging experiment should determine whether the X- rays are emitted from a few ac t ive regions or uniformly across the stellar sur face , and such observations should a l s o determine time scales fo r the v a r i a b i l i t y of these a c t i v e regions. High reso lu t ion u l t r a v i o l e t spec t r a should a l s o be usefu l i n measuring t?e Doppler s h i f t s of t r a n s i t i o n region emission l i n e s from ac t ive regions and thus t h e i r loca t ion on the stellar surface, however, such measurements w i l l be d i f f i c u l t because of the la rge l i n e widths. 3. Various authors have estiamted mass l o s s rates and flow v e l o c i t i e s f o r the cool c i rcumste l la r gas, but t he re are no measurements of the outflow or i n f a l l ve loc i t i ee f o r the hot coronal gas. It is e n t i r e l y possible tha t the X-ray emitt ing plasma is confined to closed magnetic loops tha t do not p a r t i c i p a t e i n the flow while the wind o r ig ina t e s i n magnetically open re- gions (coronal holes). requires high reso lu t ion X-ray spectroscopy, which may not be f e a s i b l e for these stars, but similar measurements f o r t he plasma at t r a n s i t i o n region temperatures requires high reso lu t ion u l t r a v i o l e t spectroscopy, which w i l l be f e a s i b l e with Space Telescope. Even with such da ta , however, the in t e r - p re ta t lon will be d i f f i c u l t as d i f f e ren t authors have in t e rp re t ed P Cygni- type p r o f i l e s f o r T Tauri stars as ind ica t ing mass inflow and outflow. Measurement of the flow v e l o c i t i e s f o r coronal plasma V I . N DWARF FLARE STARS a i What We Have Learned from E ins t e in 1. Flarer on the Sun have been studied ex tens ive ly with instruments covering the electromagnetic spectrum, and f l a r e s on M dwarf stars have been studied at op t i ca l and radio wavelengths s ince 1949. tha t X-ray observations would be c r i t i c a l i n understanding the proper t ies of the hot plasmas in f l a r e s , e e n s i t i v i t y l imi tae lons preclrrded X-ray observa- t i ons of a l l but the most ene rge t i c evente p r io r t o Eins te in . For example, While it was recognised -- 28 Heise - et a1. (1975) d e t e c t e d a large f l a r e on YZ CM (H4.3eV) and W Cet (M5.6eV) using t h e ANS sa te l l i t e , and Kahn - et al . (1979) d e t e c t e d two f l a r e s each on AD Leo (M3.5eV) and AT Mic (M4.4eV). The X-ray lumlnoeitie6 f o r the f l a r e s observed i n AD Leo and AT Mc are very l a r g e , Lx - 1.3-1.6 x 1030 ergs 8-l , as much as a f a c t o r of 4000 larger than very large solar f l a r e s . 2 , E i n s t e i n has now observed f l a r e s on Proxima Cen (Haisch et al. 1980, 19811, Y Z m h l e r - et a1. 198Z), Wolf 630 AM and RD + 64°2051 TJohnson 19811, and perhaps o t h e r M dwarfs. c o l l a b o r a t i v e observing programs invo lv ing s imultaneous o p t i c a l , u l t r a v i o l e t , and r a d i o observat ions. As a r e s u l t of t h e greater s e n s i t i v i t y of E i n s t e i n , less luminous f l a r e s can be s t u d i e d i n d e t a i l . For example, t h e f l a r e on Proxima Centaur1 observed by Haiuch - et al. had a peak luminosi ty of Lx - 7.4 x lo2’ e r g s 8-l comparable t o a very large solar f l a r e . Many of t h e s e obse rva t ions were part of The X-ray l i g h t curve and temperatures of t h i s f l a r e on Proxima Centaur1 were i n many ways similar t o solar f l a r s s . For example, t h e temperature , as c rude ly est imated from IPC pulse he igh t sytvctra, reached a peak of 17 x lo6 K e a r l y i n the f lare and t h e X-ray iuminosi ty peaked about 5 minutee after t h e peak temperature. Both p r o p e r t i e s are t y p i c a l of solar flares. From t h e X- ray decay time they pos tu l a t ed t h a t t h e f lare w a s cooled by r a d i a t i o n , but simultaneous u l t r a v i o l e t s p e c t r a are needed to determine t h e importance of conduction and expansion cool ing. For t h e extremely luminous f l a r e on AT Mic, Kahn - et al. (1979) were a b l e t o d e r i v e a single c h a r a c t e r i s t i c temperature of 30t10 x lo6 K, which is somewhat larger than the temperatures t y p i c a l l y seen during the coo l ing phase of solar f l a r e s . b) Some Important Unanswered Quest ions 1. We need to observe a cons ide rab le number of f l a r e s to see what ranges of temperature , X-ray luminosi ty , and emission measure are t y p i c a l f o r f la re even t s i n M dwarf stars of d i f f e r e n t e f f e c t i v e temperatures and ages. Monitoring by imaging instruments with energy d i s t r i b u t i o n photometry c a p a b i l i t y is needed. 2. What are the v a r i a t i o n s of temperature, e l e c t r o n d e n s i t y , X-ray luminosi ty , and emitting volume as a func t ion of time dur ing f l a r e s ? To answer t h i s ques t ion , we need a high throughput instrument with ra-.icrate spec t ral r e s o l u t i o n capabi 1 i t y. 3. What are the tu rbu len t and systematic mass motions du r ing f lares, and do these motions play an important r o l e i n t he f l a r e energy balance? Depending on whether the v e l o c i t i e s are - lo2 or - lo3 km S-’, e i t h e r high or moderate X-ray s p e c t r a l r e s o l u t i o n w i l l be needed t o s tudy the hot gas. V e l o c i t i e s of the coo le r gas should be e a s i l y s tud ied i n t h e u l t r a v i o l e t by the Space Telescopo High Resolut ion Spectrograph, 4. A c r i t i c a l ques t ion is whether flares are cooled p r i m a r i l y by radia- tion, COndUCtiOn, or expansion. This ques t ion r e q u i r e s simultaneous measure- ments by d i f f e r e n t instruments: r a d i a t i v e l o s s e s can be m a s u r e d by X-ray energy d i s t r i b u t i o n photometry, conductive l o s s e s by u l t r a v i o l e t spectrosco- py, and expansion l o s s e s by laoderate or high r e s o l u t i o n X-ray spectroscopy 29 as previous ly d iscussed . A de te rmina t ion of the t o t a l coo l ing rate d e t e r - mines the hea t ing r a t e , and thus provides va luable informat ion on t h e f lare energy source. 5. Do f l a r e s occur on warmer s t a r s ? The answer mst be yes because t h e Sun f l a r e s , but such f l a r e s are d i f f i c u l t to see o p t i c a l l y because of reduced c o n t r a s t wi th r e spec t t o t h e photospheric background and t h e proba- b l e lower frequency of f l a r e s . X-ray obse rva t ions should be a b l e to answer t h i s ques t ion as t he u i e scen t coronal background is gene ra l ly small com- pared t o f l a r e s even .or t h e Sun. Monitoring of C and K dwarfs wi th a sim- p l e X-ray imaging experiment f o r long per iods of t i m e is needed t o mswer t h i s ques t ion . VII. LUMINOUS COOL !7l’ARS a ) What We Have Learned from E i n s t e i n 1. Prior t o E i n s t e i n no nonbinary la te - type g i a n t or supe rg ian t was de t ec t ed as an X-ray source. E i n s t e i n d e t e c t e d s e v e r a l nonbinary G g i a n t s with Lx between loz8 - 10’” e r g s 8-l and two e a r l y K g i a n t s , E: Sco (KO 111-IV) and a Ser (:a 1111, with L, 0 loz8 e r g s 8-l (Vaiana et al. 1981, Ayres et a l . 1981). smaller than t h i s r a t i o f o r solar coronal holes . - The la t te r two stars have L , / L b l 9 3x1Oee, which is 2. E i n s t e i n was unable to d e t e c t X-rays from s i n g l e g i a n t s cooler than about K2 111 and G-M supe rg ian t s . For example, a Boo (K2 111) and a Tau (K5 111) were not de t ec t ed wi th upper l i m i t s i x / L b l < 3 x lo-’, about a fac- cor of 30 smaller than f o r solar coronal holes . Also the G s u p e r g i a n t s f4 Aqr (GO Ib ) and R Aqr (G2 I b ) have upper l i m i t s L , / L b l < 10’ and t h e M super- g i a n t s a Ori (M2 I a b ) and a Sco (M1 Ibi-M) have upper l i m i t s h / L b l < 2 x lo-’. These nondetec t ions l ed Ayres et al. (1981) t o propose a boundary i n t h e cool p o r t i o n of t h e HR diagram s e p a r a t i n g a reg ion ( c o n s i s t i n g of K2-M g i a n t s and C-M s u p e r g i a n t s ) i n which t h e r e is no evidence f o r hot coronae from a reg ion ( c o n s i s t i n g of DK2 g i a n t s and F-M dwarfs) in which hot coro- nae are u s u a l l y , but not always, de t ec t ed . The l o c a t i o n of t h i s boundary is similar t o t h a t s e p a r a t i n g reg ions where t r a n s i t i o n reg ions are or are not t y p i c a l l y seen and where massive coo l winds begin to appear. 3. A t p resent we do not know which of s e v e r a l poss ib l e exp lana t ions f o r t h i s boundary i n t h e HR diagram is correct. I f t h e nondetcc t ion of X-rays from stars cooler and more luminous than t h i s boundary is an in s t rumen ta l t h re sho ld e f f e c t , then the c ~ t o n a e have s u r f a c e b r igh tnesses much sma l l e r than coronal holes . and t h e X-ray emission w i l l be too s o f t f o r d e t e c t i o n by E i n s t e i n . Absorp- t i o n of s o f t X-ray emission by over ly ing cool c i r c u m s t e l l a r gas is a possi- b l e exp lana t ion f o r t h e l ack of de t ec t ed X-rays from t h e C-M supe rg ian t s but not the K2-M g i a n t s . r a t h e r extended cool (T = 10 K) envelopes t h a t do not emit X-rays. Ayres et a l . (1981) proposed the la t ter exp lana t ion , but more informat ion is needed t o confirm or r e f u t e t h i s propoeal. A l t e r v a t i v e l y , t he coronae may be cooler than - 1 ~ 1 0 ~ K F i n a l l $ , t hese stars may not have hot coronae, but 30 b) Some Important Unanswered Ques t ions 1. Do the K2-M g i a n t s and G-M supe rg ian t s have f a i n t coronae at lo5 - lo6 K? - IUE and Space Telescope, or by an extremely soft X-ray or extreme u l t r a v i o l e t imaging experiment. 8 Aqr (GO Ib), a Ay (G2 Xb), and a TrA (K4 XII) show u l t r a v i o l e t emission l i n e s formed a t 10 K. This ques t ion can be answered wi th u l t r a v i o l e t s p e c t r a , such as from Hartmann - e t al. (1980, 1981) h a w a l r e a d y found t h a t 2. What are the co rona l tempera tures f o r t h e G g i a n t s t h a t are de- t e c t e d as X-ray sources? Energy d i s t r i b u t i o n photometry can answer t h i s ques t ion , and i n p r i n c i p l e r e c a l i b r a t e d E i n s t e i n IPC pulse he igh t s p e c t r a w i l l provide t h i s in format ion on a few stars. 3. What are the e l e c t r o n d e n s i t i e s and e m i t t i n g vo lu res (1.e. loop dimensions) f o r t h e G g i a n t s ? Moderate r e s o l u t i o n X-ray s p e c t r a are needed. 4. What are the geometries of the C g i a n t coronae? Monitor ing of t h e s e stars over r o t a t i o n a l pe r iods wi th imaging experiments is needed t o answer t h i s que8 t i o n . VIII. RS CVn AND RELATED CLOSE BINARY SYSTEMS The RS CVn binary systems are detached systems wi th pe r iods of 1-14 days, g e n e r a l l y c o n s i s t i n g of a KO I V primary and a late G dwarf secondary s t a r . Hall (19?6, 1981) has reviewed t h e p r o p e r t i e s of t h e s e systems as w e l l as t h e r e l a t e d long per iod systems wi th g i a n t star components and the con tac t W UNa sysr-ems, and Popper and Ul r i ch (1977) have d i scussed t h e i r evo lu t iona ry s t a t u s . T)..s m0st s t r i k i n g p e c u l i a r i t y of the RS CVn systems is a migra t ing quas i - s inuso ida l d i s t o r t i o n i n t h e i r o p t i c a l l i g h t curves ( H a l l 1981; Rodono 1981) t h a t is g e n e r a l l y expla ined by an uneven d i s t r i b u t i o n of dark, cool photospher ic s p o t s (cf. Eaton and Hall 1979). There is evidence t h a t the chromospheric Ha and Ca 11 H and K emission l i n e s P 9 b r i g h t when the v i s i b l e hemisphere shows maximum coverage by t h e dark s p o t s ( c f . Dupree l981). a) What We Have Learned from E i n s t e i n 1. Using t h e HEAO-1 A2 experiment, Walter et al. (1980) d e t e c t e d 15 ou t of 59 systems observed wi th l u m i n o s i t i e s i n the range L, = 103°*5-1031*6 e r g s 8-l. and a s c r i b e d the nonde tec t ions to t h e s e n s i t i v i t y th re sho ld of HEAO-1 . As a consequence of its lower s e n s i t i v i t y th re sho ld the E i n s t e i n IPC has de tec t ed a t least 47 systems with L, - 1029*4-1031*5 ergs s’l and ~ o ~ ( L ~ / L B ~ ~ ) i n t h e range -4.9 t o -2.4 (c f . Walter and Bowyer 1981). ther , the k / L g o l r a t i o does not depend on the g r a v i t y of the cooler star i n the system ( u s u a l l y the more a c t i v e star) over two decades i n g r a v i t y . - Fur- 2. Walter and Bovyer (1981) showed t h a t b / L ~ ~ l - 0 , the angu la r ve- l o c i t y of t h e star wi th the most a c t i v e chromosphere in the system. S ince s i n g l e G-type stars fo l low the same r e l a t i o n s h i p (Walter 19811, the b r i g h t X-ray luminos i ty of the RS Cvn systems is not a d i r e c t consequence of 31 b i n a r i t y , but r a t h e r a r e s u l t of r ap id r o t a t i o n which is i n t u r n produced by t ida l ly-enforced synchronism of r o t a t i o n and o r b i t a l motion. Rapid r o t a t i o n presumably r e s u l t s i n s t rong dynamo-generated magnetic f i e l d s i n stars w i t h deep convect ive zones. 3. HEAO-1 observa t ions of Walter et al. (1980) and Garcia et al. (1980) i nd ica t ed t h a t RS CVn systems have r a t h e r ho t , v a r i a b l e spectra. Subsequently, Swank - ec al. (1981) were a b l e t o o b t a i n low r e s o l u t i o n s p e c t r a of 7 RS CVn systems and Algol (a con tac t e c l i p s i n g system) wi th t h e E i n s t e i n SSS d e t e c t o r . Assuming t h a t t he X-ray emission is from an o p t i c a l l y t h i n thermal plasma i n c o l l i s i o n a l i o n i z a t i o n equi l ibr ium, they found t h a t t h e s p e c t r a can be f i t by !NO components - a warm com t h e two components l i e i n t h e range Lwal.0 = 1030-1031 e r g s 8-l and Lhot = 1029*3-1031*3 e r g s 8-l. hot component v a r i e s 3 a f a c t o r of 2. The r a t i o Lhot/twarm lies i n t h e range 0.1 ( f o r t he 104 Since the hot components vary whi le t h e warm components do no t , t h e two components probably o r i g i n a t e in s e p a r a t e plasmas. nent wi th Twarn = 4-8 x lo6 K, and a hot component wi th That = 20-100 x 10 I? K. The l u m i n o s i t i e s of The warm component appears not t o vary, whi le t h e Capel la system) t o 4 ( f o r t h e 6.5d UX A r i system). 4. I n a l l l i k e l i h o o d the emitting s t r u c t u r e s are c losed magnetic loops. This hypothesis Is based on solar analogy, the appearance of l a r g e dark s t a r s p o t groups on the photospheres of t hese stars, and the i n a b i l i t y of these stars t o conf ine t h e observed hot plasma by g r a v i t y a lone. Swank e t al . (1981) and Walter et al. (1980) assumed t h e Rosner, Tucker, and Vaiana (1978) s c a l i n g l a w f o r magnetic f l u x tubes with the fa l lowing re- s u l t s . I f t h e gas p re s su re i n the loops is roughly 10 dynes cur2, similar to the l a r g e s t p re s su res seen i n s o l a r a c t i v e reg ion loops , then the loop s i z e s f o r the warm plasma are small compared t o the stellar r a d i i and t h e hot loops have sites comparable to t h e b inary sepa ra t ions . I f , on the o t h e r hand, the loop p res su res are 1100 dynes then both the warm and hot loops are smaller than t h e s te l lar r a d i i i n s ca l e . There is no compell ing evidence ye t as t o which pressure is c o r r e c t , but t he absence of l a r g e changes i n the X-ray f l u x from AR Lac dur ing primary and secondary e c l i p s e (Swank et al. 1981) sugges ts t h a t t he e m i t t i n g reg ions may be comparable t o t h e b inary s e p a r a t i o n i n t h i s system. a c t i o n s between loops from the two stars, which Simon, Linsky, and S c h i f f e r ( 1980) proposed as the mechanism re spons ib l e for flares i n these systems. - This raises the p o s s l b i l i t y of i n t e r - b) Some Important Unanswered Questions 1. What are the geometr ies of the X-ray emitting reg ions in t hese sys- tems? o r b i t with X-ray energy d i s t r i b u t i o n photometry and u l t r a v i o l e t spectroscopy can determine the l o c a t i o n of t he hot and warm loops a A t h e i r r e l a t i o n t o t h e spots . surements of e l e c t r o n d e n s i t i e s and thus p re s su res and loop s i z e s us ing appropr i a t e s c a l i n g laws. f r a c t i o n of t he a v a i l a b l e volume t h a t is f i l l e d by loops. Monitoring t h e X-ray and u l t r a v i o l e t emission dur ing a f u l l b inary Also moderate r e s o l u t i o n X-ray spectroscopy w i l l permit mea- Such d a t a w i l l a l s o provide information on t h e 2. Is the r a d i a t i o n i n the hot component indeed thermal and why is t h i s plasma so hot? The answer t o t h i s ques t ion w i l l r e q u i r e a hard X-ray 32 spectroscopy instrument t h a t can monitor t h e s e s y s t e m to s tudy t h e i r v a r i a b i l i t y time scales. 3. What are the mechanistus r e spons ib l e f o r f l a r e s i n these systems? Two k inds of obse rva t ions are needed. F i r s t , we need to monitor t h e s e sys- tems dur ing f l a r e s with moderate r e s o l u t i o n X-ray spectroscopy and u l t r a - v i o l e t spectroscopy to determine v a r i a t i o n s i n t h e X-ray and ul t raviolet f l u x e s , temperatures , and e l e c t r o n d e n s i t i e s wi th ttme. Second, we need to determine the plasma f lows , perhaps i n i n t e r a c t i n s f l u x tubes from t h e two stars or o the r binary i n t e r a c t i o n s . Such measurements r e q u i r e high resolu- t i o n X-ray and u l t r a v i o l e t spec t r a . 1x0 HOT STARS a) What We Have Learned from E i n s t e i n 1. E i n s t e i n discovered t h a t t h e 0 and B $tars are t h e b r i g h t e s t X-ray sources among a l l nondegenerate stars, d e s p i t e p r i o r p r e d i c t i o n s t h a t t h e s e stars should not have hot o u t e r atmospheres on t h e basis t h a t they l ack con- vec t ive zones and thus a c o u s t i c wave h e a t i n g processes should be i nope ra t ive . Harnden et al . (1979) and Seuard et al. (1979) repor ted t h e i n i t i a l E i n s t c i n obse rva t ions of luminous 0 stars i n t h e Cyg OB2 a s s o c i e t i o n and the reg ion around t h e q Carinae nebula, f i nd ing t h a t Lx is t y p i c a l l y *1033*7 e r g s 8-l f o r t hese stars. Subsequent obse rva t ions of ho t stars by Long and White (1980), Vaiana et al . (19811, P a l l a v i c i n i et al. (19811, and C a s s i n e l l l et al. (1981) have led t o the r e s u l t s t h a t t y p i c a l l u m i n o s i t i e s are 1031-1033 e r g s s-l f o r t he 0 dwarfs, 1027-1030*7 e r g s 8-l f o r t h e B8 V- A1 V stars, e r g s 8-l f o r t h e 0 supe rg ian t s and 30,oOO - 60,oOO K) isdated k e m r a t e dwarfs, including Sirius B (Martin et al. 1982 and references therein) and H Z 43 (tiearn et al. 1976). b) Pbtential The study of degenerate dwarf X-ray sources can provide many return. For ex-, these sources afford a laboratory in which to explore the physics of hot, dense plasmas in strorrg magnetic fields (the parameter regime is, in fact, similar to that of interest in plasma Wion reactors). We can also learn from them a great dead about the masses, internal structure, and magnetic fields of &generate dwarfs themselves. Wentially, the pulsing sources can plrwide as much infonnarion as has been obtained from the pulsing neutron star X-ray sources. Noise meawtements can be used to probe the accretion process, reflection ad repioeessing effects give dues to the geometry of the disk and the binary system, and time delw curves yield the parameters of the binary system and therebv lend insiiht into its formation and evolution. bwever, because most degenerate dwarf X-ray sources were fwnd only recently, we know very l i t t le aborrt their X-ray properties. h l y three (AM Her, SS Cyg, and U Cent) have been studied in any detail. The situation b similar in this respect to that of the stellar X-ray sources also foud by finstein (Linsky 1982). Exploration of the X-ray emission from both ha, only begm, and future X-ray astconomy missions must pmvide the data with which to urderstand it. In this review, w e amcentrate on the soft ad hard X-ray spectra produced by accreting degenerate dwarfs. We first sunmarite the 0bSe.vations in 111. We then discras the theory o f formation of the continurn spectnm in fl l l , and of emission and abwwption lines in sIV. In SV, we mention some of the inportant unresolved issues. Finally, in $VI we outline the kinds of X-ray ab6ewaticlcrs that would best advance our understanding of these sources. For reviews of the optical properties of cataclysmic variables, see Robinson (1976) and Warner (1976); for reviews of the X-ray observations, see Carmire (1979) and Codma and M a 1 (1982). Lamb (1979) and Kylafis et al. (1980) contain earlier reviews of theoretical work. II. 05SLRVATIONAL PRCPLRTIES a) luminosities and Space Densities Robably all cataclysmic variables are X-pay sources. The ones detected so far have X-ray Itminodties L c 1031 - 1033 ergs s-1. ~ r w r e of the bright (L 1OM-10)8 ergs s-1) galactic X-ray sources have been identified with degenerate bar fs . lhu~ the known accreting ciegenerate b a r f X-ray oources are l& times fainter than, e.g., the pulsing neutron stars (Lamb 1X') but c 103 times brighter than ordinary stars (Linsky 1982). 7he nearest cataclysmic variable X-ray sources lie at distances d of only 75 - 100 pc ~ o r ~ v a M~S~XI 1982). a i s i m i e s a space cimsity n * 3 x 10-7 (CVIOO pc1-3 pc' 5 Aswning a uniform distribution of sources throughout the galaxy and a galactic 39 h e n e r a t e dwarf X-ray sources in the galaxy may exceed a million. This cumpares with a total nunbet of bright (1 e 1*- 1038 ergs 5-l) neutran star sources of 4 100. b) X-Ray Spectra and Temporal Behavbr Arong accreting degenerate dwarf X-ray sources, there are two recognized classes involving magnetic degenerate dwarfs: the AM Her stars and the DQ Her stars (Lamb 1979, Patterson and Price 1981). The remaining systems show no clear-cut marifestation of a magnetic field. However, i f the past is a guide, some of these sources will be reclassified as AM Her or DQ Her stars on the basis of future observations. We may even speculate that magnetic fields are endemic in degenerate dwarfs. I f so, most, perhaps all, of the other system also contain magnetic degenerate dwarfs. However, the field strengths may be less. Below w e wzcuss the X-ray spectra and tempt,ral behavior of the AM Her stars, the DQ Her stars, and the other cataclysmic variables. i) AM Her stars Table 1 lists the seven AM Her stars that are now known and s u n m a r i t e s some of their properties. lhese stars show strong (> 10%) circular and linear polarization of their infrared dnd visible light, and are believed to be accreting magnetic degenerate dwarfs (Chanmugam d Wagner 1977, 1978; Stockman et al. 1977). The polarization (Tapia 1977a) of the visible light from AM tier, the prototype of this class, is shown in Figure 1. The X-ray spectra of these stars typically have two distinct components: an apparent black- cwnponertt with T b < 100 ev and a bremsstrahlung nwnpoclent with T b > 10 kev. The inferred blackbodv luninosity b greater than the bremsstrahlung luninosity, often bv a factor of 10 or more (cf. Tuohy et al. 1978, 1981; Szkody et al. 1.W1; Patterson et al. 1982). Figure 2 shows the soft a d hard X-ray spectrun of AM Her recently constructed from H E 0 1 observations by Rothschild et al. (1981). The bremstrahlcng spectra of these sources also show strong iron line emission at e 7 keV, as b evident in Figure 2. In these systems, the periods of the polarized light, the optical and X-ray light, and the orbital velocity curves are all the same. Thus the rotation period of the degenerate dwarf is synchronized with the orbital period of the binary system, probably due to interaction of the magnetic field of the degenerate dwarf with the companion star (loss, Katr, and Rappaport 1979). Figure 3 shows the resulting 3.1 hour 'pulse profile' of AM Her in soft X-rays ( T b et al. 1978). The source EF E r i (2AO311-227) is the second most well-studied in X-ray, of the AM Her stars. Figures 4 and 5 show i ts 1.3 hnur 'pulse profile' m soft X-rays (Pattenan et al. 1961) and its bremsstrahlung hard X-ray spectrun (White 1981). Note again the strong iron emission line at c 7 keV. ii) DQ Her stars Table 2 l ists the s e v ~ systems we have classified as OQ Her stars and sunmarizes some of their properties. DQ Her, the prototype of this class, i s believed to be an accreting magnetic degenerate dwarf (Bath, Evans, and Pringle 1974; lamb 1974). However, it shows little, i f any, polarization of i ts infrared and visible light (Swedlund, Kemp, and Wdstencmft 1974). This system underwent a nova outburst in 1934 and shows coherent small amplitude optical pulsations at 71 seconds, which are h!leved to a0 r-l . rl) d 00 . 0 m +I rl €0 d 0 r- d 0 OD V) N n 0 0 d U m m 4 . n I- (Y cu I d d .d m q E2 . N d rl l-l In ol m w . . - . A . I- (Y W . I- 4 . 0 4 d m e +I 0 m . a\ . 0 d A 0 Is. . 00 Wd +I 0 N 0 W rl w w 4 r- W 4 . 4 3 PI . . . I- m n 0 0 rl 4 ut W d d yr d t In 0 w rl w . 4 tal a c: 4 k 8 3 k 0 w JJ a a, u X a, u a 0 0 rl w 0 a, 0 c 4 -4 a 4 - t 3 m m 4 z -4 JJ -4 u) .?j cl a . . b . . 0 0 m V? N r- n 0 0 4 U m OD 4 OD \o I m m 4 0 X 0 e I OD N . . b n 0 0 d U m OD 4 . r( m d + 0 m In 0 €0 -4 IN m W aDm *m 0 n 0 0 r-l U 0 r-l +I In I- m 0 m . d m d 3 9 OP 41 . 0 I 4 Y C 0 - D " 0 c a o - L - a V w " A M Her /3U 1809*50 1 1 1 -1 -7 - 1 1 1.- - - - ... .. - .. ... . . . .... t .e e. \ ..... . . . . . . .+-..a. L. ..**e. . ' ........ . :*::;:-..: . . . . . . . . ,. . . . . . . . . . , . . . 'i . . . . . . . .t . -* . - . . . 1- 0.0 . I * a5 Phase 10 Fig. l--Circular and linear polarization of the optical light from AM Her as a function of the phase of the 3.1 b u r rotational period of the degenerate dwarf (from Tapia 1977). 42 IC IC I O - > 1 a. I0 -i y. " . 0 I 0 c g 10 0 X 3 -4 U + z w - Q IO V 5 IO IO ORIGINAL PAQE IS OF POOR QULm 1 , A M HERCULIS March - A p r i l , 1978 -+- A 2 LED I HED I Q + HED 3 + A4 LED I L E D 2 P H E A O - I 0 0 4 3 4 + A 2 YED t t 4 4 '*. 'p k + 4 - T f 0 Fig. 2--Hard and soft X-ray spectrun of AM Her (from Rothschild et al. 1981). The two distinct components with T = 30 keV and T < 40 eV are clearly visible. ORIGINAL PAGE IS OF POOR QUALITY 43 ~ " " ~ " ' " ' * " ' ~ EO, 1 . . , , , , , , , , , I , 0.5 n . * 3 I I . . . I I I I ao a2 a4 0.6 ae ID I.P ~4 PHASE Fig. 3--Soft X-ray pulse profile and hardness ratio of AM Her as a function of the phase of the 3.1 hour rotational period of the degenerate dwarf (from Tuohy et al. 1978). Fig. 4--X-ray pulse profile of EF Eri (2AO311-227) observed by Einstein as a function of orbital phase (or, equivalently, phase of the rotational period of the degenerate dwarf) (from Patterson et al. 1981). OXILLATION WAX Fig. 5--Hard X-ray spectrum of EF Er i (2A0311-227) (from White et al. 1981). Note the iron emission line at 7 keV. Fig. 6--Comparison of the pulse profile in soft X-rays and in optical light of AE Aqr through the 33 second rotation period of the degenerate dwarf (from Patterson et al. 1980). J7 - 9 . m Lnvl €I U . . . . . . . In . 0 w 42: V V .Ln d rl 0 -4 3 rl x al 0 *-4 a0 0 4 x m A X m A N. N N d N 0 d '9 rl m 0 OD cy. 1 w m N rl - 0 0 -4 U 0 0 4 m aD 4-y q n QI d r( 4 m u! m m 9 9 45 represent the rotation period of the degenerate dwarf (Patterson, Robinson, and Nather 1978, and references therein). Two other members of this class are V533 Her, which underwent a m:a outburst in 1963 and shows coherent small amplitude optical pulsations at 63 seconds (Patterson 1979a), and AE Aqr, which shows similar pulsations at 33 seconds (Patterson 1979b). Embarrassingly, neither DQ Her nor V533 Her have been detected in X rays (see Table 2). In the case of DQ Her, it has been suggested that the X rays are blocked by the disk because we are nearly in the orbital plane of the system, while in the case of V533 it can be argued that the system is too far away, and therefore too faint, to have been detected. Thankfully (for theorists), X rays have now been detected from A€ Aqr and are pulsed with the 33 second optical period (Patterson et al. 1980). Figure 6 compares the optical and soft X-ray pulse profilesof A€ Aqr. Recently, several faint galactic X-ray sources have been identified with systems that are optically similar to cataclysmic variables. They exhibit large amplitude optical and X-ray pulsations with periods 2 1OOO seconds that are believed to represent the rotation period of the accreting star (Patterson and Price 1981; Warner, O'Donoghue, and Fairall 1981; White and Marshall 1981). There is controversy as to whether these X-ray sources are actually degenerate dwarfs or are neutron stars (cf. Patterson and Price 1981, White and Marshall 1981). We believe, based on their optical appearance and their X-ray to optical luminosity ratio, that they are degenerate dwarfs. They have also been called 'interlopers' between the previously known DO Her stars, with short rotation periods of 33 - 71 seconds, and the AM Her stars, with rotation synchronous with their orbital periods of 1.2 - 2.1 hours (Patterson and Price 1981). However, we believe that they should be regarded as members of the DO Her class, in analogy with the short and long period pulsing neutron star X-ray sources, and therefore we include them in Table 2. The source H2252-035 was the first of these systems to be optically identified (Criffiths et al. 1980). Figure 7 shows i t s optical light curve (Patterson and Price 1981). Clearly visible are the optical pulsations with a period of 859 seconds, which are thnught to be produced by reprossessing of the 805 second X-ray pulse. Figures 8 and 9 show the pulse profile and the spectrum of the hard X-rays (White and Marshall 1981). The hard X-ray spectrum exhibits iron line emission at * 7 keV. Recently, a 67 minute (4022 second) periodicity has been identified in the well- studied cataclysmic variable X-ray source E X Hya (Vogt, Krzeminski, and Sterken 1980; Cilliland 1982). This period is also evident in soft X-rays but not in hard (Swank and White 1981), as shown in Figure 10. The coherence of the period over many years suggests that it may also be due to rotation of a magnetic degenerate dwarf. We have therefore included EX Hya in Table 2, but with a question mark to indicate i t s uncertain status. iii) Other cataclysmic variables Table 3 l ists 10 sources selected from the remaining 44 cataclysmic variable X-ray sources currently known (Cordova and Mason 1982). Among :hese are the prototypical dwarf novae, SS Cyg and U Gem, which undergo outbursts every * 100 days. During quiescence, both exhibit a hard X-ray spectrum with Tbr 10 - 20 keV (Mason, Cordova, and Swank 1979; Swank 1979). During outburst, the hard X-ray luminosity first increases and then decreases, the spectral temperature of the hard X-rays decreases, and an intense blackbody component with temperature T b < 100 eV appears in soft X-rays (cf. M a s o n , Cordova, and Swank 1979). Figure 11 compares the hard X- ray spectrun of SS Cy,: in quiescence and in outburst with the spectrun of AM Her. 46 -_I_.------- -- - -i--7 0- ARBITRARY PULSE PHASE Fig. 7--Fast photometry of the optical light from H2252-035 (from Patterson and Price 7981). The 859 second pulsations, corresponding to reprocessed light from a stationary point in the binary system, are clearly visible. Fig. I--Hard X-ray pulse profile of H2252-035 through the 805 second rotation period of the degenerate dwarf (from White and Marshall 1989). Fig. +-Hard X-ray spectrum of H2252-035 (from White and Marshaii 1981). %he iron emission line at * 7 keV is again clearly visible. 0 N 0 d - n 0 4 U - 0 Y 9 W u Y 6 Q N I-- ‘9 I- N n 0 0 d U 24 d a Y m N d - nn 6 0 drl uu A A O F c u m d N -1 . . u r- I- N Y 0 N *I m r v 4 QI I- 4 8 N PJ n 0 rl U 0 LD rl 0 m w m m m 3 m 0 N d d - n 0 4 U 9 N n 0 - 9 Y 0 m I OD N e d N N 0 ’I i! 5 N N OD rl - n 0 rl U A m Y Q: d 0 0 OD QJ r. I W r( - 9 n 0 0 r( U % \d 8 N 47 mcn dr- u m 48 EINSTEIN SSS l o ~ EX HVORAE Fig. 10--Soft X-ray spectrun of EX Wdra measured by Einstein and showing the necessity D . of invoking at least two components (from Swank and White 19131). i.. - 1 l . i r i 05 I 2 3 4 16' ENERGY (Lev) I O 2 103 HEAO A 2 PULSE HEIGHT SPECTRA I I AM HER - - ' YENON DETECTOR ARGON DETECTOR I III I 2 4 6 8 10 20 40 60 80 I 2 4 6 8 10 20 E (keV) E (keVJ Fig. l l - - t iard X-ray spectra of SS Cyg during quiescence (left panel) and during outburst (r igi t panel) compared with the spectrum of AM Her (from Swank 1979). The presence of ircn line emission at = 7 keV in all spectra and the absence of low energy absorption in the spertrun of SS Cyg in outburst are evident. The temporal behavM of the hard and soft X-ray kminosities of U Gem &ring an outburst is shmm in Figure 12, while Figure 13 shmms the way in which the hard X-ray spectrun of SS Cyg varies duriw an outburst. Most of the remaining cataclysmic variables stpw only a hard X-ray -t. It is not known whedwr the failure to detect a soft cuqomnt d u r i quiescence, or even dwing outburst in #*ne sources, is due to i ts absence or due to the fact that it may have 90 bw a spectral temperature that it is urobrervabk in soft X-rays. Essentially ali of the cataclysmic vari& listed in Table 3 e*hibit small amplitude quasi-periodic or aoherent optical pulsations, usually during the onset of an outburst (Robbrwn 1976). Of special interest are the J. 8 - 10 second &-periodic pulsations in SS Cyg. T k v are strongly in soft X-rays h r b g outburst, yet their coherence persizts for only 3-5 pulse periods (Codova et al. 19$0,1981). iv) lsolatedstars Sirius 6, the first degenerate dwarf disarvered, was detected as a very soft X-ray SouCe by M e w et al. (lW5). Shec+.:-ntly, wery soft X rays were also detected fmm :he hot degenerate dwarf HZ 43 (Heam et al. 1976). Both of these degenerate dwarfs are members of binaries, k the hinary separatiom are 90 large that the cowmiom are not believed to play any role in the X-ray enrission. Feige 24. another hot degenerate dwarf, has been detected in the extreme UV (Margon et al. 1976); h o w , it was not detected by the HUlrO-1 soft X-ray survey 4 unfortunately, Lbrrtein ceased operating before absewatiom of it were carried out. Table 4 lists these three swrces and sunmarires sane of their pmperties. lhe -ion fmn Sirius 6, HL 43, ad Feige 24 at optical, UV, ad X-ray waveleng-hs can be understood as WK emission fnm a hydrogen-rich [q+,&., * lo-’] atmosphere with Teff * s0,OrJo - 60,OOO K (Shiipma, 1976, Margon et al. 1976, Wessel!cs and Koester 1978, Martin et al. 1982). Ih limit in the extreme UV (200 - 800 A) for Sirius 6 (Cash, €buyer, ad L w 1978) appeared to conflict with p h o t o ~ u models for the x-ray emision and to lend support to connal models. However, UV observations yielded no evidence for a corona (Bohm-Vitense, -am, and Kapranidis 19’79) and Martin et al. (1982) have recently demmtrated that soft X- ray d a a from HEW-1, together with the optical, UV, ad extreme UV data, are consistent with PhatOspherK &ion at c 28,000 K, as shown in Figure 14. For more detailed, but earlier, reviews of extmne UV and soft X-ray emission from isdated degenerate dwarfs, see Carmire (1979) and Bowyer (1979). 111. CONTINUUM SPLCTRA a) Qualitative Rcture in the remainder of this review, w shall focus on X-ray emission by accreting degenerate dwarfs. i) Disk inflow near the star Many of the cataclysmic variable X-ray sources show clear optical and UV evidence of accretion disks. I f tk- disk extends all the way m to the stellar surface, visaous dissipation in the disk will release approximately half of the available 50 - ‘v) (Y I 0 E v) k z 3 0 0 - ORIGNc’AL PAGE is OF POOR QUALITY I I I I L I I? #) I9 20 21 22 23 OCTOBER 1977 Fig. 12--<)ptical, soft X-ray, and hard X-ray light carves of U Gem through an outburst (from M a s o n , Cordova, and Swank 1979). ORIGINAL PAGE IS OF POOR QUALllY . ' I-. I I 1 1 1 1 I I I I I I I r r i ' IO O I ro lo-' U Fig. l3--Soft X-ray spectrun of SS Cyg showing the variation in the spectrum from outburst through decline to quiescence (from Swank and White 194)l). 4 I 10 m P a& Q) 0 m 0 4 Y OFIKilPoALPaQEfS OF POOR QUMl'TY W Ln . L 0 0 0 0 a0 N 11 5 W 0 0 I- N m m 1 4 & 4 u) 12 . cy 0 0 0 0 u fI - 0 w In \o m TI N X el . rl 0 0 0 rc, II 9 CCI V 0 m 53 1-28 Fig. 14-Comparison of the flux from a 28,000 K hydrogen-rich model atmosphere with measurements and upper limits at uV, extreme UV, and soft X-rak wavelengths (from Martin et al. 1982). 54 gravitationtl energy, which will appear as blackbody radiation from the disk surfaces. The other half cf the available gravitational energy will be released in a boundary layer at the inner edge of the disk where it encounters the surface of the star, unless the star is rotating near breakup. This luminosity i s "* 4 x lo3* ( M / M O ) (R/109 cm)-' (w10-l' Mgyr -1 ) ergs s -1 , (1) L W r y = 2 R where M and R are the mass and radius of the star, and is the mass accretion rate. At moderate or high accretion rates, the boundary layer is capable of producing soft X- rays by blackbody emission (Pringle 1977). After it was found that most cataclysmic variables emit hard, but not soft, X rays during quiescence, Pringle and Savonije (1979) proposed that the boundary layer might produce hard X-ray emission by optically thin bremsstrahlung if shocks occurred there. The maximum possible shock temperature is 8 9 TS = x T ff = 2 x 10 (M/Me(R/10 cm)-' K, and thus the shocks must be strong ones. This i s difficult to achieve in the strongly sheared flow of the inner disk whose geometry would tend to favor production of a large number of cooler, oblique shocks (in principle, the disk can join onto the star without the occurrence of any shocks). To attain the rcquired strong shocks, Pringle and Savonije (1979) suggest a two-stage process in which gas that is initially mildly shocked m the M a r y layer expands into the path of, and collides with, gas sti l l circulating in the inner disk. Tylenda (1981), however, argues tha: turbulent viscosity will be a more efficient mechanism than shocks for dissipating energy in the boundary layer and that this mechanism can account for the observed high temperatures without resorting to complicated flow geometries. Knowledge of whether the boundary layer can prodace hard X-rays and, i f so, how, is important for understanding the cataclymsic variable X-ray sources. But as yet, the ideas that have been proposed have not been worked out in any detail. For up-to-date discussions o f disks, see the review bv Pringle (1981) and the paper by Tylenda (1981). ii) Radial inflow war the star I f the degenerate dwarf has a magnetic field, 3 -1 1/2 9 -5/4 B 2 2 x 10 Mgyr ) (R/10 cm) (rA/Md1/4 gauss, (3) the field will disrupt the disk and lead to approxm;z?e!v radial inflow near the star. This picture certainly applies to the AM Her and DQ Her stars, and may apply to other cataclysmic variables i f magnetic fields are endemic in degenerate dwarfs as speculated earlier. Radial inflow may also occur i f mass transfer takes place via a stellar wind rather than via R o c h e lobe averfbw. Most theoretical work has assuned radial in fbw because it is far more tractable; in the remainder of this review, we will concentrate on radial infbw. A qualitative picture of X-ray emission by radially accreting degenerate dwarfs is shown in Figure 15. k accreting matter flows toward the star, a strong standoff shock forms far enough above the star for the hot, post-shock matter to cool and come to rest at the stellar surface (Hashi 1973; Aitu 1973; Fabian, Pringle, and Rees 1976). The 0RlGINP.L PASS 1s OF POOR QUALlM . . . . . . . . . . , . , . . . . . . . . . . . e . * . . . . . . . . R P . - . - . . . . . .: '. .. . . . . . . . . ' . - * . . . . * * :. . . . * . . . . . . . . . , . . . . . . I . . . . Fig. 15--Qualitative picture of X-ray emission from an accreting degenerate dwarf. a) (bnfiguratiar of the star, the emission region, and the infalling matter. R is the stellar radius and is i s the shock radius. The straight arrows indicate infalling matter, the wiggly arm- photons. 1, is the post-shock temperature d Tb is the stellar blackbody temperature. c) Density profile of the infalling matter; p S is the post-shock density. b) Temperature profile of the infalling matter. 56 standoff distance where rs is the shock radius, R is the stellar radius, vff is the free-fall velocity, and tc\d is the time scale for cooling, due to bremsstrahlung and, i f a magnetic field is present, cyclotron emission. Roughly half of the bremsstrahlung flux is emitted outward and forms a hard X-ray component. Roughly half of the cyclotron flux is emitted outward and forms a blackbodv-limited component in the UV. The other halves of the bremsstrahlung and cyclotron fluxes are emitted inward and are reflected or absorbed by the stellar surface. The resulting blackbody flux forms a UV or soft X-ray component with where L k Lcyc, and Lbr are the luninosi t ies in the blackbady, cyclotron, and bremsstrahlung components. The total luninosity L = CMWR, or twice that given by equation (1). If we allow for the possible presence of a magnetic field, the accreting matter may be channeled onto the magnetic poles and accretion may occur Over only a fraction f o f the stellar surface. The effective accretion rate of the accreting sector is Wf, and the corresponding luminosity is L/f. X and UV radiation from magnetic degenerate dwarfs is thus a function of stellar mass M, magnetic field strength B, and effective luminosity L/f. The dependence on stellar mass is significant but is less than on the other two variables. If we specify the mass of the star, the parameter regimes encountered ale conveniently displayed on a (B,L/f)-plane, as shown in Figure 16 for a 1 M e The upper left of the plane corresponds to low magnetic field strengths and high effective luminosities (and thus high densities in the emission region). In this portion o f the plane, bremsstrahlmg cooling dominates cyclotron cooling in the hot, post-shock emission region, and the character of the X-ray emission i s essentially the same as that of a nonmagnetic degenerate dwarf. As one increases B or lowers L/f, moving toward the bwer right in Figure 16, cyclotron cooling becomes more important until eventually it dominates (Masters et al. 1977). The solid line shows the location at which this occurs, as determined from detailed numerical calculations equating tcyc and tbr, the cyclotron and bremsstrahlmg cooling time scales. This line i s approximately given by 6 36 -1 2/5 6 = 6 x 10 (Lf - ’ / lO ergss ) gauss. To the bwer right of this solid line, the magnetic field qualitatively alters the character of the X-ray emission. b) Magnetic Stars Fabian, Ptingle, and Rees (1976), masters et al. (1977), and King and Lasota (1979) have discussed the qualitative features of X-ray emission by magnetic degenerate dwarfs. Lamb and Masters (1979; see also Masters 1978) carried out detailed nunerical calculations of high h a m i c cyclotron emission from a hot plasma, and from them devebped a self-consistent, quantitative model of the X-ray and UV emission. Wada et al. (1981) have carried out a few calculations for the regime im which bremsstrahlung, 57 CYCLOTRON A - 6 7 9 1 1 .-, 1 -- - Fig. 16--Bremsstrahlung and cyzlotron emission regimes in the (L/f,R)-plane for a 1 % star (after Lamb and Masters 1979). Fig. 17--X-ray and UV spectra produced by accretion Onto a 1 M(,J star at two different accretion rates. The spectrum with L/ f = 1037 ergs s - l is in the bremsstrahlung dominated regime, while the spectrum with L / f = 1035 ergs is in the cyclotron dominated regime (from Lamb and Masters 1979). 58 not cyclotron emission, h i n a t e s (see Figure 16). i) Spectra TI * X and UV spectrun produced by accretion onto magnetic degerlerate dwarfs generally has four components: 1) a blackbody-limited UV cyclotron component produced by the hot emission region, 2) a hard X-ray bremsstrahlung component also produced by the hot emission region, 3) a hard UV or soft X-ray blackbody ccmponent produced by cyclotron and bremsstrahlung photons thzt are absorbed by the stellar surface and re-emitted, and 4) secondary radiation from infalling matter above the shock or, possibly, from the stellar surface around the emission region. The first three components are clearly visible in Figure 17, which shows spectra produced by the hot, post-shock emission region alone. Since the secondary radiation is not included, the spectra do not accurately represent the observed spectrum below 4 eV. Figure 17 shows the X and UV spectra produced by accretion at two different rates, corresponding to L/f = 1 0 3 ~ and 1037 erg s - l , onto a 1.0 Mastar having a magnetic field of 2 x lo7 gauss. First, strongly magnetic degenerate dwarfs should be intense UV sources with only a few percent of the total accretion luminosity ordinarily appearing as optical or soft and hard X-rays, and therefore easily accessible. Second, the position and relative strength of the spectral components change with variations in the accretion rate. For example, the change in accretion rate shown in Figure 17 moves the blackbob/ component from the U? into the soft X-ray region, and the luminosity of the bremsstrahlung hard X-ray coinponent increases by nearly 4 orders of magnitude while the total accretion luminosity increases only by 2. The spectra rllustrated in Figure 17 show twm important features. ii) Correlation between spectral temperature and luminosity Variations in the shape and the strength of the spectral components are a function of both mass accretion rate and magnetic field strength. They can be conveniently displayed by p' .c'ing contours on a (B,L/f)-plane. Sets of such contours are shown in Figures 18 z d 19 for a 1.0 Mastar. Bremsstrahlung and cyclotron emission dominate in the same regions as in Figure 16. In Figure 18, contours of constant shock standoff distance d = 6R/R are shown as thick solid lines. The thin solid Itnes in the bremsstrahlung-dominated region show contours of constant q = Lcyc/Lbr, while those in the cyclotron-dominated region show contours of constant le, the temperature of the bremsstrahlung hard X-ray component. In Figure 19, contours of constant E*, the peak of the blackbody-limited cyclotron component, are shown as thick solid lines while contours of constant T b the temperature of the blackbody component, are shown as dashed lines. The thin solid lines have their same meaning as in Figure 18. To the upper right of the curve labelled 'soft excess' in Figure 19, the blackbody luminosity in soft X-rays exceeds the bremsstrahlung luninosity in hard X-rays. erg s'l, radiation pressure can be important and modify the results, but because photons can easily scatter out of the accretion colunn if f << 1, the Eddington luminosity does not represent the stringent upper limit to the luminosity that it does in the case of nonmagnetic degenerate dwarfs. Below and to the left of the curve E* = 2 eV in Figure 19, the assumption d < R breaks down, as can be seen from Figure 18. I f the geometry of the hot, p o s t - s k k emission region is such that most of the flux escapes through the face rather than through the e&es of the emission region (i.e., d << J(Zf)R), then Compton degradation of the bremsstrahlung hard X-ray component will occur i f L/f excetut s d 7 erg s'l. Such degradation is identical to that encountered Near and above L/f = LE = 1.4 x 59 ORlGlNM PAGE Is OF POOR QUALW Fig. 18--Contours in the (L/f,B)-plane for a 1 Mastar. For an explanation of the various lines, see the text. Fig. l!+-More contours in the (L/f,B)-plane for a 1 Mgstar (from Lamb and Masters 1979). For an explanation of the various lines, see the text. in nonmagnetic degenerate dwarfs, and will be discussed below. Figure 19 illustrates a third importarc feature of X-ray emission im magnetic degenerate dwarfs: observations of the qualitativr features of the X and UV spectrum can Cetwnine fairly accurately the physical conditions in the emission region, including the value of the nrabvetic field. c) Nonmagnetic Stars Studies of X-ray emission from accreting nonmagnetic degenerate dwarfs include those by Hoshi (1973), Aizu (1973), Hayakawa (1973), DeCregoria (1974), kyakawa and Hoshi (1976), Fabian, Pringle, and Rees (1976), Katz (1977), and Kylafis and Lamb (1979, 1982a,b). These calculations are applicable, even if a magnetic field is present, as long as the accretion flow is approximately radial and bremsstrahlung cooling dominates cyclotron cooling in the X-ray emission region (recall Figure 16). Thus they are relevant to the AM tier stars, such as AM Her itself, which has a magnetic fieid 8 * 2 x lo7 gauss (Lamb and Masters 1979; Schmidt, Stockman, and Margon 1981; Latham, Liebert, and Steiner 1931), and W hp, which has a magnetic field B * 3 x IO7 gauss (Visvanathan and Wickramasinghe 1979; Stockman, Liebert, and Bond 1979), as well as to the 00 Her stars. i ) hpectra The X and UV spectrm produced by accretion onto nomagnetic degenerate dwarfs generally has three components: 1) a hard X-ray bremsstrahlung component produced by the hot, post-shock emission region, 2) a soft X-ray blackbody component p d u c e d by bremsstrahlung photons that are absorbed by the stellar surface and re-emitted, and 3) secondary radiation produced by Compton heating of infalling matter above the shoc k . These components are clearly visible in Figure 20, which shows six spectra that cpan the entire range of accretion rates. Figure 21 shows for comparison three similar spectra when nuclear burning occurs at the accretion rate (see below). At low accretion rates, TeS < 1 and the observed hard X-ray spectrum is essentially the same as that produced in the emission region. As the accretion rate i s increased, le, exceeds unity and Compton scattering begins to degrade the spectrum (Illarionov and Sunvaev 1972). The blackbdy component then contains a contribution from bremsstrahlung photons which are backscattered by the accreting matter and absorbed by the stellar surface. The secondary radiation, which arises from accreting matter heated by the Compton scattering of the bremsstrahlung photons, is important only when degradation of the bremsstrahlung is substantial. As the accretion rate is increased further, this degradation becomes more severe. Finally, due to the combined effects of degradation and weakening of the shock by radiation pressure, the bremsstrahlung component disappears altogether. The star then ceases to be a hard (i.e., lobs > 2 keV) X-ray source. Figure 20 illustrates two important features of X-ray emission from nonmagnetic degenerate dwarfs. First, an intense blackbody soft X-ray component is always present. Second, at high accretion rates Compton degradation leads to low spectral temperatures even for high mass stars. ii) Correlation bet ween spectral temperature and Iminosity The resulting correlation between T h S and Lh is S h o w n in Figure 22 for stars of mass M = 0.2-1.2 M g Note that the accretion rate increases as one moves from upper left to lower right along the curves. For sources found in the lower right of the figure, Fig. 20-X and UV spectra produced by accretion onto a 1 Me star for six different accretion rates (from Kylafis and Lamb 1982a). The dashed line shows the changing cutoff dw to Compton degraaation. 61 Fig. 2l--Comparison of X and UV spectra produced by accretion onto a 1 Me star with nuclear burning at the accretion rate (solid curves) and without nuclear burning (dashed curves) (from Weast et at. 1982). 62 n > a, 1L W Fig. 22-Correlation between T d s and Lh for stars with masses 0.2 - 1.2 Ma (from Kylafis atrd Lamb 1982a). The dadred lines give the same correlation when the contribLtion of the blackbody component is includtd in Lh. a b 4 : b --7 ‘hard i e rq S ’ ) T 4 Fig. 23--Correlatbn between Tabs and Lh for stars with masses 0.2 - 1.2 Ma (from Weast et ai. 1982). The dashed lines give the same correlation when the contribution of the blackbody component is omitted. 63 an increase in T h and Lh therefore cotresponds to a decrease in the accretion rate: tabs and Lh increase since the smaller accretion rate lessens campton degradation of the hard X-ray spectnm. Figwe 22 illustrates the dramatic variation in the spectral temperature at high a c c r e t h r a t s and the pronounced correlation between X-ray spectral temperature and luninosity. d) Effectsof Nuclear Burning The energy liberated by nuclear burning of matter accreting onto degenerate dwarfs can be more than an clrder of magnitude greater than that available from the release of gravitational energy. I f burning occurs quiescently, the resulting energy is transported to the stellar surface ard produces an interne Hac- soft X-ray flux. S t m nuclear burning has therefore recently received a great deal of attention as a possible explanation of the intense blwldxxJy soft X-ray componemc inferred in the AM I4er stars (Raymond et al. 1979, Patter- et al. 1982) d in dhet cataclyrmic variables, such as SS Cyg and U Gem, during outburst (Fabbiano et al. 1981). i) Conditions for steady nuclear burning Unfortunately, the coditions under whiih steady nuclear burning can occur are poorly uwkrrtood. Detailed spherically symmeCric cakulations by Paczynski and Zyaokow (1978). Sian, Aciemo, and Turmhek (1978), and Sion, Acierno, and Tomczyk (1979) show that i f the degenerate dwarf is initially d d and the accretion rate is nat too high, the accreting matter becomes higMy degenerate before it ignites. E l e c t r o n d u c t i o n then rapidly tramports energy away into the core, and it must be heated before ingition can occur. I f the degenerate dwarf is hot, or if the accretion rate is high, the m e n in the accreting matter ooon igniter due to conpressional heating. In either case, eventually a violent nuclear outburst ens~es. Such outbursts are believed to acccoult for IN ae (cf. karrfield, Sparks, and Truran 1974). The outbursts are separated bv quiescent periods, in which nuclear burning occurs steadily at ody a small fraction d the accretion rate. The qJiescent periods are shorter for higher acc re th rates and can last fram @ 20 years or less (Sion et al. 1979) tn > 107 years (Paczynski and Zvkow 1978). For a narrow range of higher accretion rates, steady nuclear burning is possiMe at the rate of accretion (e. g. 1.0 - 2.7 x 10-7 %yr-l for a 0.8 Me) st3r; Paczynski and Zvtkow 3978). S t i l higher accretion rates lead to envelope expansion and the formation of a red giant with a degenerate awe. Dep le th of CNO nuclei m the accreting matter and thr burning rxion by diffusion can lead to buning via the p p chain rather than via the more tenlperature sensitive CNO-cycle ( S t a r r f i a Truran, and Sparks 1981), and stabilize the burning at higher accretm rates. However, theoretical investigat;m show that such rapid depletion i s rnlikely (Fujimoto and Trurar 1981; Papabizmu, Pingle, and MacDonald 1982). Effects due to non-spherical geometries also warrant investigation. For example, m the AM Her and DQ Htr stws a strong magnetic field channels the accreting matter onto the magnetic poles. I f t k matter is confined and bums aver a d y a m a l l fraction of the stellar surface, tk kming might be stabilized by the rapid transpc of energf horizontally. ii) Effects on A-tu, emission 64 The effects of nuclear burning on X-ray emission by nonmagnetic degenerate dwarfs have been investigated in detail bv lmamura et al. (1979, 1982) and Weast e t al. (1979, 1982). lhe accreting matter does not burn in the hot X-ray emission region, but may do so deeper in the envelope of the star. The energy thus liberated is transported to the stellar surface and enhances the blackbody f l w in soft X-rays. l h i s i:ux of soft X-rav photons cools the X-ray emission region by inverse Compton scattering. & a result, the hard X-ray luminosity IS often an order of magnitude less than it muid be in the absence of nuclear burning, the hard X-ray spectrun is softer, and the soft X-ray luminosity can be 100 times tlre hard X-ray luninosity. Figure 21 compares the X-ray spectra of a 1 Mestar in which nuclear burning occurs at the accretion rate to the spectra in the absence of burning. l h e three spectra shown span the entire range of accretion rates. Figure 23 shows the correlation between T b and Lh when nuclear burning occurs at the accretion rate for stars of mass 0.2 - 1.2 M e These Cuivs should be compared with those in Figure 22, which assunes no nuclear burning. The effects of nuclear burning on X-ray emission by magnetic degenerate dwarfs are not expected to he as dramatic. As long as cooling by cyclotron emission dominates cooling try inverse Compton scattering of the blackbody photons, the cyclotron UV and bremsstrahlung hard X-ray lununosities will be l i t t le changed. The spectral temperaturs of these amp one^ . t s will also be l i t t le affected. The blackbody soft X- ray lunitmsi?b will, however, be much larger. IV. L INk SPECTRA a) Ionization Structure The cimunstellar ionization structure of degenerate dwarf X-ray sources has been calculated analytically by Hayakawa (1973) and more recently bv Kylafis and Lamb (l982b). 'I'hese calculations assme spherical symmetry, and assune that the optical depth to absorption is small. The degenerate dwarf X-ray oources detected so far have low luninosities and b w accretion rates. Therefore, the analytical calculations are valid, provided that the accretion flow is approximately radial. The calculations by Kylafis and Lamb (1982b) show that the black- soft X-ray flux ionizes # t , He, and C out to distances large canpared with a typical binary separation. Furthermore, for high mass stars and low accretion rates, the bremsstrahlurg hard X-ray flux ionizes heavy elements out to considerable distances. These features are illustrated in Figures 24 and 25, which show 12, the radius at which the element with charge Z is half ionized and half neutral, as a function of mass accretion rate for a 1.2 Mastar- Figure 24 shows the effect of the blackbody soft X- ray flux, while Figure 25 shows the effect of the bremsstrahlung hard X-ray flux. In both figures, the solid lines correspond to no nuclear burning and the dashed lines to nuclear burning at the accretion rate. The absorption optical depth r o Z at the ionization edges of heavy elements remains small until the accretion rate exceeds about 3 x 10'3 but thereafter increases rapidly, as shown in Figure 26. Compton scattering and the resulting degradation of the hard X-ray spectrum occurs primarily close to the star, while most of the absorption m a i s relatively far from the star, as illustrated in Figure 27. lhus the amount of Compton degradation is less sensitive, and the amount of absorption more sensitive, to the distributicm of accreting matter. b) Absorption Features Figure 28 shows the emergent had X-ray spectnm fran a 1.2 Mg star for four different accretion rates as calculated analytically bv Kylafis and Lamb (1982b), taking absorption into accornt. Ross and Fabian (1-1) have carried out detailed nmerical calculations of the emergent spectrun f m a 1.0 M e star for three different accretion rates. The latter calculations treat the atomic physics carefully and are valid even for large absorption optical depth. The results are shown in Figures 29-31. Note both the absorption K-edges due to 0 V l l l (0.87 keV), Si XIV (2.7 keV), and Fe XXI-XXVI (8.2 - 9.3 keV), and the emission lines, broadened by Compron scattering, due to the Ka lines of 0 V l l l (0.65 keV), Si XIV (2.0 keV), and Fe XXV (6.7 keV). c) Emission lines The temperatures in the X-ray emission regions of degenerate dwalf X-ray sources are high enough (> 10 keV) to produce thermal emission lines, including those of Fe at * 7 keV, with significant equivalent widths. Emission lines can also be produced by fluorescence in the accreting matter above the X-ray emission region, as seen in Figures 29-31. Fluorescent emission lines may also be produced by X-rays striking the stellar surface sur rd ing the emission region, the disk, a d even the awnpanion star. The emission lines may be broadened by 1) thermal aoppler broadening, 2 ) Compton scattering, ad 3) Doppler broadening &e to bulk streaming velocities. lhermitl OOQpler braadening produces a relative line width, ( 7 ) Av/v (2kT/% c2) . The resulting width is s 0.5 keV for the s 7 keV Fe lines i f they are formed in an X-ray emission region with temperature Te s 10 keV. Compton scattering produces a relative line width 1/2 Thus the s 7 keV Fe emission lines will be relatively broad even if the electron scattering optical depth thmqgh accreting matter is only modest. hppler broadening due to bulk motion produces a relative line width where in the last s t e p we have scaled from the freefall velocity at the surface of a 1 Me star. Doppler broadening due to bulk motion is therefore generally less than thermal Cbppler broadening and Compton scattering. These 1 4 t s imply that the X- ray emission lines produced by degenerate dwarfs are relatively broad. HDwever, as noted above, emission lines can be p d u c e d by recombination and fluorescence in accreting matter far from the star. In this case, the t-ratures may be bw, and the electron scattering optical depth small. I f so, narrow emission lines can be produced. v. ISSUES 66 Fig. 24--Circunstellar ionization structure as a function of accretion rate produced by the blackbo& soft X-ray flux of a 1.2 @star (from Kylafis and Lamb 1982b). The quantity Iz, the radius at which the element with charge 2 is half ionized, is shown assuming no nuclear burning (solid lines) and nuclear burning at the accretion rate (dashed lines). Fig. ZS--CircrroteIlar ionization structure as a function of accretion rate pioduced by the bremsstrahlung hard X-ray flux of a 1.2 M a star (from Kylafis and Lamb 1982b). The curves have the same meaning as in Fig, 24. 67 , . Fig. 26--Optical depth T: at the absorption edge of a given dement as a function of accretion rate for a 1.2 Mestar without nuclear burning (from Kylafb and Lamb 1982b). Fig. 27--Fractional electron scattering and absorption optical depths reached at a given radius for a 1 M e star accreting at a rate 0.1 IE (from Kylafis and Lamb 1982b). Note that the electron scattering optical depth increases rapidly near the star, while the absorption optical depths remain small until larger radii. 68 ORIGINAL P K E IS OF POOR Q U A L l n 10 10) IO' f , o ' L . . . , , . . . . 10-1 E Lh.V) Fig. 28--Bremsstrahlung hard X-ray spectra produced by accretion onto a 1.2 MQ star at 4 different accretion rates, taking into account the effects of absorption analytically (from Kylafis and Lamb 1982b). Fig. 29--X and UV spectrum produced by a 1 Mastar at an accretion rate 0.045 h ; l ~ (T = 3) taking into account the ef feca of photoabsorption through detailed numerical calculations (from Kos and Fabian 1980). ,3 10 ENERGY ( r e v ) Fig. 3O--Same as Fig. 29 for an accretion rate 0.090 6 l ~ (T = 6) (from Ross and Fabian 190). Fig. 31--Same as Fig. 29 for an accretion rate 0.15 6, ( T I 10) (from Ross and Fabian 190). 69 Among the important unresolved issues concerning degenerate dwarf X-ray sources are the folbwing. a) Magnetic Fields Do only a few degenerate dwarfs have magnetic fields, and are the AM Her and DQ Her stars the only cataclysmic variables with magnetic fields? Or are magnetic fields endemic in degenerate dwarfs, and therefore in cataclysmic variables? If so, what are the field strengths? Are they large enough to affect the disk and the accretion flow near the stellar surface? b) Origin of Hard X Rays What is the origin of the hard X-rays emitted by cataclysmic variables? Are they produced by optically thin emission in the boundary layer between the disk and the star? If so, are the required high temperatures achieved by strong shocks, turbulence, or some other mechanism? Alternatively, are magnetic fields present in these sources sufficient to disrupt the disk near the star, p d u c i n g quasi-radial inflow and a strong shock? c) Origin of Soft X Rays What is the origin of the intense blackbody soft X-ray emission inferred in the AM Her stars? How can it be so large compared to the optical and hard X-ray emission? Is i ts origin the same as the soft X-ray emission seen io SS Cyg and U Gem at outburst, or is it different? d) Nuclear Burning Under what conditions is steady nuclear burning possible? If it is generally not possible, as many calculations suggest, why are outbursts not more evident? Can cteady burning occur more easily in non-spherical situations, as in the AM Her stars? If so, could it account for the intense blackbody soft X-ray emission in these stars? Could it acmunt for the blackbody soft X-ray emission seen in cataclysmic variables like SS Cyg and U Gem? e) Long Period hlsing Sources What is the nature of the long period pulsing sources? Are they actually degenerate dwarfs, or are they neutron stars? Why are they rotating so slowly; that is, why have they not been spun up more by their accretion torque? Does their rotation rate increase and then drxrease with time, like the previously known pulsing neutron star X-ray sources with long periods? f ) Cyclotron-Dominated Sources Although the AM Her stars are strongly magnetic, bremsstralung, not cyclotron emission, is the dominant cooling mechanism in the X-ray emission region (because the 70 accretion flow is channeled onto such a small fraction f E 10-3 of the stellar surface that the density in the emission region is very high). Where are the sources in which cyclotron cooling dominates? W i l l they be found bv a UV or extreme UV survey? g) High Luminosity burces To date no high lminosity (L c 1036 - 1038 ergs s-1) X-ray source has been unequivocally identified with a degenerate dwarf, and the future does not look promising. Are there no high luminosity degenerate dwarf X-ray sources? I f not, why not ? From these brief remarks, it should be evident that we have only begun to explore the nature of degenerate X-ray sources. We must rely on future X-ray astronomy missions to provide the data needed to understand them. In the following section, we outline the kind of instrunents that would best advance our knowledge. VI. OBSERVATIONAL NEEDS a) High Throughput Because degenerate dwarf X-ray sources are faint, the most important attribute of any instrument designed to study them effectively is high throughput. This requirement implies that the instrument should have a large area and a low background rate. A low background rate implies, almost inevitably, the necessity of a focusing instrument. A focusing instrunent is also desirable from the standpoint of source confusion, which could be a problem at lower energies. OQ;. b) Pointing Capability and Flexibility A central characteristic of degenerate dwarf X-ray sources, like other compact X - ray sources, is their time variability. They show quasi-periodicities on time scales ranging from several seconds to a thousand seconds, pulsing due to rotation periods ranging from 33 to as much as 4000 seconds. flaring behavior, variability correlated with the binary period, and, of course, the outbursts from which the cataclysmic variables derive their name. bint ing capability is essential for any instrument which is to study them successfully, and flexibility (w, that one can move onto the source when it goes into outburst, for example) i s desirable. c) Hard and Soft X-Ray Spectral Sensitivity k we have seen, many, and perhap all, degenerate dwarf X-ray sources have two distinct cmponents, one with Tbr > 10 keV and another with T& < 100 eV. k a result, a soft X-iay capability is important and should extend down to at least 0.1 - 0.25 keV. Conversely, a number of sources have hard X-ray spectra with temperatures as high as 30 keV (e+, AM Her). Therefore a hard X-ray capability extending up to at least \F 35 71 AeV, and possibly beyond, would be highly desirable. d) Broad-Band Spectral Measurements Correlations between the shapes and intensities of both the hard and soft X-ray components have been seen, for example, as a m r c e declines f ran outburst. Althougn many of these correlations are not yet understood, theoretical work indicates that they are potentially a powerful source of information about the physical conditions in the X- ray emission region, such as temperature, density, magnetic field strength, and mass accretion rate. Therefore broad band spectral measurements have been, and will continue to be, very useful. e) Moderate Spectral Resclution Further studies of the iron emission lines in these sources may yield information about the X-ray emission region, the accretion flow, and the geometry of the binary system. Other emission lines, if present, could provide similar information. 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I. , 230, 832. Sion, E. M., Acierno, M. J., and Turnshek, D. A. 1978, Ap. I., 220, 636. Starrfield, S., Sparks, W. M., and Truran, 1. W. 1974, Ap. 1. (Suppl.), 28, 247. Starrfield, S., Truran, 1. VJ., and Sparks, 1981, Ap. J. (Letters), 243, L27. Steiner, 1. E. 1981, private communication. Steiner, 1. E., khwartz, D. A., lablonski, F. I . , Busko, I. C., Watson, M. G., he , 1- Po, and McHardy, 1. M. 1981, Ap. J. (Letters), 249, L21. Stockman, H. S., Schmidt, kC. D., Angel, J. R. Po, Liebert, I., Tapia, So, and Beaver, E. A. 1977, Ap. I., 217, 815. Stockman, H. S., Liebert, I . , and Bond, H. E. 1979, in IAU Colloquium 53, White Dwarfs and Variable Degenerate Stars, ed. H. M . Van Horn and V. Weidemann ‘Rochester, N.Y.: University or Rochester), p. 334. Stockman, ti., Liebert, I . , Tapia, S., Green, R., Williams, R., and Ferguson, D. 1981, IAU Circ. No. 3616. Swank, J. H. 1979, IAU Colloq. No. 53, White Dwarfs and Variable Degenerate Stars, ed. ti. M. Van Horn and V. 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Q. 1982, submitted to Ap. 1. Wesselius, P., and Koester, D. '978, Astron. Ap., 70, 745. White, N. E. 1981, Ap. J. (Letters), 244, L85. White, N. E.; and Mar-,iall, F. E. 1981, Ap. 1. (Letters), 249, L25. Young, P., and Schneider, D. P. 1979, Ap. I . , 230, 502. ed. P. Eggleton, S. Mitton, and 1. Whelan (Dordrecht: Keidel), p. 85. k -. NEYFRON STAR BLVARIBS, PULSARS, AND BURST SOURCES' F. K. Lamb liepartments of Physics and Astronomy University of Illinais at Urbana-Champaign ABSTRACT Some impoztar.' but as yet unresolved issues involving ncutron star binaries, pulsars, and burst sources are described. Attention is drawn to the types of observations most likely to tasolve them. Many of these observations are likely to be carried otit during the next decade by one or more missions that have been approved or proposed. carry out sensitive flux measurements wizh an imaging detector and broad-band spectroscopic stud:es in the energy range 30-150 keV. There is also a need for soft X-ray and X-ray observations with an instrument which has arcminute angular resolution and an effective area substantially greater than that of ROSAT o r EXOSAT. Missing so far is an opportunity to I . INTRODUCTION Many interesting and inportant questions about nectron star binaries, pulsars, and burst sources are still unanswered. As examples, twenty s x h questions are discussed in SII, together with the types cf observations most likely to resolve them. Altliough plausible interpretations have been proposed €or almost all interesting observations, and i u ?any cases widely accepted, these interpretatior.8 are often based on very fragile evidence. Thus, our apparent understanding of such observations could be reversed very easily. Some of the questions considered are of this type. Key observations suggestad by these questions are compared with the opportunities offered by approved and proposed missions in $111. In di.:ussing each question, I have referred to several of the most recent papers as well as one or two recent review articles. Where an approFriate review article is unavailable or unknown to me, I have provided more extensive references. Photons in the eqergy ranges 0.1-10, 2-60, and 30- 150 keV are reCerred to as soft X-rays, X-rays, and hard X-rays, respectively. Spectroscopic studies are *ategorieed 3s lorresolution (E!& < l o ) , moderate-reeo1ut.m (E/AP - 10-100). or high-resolution ( E / X > 100). 'Supparted in part by MSF grant PHY 80-25605 and NASA grant WSC, 7653. 78 11. CURRENT ISSUES a) Pulsing X-ray Stclrces 1. Are these sources disk-fed or wind-fed? -- By this 1 mean. does the neutron star accrete matter from a disk or from a more radial fits': In considering this question, one should not assume that a neutron star with a companion that is losing mass via a wind is necessarily wind-fed, since such a companion may transfer matter to the neutron star with sufficient angular momentum to form a disk. Furthermore, the initial phase of critical-lobe overflow of ma-sive stars, which usually also have vinds, t s adequate to power the binary X-ray sources with orbital periods less than five days without smothering thew, and leads to reasonable X-ray source ltfetimes on the order of lo4 - lo6 yr (Paceynski 1976; Savonije 1978, 1979). The currently availsble evidence suggests that most pulsing X-ray sources are ted by uccretion disks, although this conclusion is not yet secure (see Ghosh and Lamb 1979; Elsner, Ghosh, and Lamb 1980). An answer to this question is important for understanding the mass transfer process in binaries, reprocessing of X-radiatlon within the accretion flow, the gross structure of the magnetosphere, and the beh-:-ior of the pulse frequency with time. The types of observations most likely ta furnish information that will help to provide the answer are high-resolution ultraviolet and soft X-ray spectroscopy, and X-ray timing studies. Ultraviolet and soft X-ray spectroscopy could sample the accretion flow at and outslde the magnetospheric boundary. while tieing studies could measure the specific angular momentum of the inflowing plasma. studies, Figure 1 shows the relationship between the pulse period, period dcrlvative, and acc7etlon lumlnosity predicted by the most detailed current theory of disk accr:tion. A sequence of measurements of these quantities which traced out a ccrve like those shown would confirm thut the source in qwstion was disk-fed and also determine its magnetic moment. As an example of what may be possible with tiring 2. How do some sources reach lonn oeriods and whv do such wriods persist? -- The mecha.?isms by which the long-period pulsing X-ray sources were spun down initially is not yet clear, although several possibilities have been proposed (see Davies, Fabian, and Fringle 1979; Chosh and Lamb 1979; Elsner, Chosh, and Lamb 1980; and references therein). Long periods are most likely the result of alternating episodss of spin-up and spin-down. Further support for this hypothesis has recently been provided by the Hakucho observations cf Vela X-1 shown in Figure 2, which reveal alternattng episodes of spin-up and spin-down. In disk-fed sourced, such behavior finds a natural explanation in the magnetic braking that occurs when the accretion rate falis (Ghosh and Lamb 1979; Elsner, Ghosh, and Lamb 1980). In wind-fed sources, such behavior might b;. explained by reversals in the circulation of the matter accreted by the neutron star. Despite the profoundly different implications of these two explanations. it has so far not been possible to cqambiguously identify the cause of alternating intervals of spin-up and spin-down in any long-period source. Thus, for excaple, the episodes seen in Vela X-1 may be due to accretion by the neutron star of vortices in the wind from its compasion, but they are alsc 79 consistent with spin-up and spin-down due to accretion from a disk, assuming variations In the laass accretion rate of a few percent; such small variations in the accretion rate are not Lnconsistent with the constraints on X-ray flux variations reported to date. Answers to these two questions are important for understanding the rate of formation of neutron star binaries, the typical lifetimes of X-ray binaries, and the properties of pulsars formed from massive X-ray binaries. Among the more promising observations with which to explore these questions are X-ray timing of the low states of the long period sources and long-term monitoring of their luminosity behavior by, for exaeiple, a sky monitoring erperisent. In connection with the latter, it is worth noting that the behavior on tire scales longer than one or two months is known only for a handful of sources. As an example of what Is expected, Figure 3 shows the dif ferent relationship between the luminosity and pulse frequency predicted for disk-fed sources which have small 1um;iosity variations, like those reported in Her X-1, and sources which e>..iibit shor . intense flares, like those reported in A0535+26. 3 , What are the causes of pulse period fluctuations? - The reasons for the pulse period fluctuations observed in Her X-1, Cen X-3, Vela X-1 a d other sources could be due to fluctuations in the accretion torque acting on the crust, episodic unpinning of vortices in the neutron superfluid thought to interpenetrate the inner crust, repeated fracturing of the crust, or stochastic spin-up of the neutron superfluid expected in the core of the star (Lamb, Pines, and Shaham 1978; for a review, see Lamb 1979). At present it is not even kcown whether the observed period fluctuations are produced by processes outside or inside these stars. Answers to this question could provide important information about fluctuations in the accretion flow or, alternatively, about the internal dynamics of neutron stars. The most promising observational approach here is to look for correlations between the X-ray luminosity and the spin-up rate, since lluctuations in the accre:ion luminosity may accompany fluctuations in the accretion torque, whereas short-term fluctuations in the luminosity are not expected as a result of iiiternal processes. 4. What are the respective roles of ma,qnetospheric and surface plasmas in forming pulse shapes and spectra? - There is some evidence that both play a role. Thus, €or example, the G p l e x wacreforsz seen in A053-26 and Vela X-1 (hce Fig. 4) have been Interpreted as the tssrrlt of cyclotron scattering by streams of accreting plasma above the stel?sr surface (Elsner and Lamb 1976), while spectral features reported in Her Y-1 (see Fig. 5 ) . 4U0115+63 (Wheaton et al. ' 3 7 9 1 , and 4111626-67 (Pravdo et 81. 1979) have been interpreted as the remilt ti cyclotron scattering or emission (for reviews, see Lamb 1977; Stat,' * io r t sl. 1981; and Trumper 1982). Theoretical work has so far focussed primarily tn the role of plasma at the s;el!ar surface (see, for example, Pig. 6 and Nagel 1981a.b; for a review, sze Hesedroe 1982), but plasma above the surface is also expected to play an Important rQ;e (Lamb 1977), as is plasma near the magnetospheric boundary (McCray and Lamb 1976; Basko and Sunyaev 1976; HcCray et al. 1982). Answers to this question are important for understanding the patterns of plasma flow within the magnetosphere, the characteristic flow velocities within the magnetosphere, inagnetic field geometries, and the physical conditions in the emission regions. theoretlcal problem, so observational guidance is especially important. This is an extremely difficult A key discriminant between magnetospheric and surface plasma is the time scale of fluctuations in the pulse waveform or spectral features. The dynamical time scale for plasma near the magnetospheric boundary is expected to be 0.1 - 1 s, and anisotropies in the emission pattern or spectral features caused by plasma there are expected to show fluctuations on a similar time scale. In contrast, the dynamical time scale near the stellar surface is - 0.1 - 1 ms, and angrtlar and spectral features produced by this plasma are expected to show fluctuations on this time scala. can probably be studied best by moderate resolution soft X-ray spectroscopy and high-resolution spectroscopy at the 7 keV iron lines, whereas the surface plasma can probably be studied best by phase-resolved spectroscopy at X-ray and hard X-ray energies. The magnetospheric plasma 5. What are the surface magnetic fields and dipole -Pent9 of these necitron stars? - Theorettcal intcrpretatlon of the observed s in-up rates, 1031 C cm3, although the actual value for any given source is uncertain (Ghosh and Lamb 1979). strengths - 4-6 x lo1* C in Her X-1 and - 2-3 x 10l2 G in 4U0115+63, while the variations of pulse shape with energy in A053926 and Vela X-1 have been interpreted as the reslrlt of cyclotron scattering by - 105-106 cm above the stellar surface (for references, see question 4, above). assuming disk accretion, yields dipole moments in the range 10 59 - bv small-scale magnetic loops of strength - 1-2 x 10 14 C at distances Estimates based on reported spectral features give field lasma streams channelea Nore iformation on this issue is importhnt for understanding the evolution magnetic flelds in pulsars (see Lamb 1981a) and X- and gamma-ray burst sour 's (see below). Measures of dipole field strengths can be obtained for stars which are disk-fed, if theoretical curve3 like those shown in Figure 1 are confftmed and a sufficient set of spin-up rates, pulse frequencies, and accretion luminosities are assembled for eack source so that the particular curve followed by a given cource can be determined. possible to estimate surface magnetic field s*.rengths using X-ray and hard X- ray observations of pulse waveforms and spectra, although the problem of Lnterpreting these obeervations appears to be much more difficult than that of interpreting timing observattons. In borne cases it may be 6. What is the equatic? of s'ate of-matter at very high densities? - A substantial effort during thc past decade has led to significant observational constraints on the equation of state of neutron star matter (for reviews, see Baym and pethick 1979; Pines 1980; Lamb 1981b). Thus, for example, it has been possible to rule out several cf the sbfter equations of state that have been proposed. However, iarge uncertainties still remain. A precise knowledge of the equation of state is important for identifying black hole candidates, since the only presently known property of black holes that distinguishes them from nonmagnetic neutron stars is their ewss, which amy exceed the maximum stable a s s of neutron stars. The equation of state is, of course, also of interest for the information it provides about the interactions of hadrons in dense ewtter. Further progress will require reflnements in our understanding of a variety of aapects cf pulsing and bursting sources and is likely to be slower. make a substantial contribution. In this connection, it is worth remembering that the orbits of only about half of the known systems of this type have been measured accurately. Xeasurements of the thermal fluxes from neutron stars, like those described below (question ll), could also make an important contribution, since the cooling rate of a hot neutron star is sensitive to the equation of state. New measurements of the orbits of neutron star binary systems can 7. What are the internal dynaraical properties of neutron stars? -- So far, there is no evidence for internal degrees of freedom fn neutron star X- ray sources (see Boynton 1981; Lamb 1981). This is puzzling, given the evLdence for a weakly coupled superfluid neutron component in pulsars and the variety of internal degr-s of freedom expected theoretically (Lamb, Pines, and Shaham 1978). X-ray timing observations are the most promising appicttch here (see Lamb 1979). Figure 7 shows the very different power transfer functions for pulse frequency fluctuations of a neutron star with and without an internal, finite- frequency norma' mode. b) "Galactic Eulge Sources" 8. How are they forsed m d how do the;- evolve? -- The bright galactic bulge sources are widely tzl.ir?ved to be binary svstems formed by capture processes (see 7.ewin and Clark 1980). Hov-ver, there 1s evidence that the majority of buA&e scurces which are not currently surrounded by globular clusters may require R mechanism of formation other than capture of a general field star by a compact object (Lightman and Criiicilay 1982). Moreover, the evolution of a white dwarf binary system into a neutron star binary does cot . . produce a source rith the high X-ray luminosities observed (Rappaport and Webhlnk 1982). Jnss , w?\y are globular ciuster X-ray soorcts so much more common in M3J than in our own galaxy? Both in our galaxy and in M31, the globular cluster sources ere groupdd near the plane, suggestipd that passage through the galactic plane plays an important role in forming oi activating these sources. The most promising approach to studying the formstion and evolution of the "bulge" sources is to study their appearance in nearby galaxies using optical and X-ray instruments. 9. Why ib there never more than @ne bright, compact X-ray zuurce observed in a globular cluster? -- Lightman and Crindlay (1982) have shom that the data ate not inconsistent with a probability of observing an X-ray source in a globular cluster which varies as the inverse of the time scale for two-body 82 binary formation, as would be expected if this is the formation process. Assuming that this Is the formation process, they show further that only one X-ray source is to expected in any one cluster. Direct evidence of the binary character of the globular cluster sources would help to secure this argument. There are two promising approaches here: long-term monitoring of X-ray flux levels to discover the modulation with binary phase that must be present at some level, if these sources are indeed binaries, and optical and X-ray studies of nearby galaxies, such as M31. 10. How large are their surface magnetic fields? -- It has been widely argued that the reason no pulsations have been seen in these sources is that the magnetic fields of these neutron stars are extremely weak. is a plausible hypothesis (see Lewin and Clark 1980), it becomes less comfortable when one notes that Her X-1, which is believed to be old, apparently has a magnetic field that is still - 5 x lor2 G (see Lamb 1981a), that the most plausible explanation for the temporal "footprints" of the X-ray burst sources is channeling of fuel by magnetic fields, and that the two mos: promising models of gamma-ray bursters, episodic accretion and thermonuclear explosions on neutron stars which are presumably rather old, both require surface magnetic fields 2 1Ol2 G (see question 17). illsuer to this question remains uncertain. Although this - 108 years Thus, the The question can be addressed by sensitive X-ray timing observations, to look harder €or periodic flux variations, and sensitive X-ray and hard X-ray spectroscopy, to search for any evidence of magnetic fields manifested by spectral features. c) Pulsars 11. What are the surface temperatures of pulsars? -- Pulsars are expected to have surface temperatures in the range 10' - 10" IC due to compressional heating when they were formed (see, for example, Van Riper and Arnett 1978; Bowers and Wilson 1980; Epstein and Pethick 1981; Richardson et al. 1982). bombardment of their surfaces by particles and gama-rays (see, for example, Cheng 1981), heating of their interiors due to friction beween the solid crust and the neutron liquid (see Greenstein 1981), and release of energy in crustal or core fracture events (see Fines, Shaham, and Rudetrsan 1972) or in episodes of vortex unpinning (see Alpar et al. 1981). Stimulated in part by the much more sensitive observations made possible by EINSTEIN, substantial iaprovements hsve rscently be-n made !n detailed calculations of neutron star cooling (Glen and Sutherland 1980; Van Riper and Lamb 1981; Gudmundsson, Pethick, and Epstein 1982; Richardson, et al. 1982) as well as in understanding which energy transport processes affect the results most strongly (Cudmur.dsson, Epstein, and Pethick 1982). Figure 8 shows a comparison of the theoretical cooling curves of Van Riper and Lamb with observat-ions . Imoortant new results on neutron star surface emission have been obtained with the EINSTEIN observatory (for reviews, see Helfand, Chanan, and Novick 1980; Helfand 1981). Observations of about fifty supernova remnants, including seven remnants of historical supernovae, have placed stringent upper limits on the luminosities of any compact source in all but four. In the four 83 remnants where flux is detected above the nebular background, the identification of the flux with thermal emission from the surface of the pulsar is uncertain (see, for example, Tuohy and Garmire 1980). A different approach is the survey of a selected group of known pulsars within a distance of 300 pc. Of the eighteen pulsars observed, six were detected. Although a detailed analysis of these observations is still in progress, a prelimiiiary analysis of data on PSR1055-52 indicates a luminosity in soft X-rays 4 x 1033 erg s-1, corresponding to - 0.1 of its rotational energy loss rate. The data is consistent with a blackbody of temperature - 1 x lo6 K. Further progress i n determining the surface temperatures of pulsars would give us information about the formation of neutron stars, ongoing processes at their surfaces and in their interiors, and the equation of state of neutron star matter. Thus, for example, neutron stars cool much more quickly if they contain condensed pions or quark matter. The most promising observational approach here is soft X-ray imaging of additional nearby pulsars with high sensitivity and sufficient time resolution to test for pulsations. 12. What are the physical processes rssponsible for converting rotational 5 into X-rays and gamma-rays? -- A variety of theoretical models predict the conversion of rotational energy to photons with energies up to and including gamma-rays (Elitzur 1979; Hardee 1979; Harding, Tademaru, and Esposito 1978; Ayasli and Ogelman 1980; Arons 1981; Cheng 1981; Rudefman 1981; for reviews, see Ruderman 1980; Mlchel 1982). Although longer period pulsars may be unable to produce a dense pair plasma above their polar caps, so that B larger fraction of their luminosity escapes as X- and gamma-rays, shorter period pulsars tend to have a much larger total luminosity. Thus, on balance short pertod pulsars are favored as X- and gamma-ray emitters. This conclusion is consistent with the fact that so far only the Crab and Vela pulsars are confirmed sources of X- and gamma-radiation (see Manchester and Taylor 1977; Bennett et al. 1977; Swanenberg et al. 1981). The reported detection of PSR1818-04 and PSR1747-46 above 35 MeV by SAS-2 (Ogelman 3t al. 1976) was not confirmed by COS-E (Mayer-Hasselwander et al. 1980), while Knight et el. (1982) using HEAO-1 were unable to confirm the detection of PSR1822-09 in hard X-rays reported by Mandrou, Vedrenno, and Hasnou (1980). For a recent review of aamma-ray observations, see Buccheri (1981). Answers to this question would shed important light on the maanetic field of the neutron star, the electrodynamics of the pulsar and the surrounding medium, the geometry of the emtssion region, and the properties of the emitting particles. Broad-band spectroscopic observations of known nearby pulsars with greatly increased sensitivity at X-ray and hard X-ray energies appear the most llkely to assist in ansuering this question. be the discovery of one or more very nearby neutron stars with magnetic fields that are too weak or rotation periods that are too long to produce a dense pair plasma. According to some models, such stars would be expected to be "radio quiet," but might still emit an observable flux of hard X-rays or gama-rays. Of particular importance would d) X-Rav Burst Sources 13. Are they binary systems? -- The X-ray burst sources are widely believed to be binaries (see the review by Lewin and Joss 1981), but direct evidence for binary membership has been discovered only recently. The transient X-ray burst sources Cen X-4 and Aql X-1 (Matsuoka et al. 1980; Koyama et al. 1981) have, in quiescence, optical counterparts which exhibit stellar spectra, a circumstance that has been taken as evidence for their binary nature (van Paradijs et al. 1980; Thorstensen, Charles, and Bowyer 1978). More recently, Walter et al. (1981) and Whtte and Swank (1981) have reported the discovery of periodic absorption events in 4U1915-05 (= MXB1916- 05) with a period of 50 minutes, which they interpret as the binary period of this system. This may be the first direct evidence for the binary nature of burst sources. It is important to confirm the binary character of these sources arld to determine their orbital parameters, both because their orig in is at present still uncertatn and because it is difficult to develop convincing models of their opttcal and X-ray emission without this information. The most promising approach is likely to be further searches for evidence of X-ray variation with binary phase. bu.st sources in nearbj galaxies would also make a valuable contribution. Further optical and X-ray studies of reasonable qualitative agreement with the observed properties of so-called type I bursts, but a number of disturbtng discrepancies remain (for reviews, see Lewin and Joss 1981; Lewin 1982). Thus. lor example, more detailed calculations (Taam and Picklum 1979; Taaa 1980, 1981, 1982; Ayasli and Joss 1982) give a wid2r variety of burst time scales and temperatures than have usually been associated with "standard" type I bursts. Some characteristic properties of burst profiles, such as the double-peaked bursts observed from some sources, have not yet been accounted for. Perhay a more serious difficulty for current models is the observation of bursts separated by as little as e-10 minutes (see Hayakawa 1981 and references therein). Such short intervals seem difficult to explain unless the nurlear fuel is burned incompletely (Lqmb and Lamb 1978), contrary to the results of most current modelc. The peak burst luminosities and temperatures given by current models (Taam 1982; Ayasli and Joss 198;), which assuLc blackbody emission, are signiflcantly lower &..an those observed (see Fig. 9). In one calcula'ion, Taam (i482) finds that the photospt.ere expands to a radius of at least 50 km, producing a very soft burst. Peak luminosities reportedly agree bet;er with observation i f the star assumed to have a mab.ietic field of - 3 x 10'2 G, which reduces the opacitv of the -.:rface material, but such models conflict with the idea that strvngly nragne ir stars don't burst. It ia interesting to note, however, that most uqdels of t t = Rapld Burster aedume this star has an app.ec1able maqnettc field in order t repetitive type I1 bursts, yet the Rapid Surster also produces nornal-loAing type I burpte. Furthermore, strong madnet!c fields are trken tcr be essential in most gama-ray burst modeh. produce the rapiLly This raises the que.stiou of why the neutron 85 stars that produce X-ray and gamma-ray bursts should have different magnetic fields . Some of these discrepancies may be resolved when models are developed which include factors, such as realistic initial temperature and composition profiles, propagation of detonation or deflagration fronts through the nuclear fuel, hydrodynamic motions,'expansion of the photosphere and mass loss, and radiative transfer through the atmosphere and infalling matter, that are known to be important for at least some bursts. A promising observational approach may be to study, at X-ray and hard X- ray energies, a much wider class of fast transients than the "standard" type I bursts. c, 15. How are individual burst sources to be interpreted? -- Still more challenging than acounting for the properties of the type I burst sources as a class is to account for the properties of an individual burst source. Among the quantities to be determined are the surface magnetic field, the thermal history of the star, and the history and current value of the mass accretion rate. So far no theoretical calculations have followed a succession of bursts. flashing neutron stars to reach a quasi-stationary thermal state (Lamb and Lamb 19781, while time scales much longer than the duration of single burst are required for the envelope to reach such a state, bursting neutron stars may always be evolving thermally. Given that time scales - 102-103 years are required for the cores of Long-term aonitoring of burst activity and inactivity, and accurate measurements of burst and persiste t luminosities, may help to resolve these issues. 16. What is the gating mechanism for the Rapid Burster? -- The magnetospheric and thermal instability model, which asqumes radial inflow, gives qualitative agreement with the behavior of the Rapid Burster (Lamb et al. 1977), but raises many questions (for a brief revfew of Rapid Burster models, see Lewin and Joss 1981). Indirect evidence for disk accretion in other bursters caste doubt on the assamption of approximately radial inflow made in this model, although the Rapid Burster clearly has unique properties. Perhaps a more serious difficulty for this model is the observation by Hakucho of flat-topped type 11 bursts lasting as long as 10 minutes with intervals between bursts c - ' at least 20-30 minutes (see Fig. 10; Hayakawa 1981; Oda 1982; and references therein). Such large intervals between bursts would require a very large bfnarv separation in radial flow models. More theoretical wort. is needed on those instabilities of accretion disks which may have the required very long time scales. Another puzzle is the relatively small influence which the type I bursts, which are believed to be due to thermonuclear flashes, have on the type 11 bursts, which are believed to be due to instability in the accretion flow. The small size of the effect (see Fig . 11) favors models involving disk accretion, since disk flows are less affected by radiation from the surface of the star. It is interesting to note that most models of the Rapid Burster invoke an appreciable magnetic field, whereas models of type I bursters usually neglect magnetic fields (see below). 86 Optical and X-ray observations to constrain the parameters of the system, assuming that it is a binary, or to determi.ie the mode of mass transfer, would be particularly important. band spectroscoptc studies of the interaction between type I and type I1 mrsts. One promising approach is X-ray timing and broad- e) Gamma-Ray Burst Sources 17. What are they? -- An array of Earth-orbiting atd interplanetary spacecraft have now provided confirmed positions of six gamma-ray bursts with arcminute accuracy (see Hurley 1082). positions which are either unconfirmed or less accurate than this. radio, X-ray, and optical candidates have been found in the error boxes, no clear association between gamma-ray bursts and other forms of emission has merged. The event of 1979 March 5 was unique; its identification with the supernova remnant N49 in the Large Magellanic Cloud remains controversial (see Cline 1982). spacecraft timing during the next few years, but given the sizable number of positions of this accuracy already available, one cannot expect a major breakthrough in determ'ntng the nature of these sources. Many other bursts have yielded Although Arcmimite positions will continue to be accumulated by multiple Despite the absence of unambiguous identificatfons with known astrophysical objects, a consensus view that the bursts come from strongly magnetic neutron stars bas emerged. consensus have recently been summarized by Lamb (1982). properties of the 1979 March 5 event which, though unique, has had a major impact on thinktng about gamma-ray bursts in general; theoretical arguments which suggest the presence of a magnetic field of Petschek 1981); and the reports by Mazets and his colleagues of spectral features (see the review by Teegarden 1982). The developments that have led to this They include the G (Colgate and The gamma-ray burst sources are one of the few major discoveries of the last decade whose natures remain a puzzle. Confirmation that they are strongly magnetic neutron stars would clear the way for a further advance in our understanding of them. Arcminute positions based on multiple spacecraft timing may produce an Another possibility is the serendipitous positioning of faint identification, but it is clear that still smaller error boxes will generally be required. gamma-ray burst sources by sensitive hard X-ray imaging detectors, if the latter are flown for t!ie purpose of studying bright, known X-ray sources. 18. What powers thea? -- At present the most promising energy sources are accretion onto neutron stars (see, for example, Lamb, Lamb, and Pines 1973; Colgate and Petschek 1981) or nuclear outbursts on such stars (Woosely and Taam 1976; Woosely and Wallace 1982). It is interesting to contrast the latter models, which discuss strong surface magnetic fields, "pools" of fuel on the star, and propagating burning fronts, with the thermonuclear flash models of X-ray bursts, which do not mention such phenomena (for reviews, see Lamb 1982; Woosley 1982). Hard X-ray spectrmcopy may be the most promising method of addressing this question. 87 19. What is the origin of the continuum emission? -- The expression that has generally been used to characterize the continuum spectra of gamma-ray bursts (see Teegarden 1982) corresponds to the spectrum of opticallythin thermal bremsstrahlung, if the energy dependence of the Gaunt factor, relativistic corrections, and magnetic field effects are ignored. These are, however, substantial effects for the energies and magnetic fleld strengths of interest. Few attempts to fit an actual bremsstrahlung emission spectrum to the observations have been reported to date. Actually, for the source to be optically-thin would require an aspect ratio Z lO3:l (Bussard and Lamb 1982; Lamb 1982). sheets or filaments of plasma, but f t is hard to see why such a distribution should always occur, as would be necessary to account for the apparent universality of the spectral shape. Such a ratio could arise if the bursts are produced by thin Another possibility is that the bursts are made up of a sequence of brief The average count-rate flickers, each shorter than the 4s detector acclimulation time, which emit Comptonized thermal spectra with falling temperatures. spectrum of such a source is quite simtlar to ttrat of optically-thin bremsstrahlung, as painted out by Bussard and Lamb (1982). This issue can only be reselved by hard X-ray and gamma-ray spectroscopy with better spectral and temporal resolution. 20. What are the origins of observed spectral features? -- Rekorted spectral features are Zenerally of two types: extinction in the energy range 10-50 keV, and excess emission in the energy range 300-650 keV (again see Teegarden 1982). The lower energy fea’ures could be due to cyclotron scattering, if the magnetic field in the source varies by at least a factor of two (Bussard and Lamb 19R2), or a time-varying low energy cutoff (Lamb 1982). The extinction ,,annot be due to photoelectric absorption by iron or nickel unless the magnetic field is Z 2 x loL3 G. The higher energy features are widely interpreted as redshifted electron- positron annihilation radiation. However, the shapes of these features are poo:ly determined at present, and hence their interpretation remains in doubt. If the lower energy features are due to cyclotron scattering, more accurate determinations of the 300-650 keV emission features could provide a cross-check QTI the inferred magnetic field strength, since ma netic broadening and one-photon annihilation become important for fields Z 10 f3 G (see Fig. 12). Again, the only observatlons with real promise of resolving these issues are hard X-ray and gamma-ray spectroscopic studies with better spectral and temporal resolution. 111. DEVELOPMENTS DURING THE NEXT DECADE Table 1 list? the various types of observations that have been cited above as promising means to answer the questions that have been posed, together with approved or proposed missions that are capable of making the type of observation in question during the next decade. suggests tb,at the most discovery space is available to an instrument capable of carrying out hard X-ray imaging and spectroscopy. There i* also a need for Thie accounting X-ray and soft X-ray observations with an instrument which has arcminute angular reso1ut:on and an effective area substantially larger than that of ROSAT or EXOSAT. I thank Roger Bussard, Don Lamb, Christopher Pethick, and Gregory Zylstra for their help in the preparation of this review, and Larry Smarr for sharing EINSTEIN results on M31 in advance of publication. TABLE 1 APPROVED OR PROPOSED MISSIONS ~~~ ~~ 1 Suggested Observation Missione Soft X-ray imaging ROSAT, EXOSAT Soft X-ray spectroscopy ROSAT, EXOSAT X-ray timing XTE, EXOSAT, ASTRO-C X-ray (2-60 keV) sptctroscopy XTE, EXOSAT, :STRO-B X-ray sky monitoring XTE, ASTRO-8, ASTRO-C Lron-line spectroscopy Hard X-ray imging Hard X-ray spectroscopy Studres of nearby galaxies (XTE, EXOSAT, ASwRO-B) ------ Gamma-ray spectroscopy GRO 'Missions whose nrimary objectives are other than the suggested observations listed here are shown ir. parenthesis. 90 Alpar, H, I.., Anderson, P. W., Pines, D., and Shaham, J. 1981, Ap. J., -9 249 Arnett, W. D. 1980, in Proc. 9th Texas Symp. on Relativistic Astrophysics, Arons, J . 1981, Ap. J., zb8, - 1099. Ayasli, S. and Joss, P. C. 1982, Ap. J . , in press. Ayasli, S. and Ogelman, 8. 1980, Ap. J., 237, 227. Baym, G. and Pethick, C. J. 1979, Ann. Rev. AstrG. Astrophys., 17, 415. kannett, K., et al. 1977, Astr. Ap., 61, 279. Lowers, R. L. and Wilson, J. 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Walter, F. M., Bouyer, S., Mason, i;. n., Clarke, J. T., Henry, J. P., Salpern, Wheaton, W. A., Doty, J. P., Pcimini, F. A., Cooke, 8. A., Dobson, C. A., J. 1982, in Gama-Ray Transients and Related Astrophysical J., Pietsch, W., Reppin, C., Voges, W., Staubert, R., and - - Wamsteker, W. 1980, Ap. J., 241, L161. J., and Grindlay, J. 1981, preprint. Goldman, A. , Hecht, ' . , Hoffman, J. A., Howe, S. K., Zcheepmaker, A., Rothschild, R., Knight, P. K., Nolan, P., and Peterson, L. E. 1979, Nature, 282, 240. - White, N. E. and Swank, J. H. 1981, preprint. Woosley, S. E. 1982, in Gamma-Ray Transients and Related Astrophysical Phenomena, AIP Conference Proceedings No. 77, ed. R. E. tingenfelter, H. S. Hudson, and D. M. Worrall (W: AIP), p. 273. Wooeley, S. E. and Taam, R. E. 1976, Nature, 263, 101. Woosley, S. E. and Wallace, R. K. 1982, Ap. J., in press. - 93 PICURE CAPTIONS Fig. 1. Data from nine pulsing X-ray sources plotted against theoretical spin-up curves for d!sk accretion (ftw Chosh, Lamb, and Zylstra 1982). Here P afid P are the pulse period and period derivative, respectively, and L is the accretion luginosity. Rach curve is labeled with the assumed stellar magnetic moment in units of 1030 c cm3. Fig. 2. The variation of the pulse period of Vela X-1 (from Hapakawa 1981). The Stt8fght line represents a constant snin-down rate PIP = -1.5 x 10-4 yr-1. Pig. 3. Two examples of luminosity variations and the resulting pulse frequency behavior, Illustrating the effects of different magnetic moments and lusinosity patterns (from Elsner, Chosh, and Lamb 1980). Also shown is the observed behavior of A0535t26. Fig. 4. Pulse waveforam of Vela X-1 observed at different energies (see Rappaport and Joas 1981). Fig. 5. Deconvoluted Her X-l-.spectrum from the 1976 MPI/AfT balloon observation (from Trumper et al. 1978). Pig. 6. Left and right panels show pulse shapes obtained by convolving model beam shapes with the rotation of the neutron star for two different sets of inclinat ion angles (from Mikziiros and Bonazzola 1981). Fig. 7. Theoretical power transfer functions for a two-component neutron star model (left) and a generalized two-coaponent model with a finite-frequency internal mode (right). The power transfer function describes the amplitude of pulse frequency fluctuations excited by an internal or external fluctuating torque of fixed strength (from Lamb 198Lb). Fig. 8. Theoretical cooling curves compared with observations (from Van Riper and Laah 1981). bounded b stiff equation of state; light shading: star with a pion condensate; no shading: stat with free quarks. Also shown are detections (dots) and upper limits (arrows) obtained from EINSTEIN soft X-ray observations of pulsars and supernova remanants. rectangle characterizes the upper limits that have been obtained for 7 nearby pulsars. The results for each star are shown as regions the cooling curves for zero maanetic field and B = 4.4 x 101 z C. Dark shading: soft equation of state; medium shading: The cross-hatched Pig. 9. Variation of the surface luminosity with time for a burst produced by a combined hydrogen-helium shell flash (from Team 1982). The effective temperature is given at the peak of the burst, and after one e-folding time. 94 Fig. 10. Time profiles of a typical trapezoidal buret from the Rapid Burster as seen in two different energy channels (from Bapakawa 1981). Fig. 11. Bursts from the Rapid Burster (from Lewin and Joss 1981). I bursts (marked as "epecial") occur independently of the sequence of the rapidly repetittve type I1 bursts (numbered separately). The type Fig. 12. Rate coefficients for nonrelativistic electron-positron annihilation into one or two photons, at3 a function of megnetic field etrelrgth (from Bussatd and Lamb 1982). 2 0 .E 4 Y 82 -2 J -4 -6 1 I I 0 I + 63 IO L 2 Figure 1 96 ORIGINAL PAGE IS OF POOR QUALITY C m co cv * - a - + a S \. U r n tn L (3 6 t W Q € .- I- Figure L 97 4 pjg- 1.0 - TIME ( Y E A M eo-- 1 I 1 J 0 S 15 TIME MARS) Figure 3 98 1 1 1 1 1 1 1 1 1 l 1 1 ! 1 1 1 1 1 1 - - f 1.2 -3 keV 50C xx>c 15oc lOOC 500 2000 ’ \ 15’3 C v) c s lo00 s 500 2000 I500 1000 500 1000 500 I I 2 x 282.9s - 6-12keV 99 HERX-1 1 t IO0 IO’ lo2 lo3 PtlOTON ENERGY IN KEV Figure 5 xnw Figure 6 I I J I 1 - m I 101 103 Figdre 9 '0-24*5 keV I 1.5-10 keV 1 2 4 6 8 IO 12 TIME AFTER UT 3:26:15 (MIN.) AUGUST 11 , 1979 Figure LO 10s 0 (D 0 I(v 106 g? 107 "1P82' 26059 X-RAY SPECTROSCOPIC OBSERVATIONS AND HOOELING OF SUPERNOVA REHNANTS 3. Michael Shull University o f Colorado Laboratory f o r Atmospheric and Space Physics and doiqt I n s t i t u t e f o r Laboratory Astrophysics X-ray spectroscopy can be a valuable too l i n in terpret ing the temperatures, densities, and elemental abundances i n young supernova remnants. Because of the large enrichment o f the remnant ejecta i n nucleosynthetic products o f the s t e l l a r explosion, emission l ines from He-like and H-l ike ion izat ion stages of many heavy elements are prominent i n HEAO-2 spectra. Precise abundance deter- minations, however, remain elusive because o f the need f o r non-ionization equi- l ibr ium spectral modeling. This review describes the recent x-ray observations o f young remnants and t h e i r theoretical interpretat ion. A number o f questions remain, concerning the nature of the b las t wave interact ion w i th the i n t e r s t e l l a r gas and grains and o f atomic processes i n these hot plasmas. Future x-ray spectrometers wi th high co l l ec t i ng area (lo3 cm2), moderate spectral resolut ion (€/A€ % 100) and good spat ia l resolut ion (5-10") can make important contr ibutions t o our under- standing o f supernova remnants i n the M i l k y W a y and neighboring galaxies and o f t h e i r r o l e i n the global chemfcal and dynamical evolut ion o f the i n te rs te l - 1 a r medi um. 108 I. INTRODUCTION I f X-ray spectroscopy i s t o be used as a diagnostic too l fo r astrophysical plasmas outside the solar system, it w i l l most l i k e l y prove i t s e l f i n observa- t ions of young supernova remnants (SNRs). Heated t o tempratures of 1-10 keV and enriched with the heavy element products o f nucleosynthesis, the gas i n these remnants emi ts prodigiously i n X-ray l ines ( H o l t 1980). Models o f the emission from elements wi th atomic number 6 - - < 2 < 28 provide measures o f plasma temperature, elemental abundances, and the degree o f approach t o ionizat ion equil ibrium. These observations y i e l d valuable information on the type and structure o f s t c l l a r explosion, the nucleosynthetic products, and the subsequent interact ion o f the b last wave with i n t e r s t e l l a r gas and grains. future X-ray spectrometers can make further c tx t r ibut ions t o our knowledge o f SNRs i n the Milky Way and neighboring galaxiss. brl’ef summary o f the past observations, the theorzt ical modeling, the primary unanswered questions, and the requirements f o r tuture instruments. This review presents a I I. OBSERVATIONS Most young remnants (t < 1000 years) have been observed by the HEAO-2 Imaging Proportional Counter ( IPC) o r by the High Resolution Imager (HRI). Shel l - l ike emission was found i n the remnants Tycho, Cas A, Kepler, and others; i n a few cases such as the Crab Nebula and Vela, the remnant shel ls are f i l l e d . Although the spherical morphology i s reassuring evidence for the b last wave model o f SNRs, it i s yet unclear whether the detai led emission structure i s best described by the Sedov adiabatic s i m i l a r i t y solut ion (Sedov 1959; Taylor 1950), o r by reverse-shocked ejecta (McKee 1974; Gull 1975; Kirschner and Cheval i e r 1977). 4- Two o f the HE . . .. - .. _. 109 ,0-2 spectrometers have resolw& prominent emission l i n e s from heavy elements i n young SNRs. The Sol id State Spectrometer (SSS), wi th e f fec t i ve area %lo0 cm , observed remnants i n our galaxy and the Large flagellanic Cloud with an energy resolut ion A€ z 160 eV f r o m 0.6 t o 4.5 keV (Becker e t a l . 1980ab). The strongest l ines (Figure 1) ar ise from He-like ionizat ion stages o f Ne, Mg, 2 I I S i , S, A r , and Cas as w e l l as f r o m blended L-shell l ines o f Fe near 1 keV. i Figure 1. Einstein SSS experimental spectra from four young SNRs (Holt 1980). 2 The Focal P law Crystal Spectrometer (FPCS), wi th ef fect ive area ~ 2 - 3 cm , pushed X-ray spectroscopy even further, sepirrating the He- l i ke " t r i p l e t " 1 ines (resonance, forbidden, and intercofubination) o f 0 V I 1 and Ne I X i n Puppis A 110 ORIGINAL PAGE lS OF POOR Quarn (Winkler - - et al. 1981) and resolving the H-like La and Lp lines of 0 VI11 and Ne X and several lines o f Fe X V I I (Figure 2). EINSTEIN OBSERVATORY Y. L 1. CRYSTAL SPECTROMETER IIML#rtw lib d n 5 Zlb I S8 '9.0 1.b ,... l.l(h,r.y . . . . I . I A. , 1 . . . I , . . . , . PUPPIS A I- ! I . - $ - 0 ~ . . . . . . , r r m . . . . , . . . ~ . ~ F m , . . . . . . . . . , . . r r r r m , . . . . ~ mo m m a0 Boo s30 lQl0 loso c*cRev cm Figure 2. HEAO-2 (FPCS) spectra of the Puppis A SNR (Winkler et al. 1981), showing He-like "triplet' lines (marked F, I, RX H-like La and Lp lines o f 0 VI11 and Ne X , and several L-shell lines of Fe X V I I . Dashed lfnes fndfcate background. 111 Preliminary coronal equi l ibr ium analyses o f the SSS data (Becker et 1980ab) required two temperature components t o f i t the spectra: a "hard" compo- nent w i th kT % 5-10 keV t o f i t the continuum and a "sof t" component with kT % 0.5 keV t o f i t the H- l ike t o He-like l i n e rat ios. f o r Tycho suggested tha t S i and S were enhanced by factors 4 0 over t h e i r cosmic abundances, but tha t Fe was underabundant by a factor 0.15. Even stranger were These equilibriURl models the derived abundance enhancements o f Ca (x 76) and A r (x 35). A l l o f these abundances are suspect, however, because o f the possibi 1 i ty o f substantial departures from ion izat ion equilibrium. 111. THEORETICAL MODELING Spectral emission models o f hot, opt ica l ly - th in , low-density plasmas (Shapiro and Moore 1976; Raymond and Smith 1977; Shull 1981a) are now a famil- i a r too l i n x-ray astronomy f o r der iv ing dances i n solar f lares, s t e l l a r coronae, accurate avai lable rates f o r c o l i is ional the equil ibr ium ion izat ion f ract ions o f then generate the spectral emissiv i ty o f the temperatures, densities, and abun- and in t rac lus te r gas. Using the most on i ra t ion and recombination t o compute the abundant elements, these models the hot plasma i n the x-ray continuum and emission l ines. Coupled w i th a X-square f i t t i n g program, these spectral codes y i e l d the plasma temperature and elemental abundances. The appl icat ion o f ion izat ion equi l ibr ium ( I E ) models t o young SNRs i s questionable f o r three major reasons: 1. The SNR ages are of ten comparable t o the character is t ic co l l i s i ona l ion izat ion time f o r He-like and H-l ike ions. 2. The dramatical ly d i f f e ren t temperature components (4 keV and ~ 0 . 5 keV) required t o f i t the continuum and l ines, respectively, suggest an ion iz ing plasma i n a t ransient stage. The r a t i o o f the resonance l i n e t o forbidden plus intercombination l i n e o f He-l ike 0 V I 1 and Ne I X (Winkler e t a l . 1981) i s too high f o r ion izat ion equi l ibr ium (Pradhan and Shun 1.1). 3. 112 ORIGINAL PAGE fS OF POOR QUALITY As shown by I t o h (1977), non-ionization-equilibrium (NIE) e f fects may hdve a s ign i f i can t e f fec t on the emissivity o f young remnants. I f the shocked plasma has not had su f f i c i en t time t o ionize S i o r S t o t h e i r equi l ibr ium state a t temperature kT + 5 keV, the He-like stages (aad the resu l t ing emission l i n e strengths) may f a r exceed t h e i r values i n equilibrium. NIE models nf X-ray emission from a hot plasma are f a r more complicated than I€ models, because they involve the calculat ion o f the time-dependent ionizat ion h is tory o f every parcel o f emit t ing piasma. !:or young SNRs, NIE modeling requires the coupling o f gas hydrodynamics w i th a time-dependent ion i - zation code and a spectral emission code (Shull 1981b). Recently computed NIE models o f young remnants have been used (Shull and Szymkowiak 1981) t o derive elemental abundances from the SSS data (Figure 3). x" IOE 2 i \ s I I I l l 1 T~&*s SNR I 1 I l l I 1 I l l ] GSFC- SSS Si f IOL 1 10E 0 l0Cl 0.u 0.5 0.6 0 . 7 0 . e 1 . 0 1.2 ~ . Y I . s ~ . w . o 2.5 3 . 0 3 . 5 9 . o u . 5 CNLRGI i n t w Figure 3. Einstein SSS data (crosses) and NIE model ( s3 l i d l i r e s ) f o r Tycho's SNR (Shull and Szymkowiak 1981). Emitt ing elements are marked near t h e i r strongest l ines. 113 Table 1 compares the abundances f o r Tycho derived from a single-velocity NIE blast wave model with those derived from a 2-temperature I€ model. Although S i and S ?.re s t i l l enhanced i n the NIE model, Fe i s no longer underabundant, and Ca and A r have more r e a l i s t i c abundance enhancements. Although a great deal o f fur ther work remains i n t h i s area, it appears that NIE models wi th a single veloci ty can achieve nearly as good a f i t t o the Tycho spectrum as 2- temperature IE models. The Ca, A r , and Fe abundances appear more r e a l i s t i c , and the single ve loc i ty parameter i s physical ly simpler than the multi-component I€ f i t . Several theoretical questions remain t o be answered. 1. What i s the ra t io , T./T o f ion t o electron temperatures behind the b las t wave? (Pravho 8ni Smith 1979). 2. Do NIE effects f u l l y explain the low r a t i o o f He-like forbidden and intercombination l ines t o the resonance l ine? 3. What fract ions o f emission come from the b las t wave and from reverse- shocked ejecta? What. are the ejecta masses? 4. How sensit ive are the derived NIE abundances t o the assumed dynamical model (the range o f densities and temperatures) o f the remnant? IV. FUTURE STUDIES Further observational advances i n answering the four questions posed i n Section I11 w i l l undoubtedly require x-ray instruments with higher sensi t iv i ty , greater spat ia l resolution, and better spectral resolution. Nowever, i n design- ing such instruments, one must decide which o f these requirements are essential and which are only advantageous. Let us examine the four questions i n order. The degree o f ion-electron temperature equi l ibrat ion may be studied by measuring the hard x-ray (5 - 30 keV) continuum i n SNRs (Pravdo and Smith 1979; McKee and Hcllenbach 1980). A t the low densities character ist ic o f remnants. the Coulom9 equi l ibrat ion time (Spi t ter 1962) may be qui te long. Plasma insta- b i l i t i e s behind co l l is ion less shock fronts may equi l ibrate ll and fe on a 114 f as te r scale (McKee 1974). To study these e f f e c t s observational ly, one requires h igh spa t ia l reso lu t ion t o i d e n t i f y the l oca t i on o f the hard continuum emission r e l a t i v e t o the b l a s t wave. An observational study o f He- l i ke 1 ine r a t i o s requires subs tan t ia l l y great- e r spectral reso lu t ion than t h a t af forded by the SSS. While the FPCS had the needed resolut ion, i t was only capable o f studying the b r igh tes t remnants. Therefore, these observations probably requ i re a d i f f e r e n t type o f spectrograph w i t h both good energy reso lu t ion and h igh e f f e c t i v e area. the energies o f the He- l ike resonance ? ines and the energy resolut ions necessary t o separate the forbidden and intercombination l ines. Although 0 V I 1 and Ne I X requi re the lowest reso lu t ion (* loo), the S i XI11 and S XV l i n e s are f a r strong- e r i n most remnants. show a progressive departure f rom equ i l ib r ium w i t h increasing atomic number 2. However, i t i s probably most f r u i t f u l t o study the 0 and Ne l i n e s w i t h a sensi- t i v e instrument having good spa t ia l reso lu t ion i n order t o i d e n t i f y var ia t ions i n r a t i o s from spec i f i c regions i r \ a large number o f remnants. Table 1 presents A study o f l i n e r a t i o s f o r a range o f elements should The answer t o the t h i r d quest;on--that o f the r e l a t i v e cont r ibu t ion of "b las t wave" and "ejecta" x-ray emission-is the most dependent o f the four on h igh spat ia l resolut ion. The e jec ta appear t o be unresolved on the HRI images (Seward 1981). Therefore, i n order t o measure the heavy element abundances i n the t r u e "b las t wave" and d is t ingu ish them from the metal enriched ejecta, one must be able t o ob ta in moderate energ/ reso lu t ion spectra o f small regions of the remnant. The mass determinatlons o f these e jec ta requ i re accurate abun- dances, since the emiss iv i ty o f the plasma i s g rea t l y enhanced by metal enr ich- ment and N I E ef fects . Because the x-ray intensity from these ;mall regions w i l l be less than t h a t from the l a r g e - f i e l d SSS data, 3 high c o l l e c t i n g area i s required. Question four may be the most important s c i e n t i f i c issue concerning SMs f o r the overa l l astrophysical community. I f the x-ray spectra o f young remnants are t o be used as diagnostics o f s t e l l a r evolut ion, s t e l l a r explosions, and nucleosynthesis, x-ray astronomers must increase the accuracy o f the spectral data as wel l as r e f i n e the theore t ica l models. The HEAO-2 SSS provided remark- ably dood l i n e determinations, considering i t s moderate energy resolut ion, E/AE ..O-20. That t h i s was possible was due p r i m a r i l y t o i t s large e f fec t i ve area (% 100 cm ). Because the major uncer ta in t ies i n the modeling o f SNR x-ray spectra invo lve the e f fec ts of non-equil ibr ium ion i za t i on ( i .e. time-dependent temperature and densi ty h i s to ry o f the plasma), f u tu re observations can best help by prov id ing high spectral reso lu t ioa - and a largn c o l l e c t i n g area. Resolv- i ng the Fe L-complex near 1 keV, as wel l as higher exc i ta t i on l i n e s o f He-l ike and H- l ike ions o f Ne, Mg, S i , and S, o r determining the energy cent ro id o f the Fe K- l ine complex near 7 keV can b e t t e r const ra in the model f i t s . Unt i l t h i s i s done, fur ther refinements i n the theore t ica l modeling are probably unwarranted. 2 The conclusion drawn frs;;, the assessment o f the four s c i e n t i f i c questions 3 2 appears t o be t h a t large c o l l e c t i n g area (* 10 cm ) and moderate spectral 2 reso lu t ion (E/AE of SNRs. Moderate spat ia l resolut ion, o f order 5", would be advantageous fo r some spec i f i c problems i n ind iv idua l remnants. However, unless one achieves the high e f f e c t i v e area and spectral resolut ioo, SNR studies w i l l be l i m i t e d t o only a few b r i g h t sources. While de ta i i ed studies of these sources a t h igh spat ia l reso lu t ion w i l l y i e l d useful informat ion on the masses and remnant structure, the s c i e n t i f i c top ic o f greatest general interest-an abundance analysis o f a large sample o f remnants i n our galaxy and the Magellanic Clouds- w i l l almost c e r t a i n l y requ i re the s a c r i f i c e o f spa t ia l reso lu t ion t o achieve the required spectral reso lu t ion and c e n s i t i v i t y . 10 ) are needed t o make fuyther progress i n spectral studies I I 116 Ib 1 0.37 IrO 0.1 2.0 si 6.0 7.6 5 u. 5 6.5 Ar 34.6 3.2 ca 76 2.6 Fe 0.u 2. i *- relative to solar values, dctmid fmm &fi t t ing to -2 SSS data- H, Ha, and Ello an a s s d to be in solar elndime. T M U 2. HE-LIKE LIMS LLOlEIQT EE(R-f)" AE(RI)* RESOUnIW 0 574 cy U eV 5.4 av 110 Re 922 17 6.8 140 51 la65 2s 11 170 5 2461 30 13 150 Fc 6702 64 26 2Go ~ ~ "Enargy ER of tha He-lfke resonance l ine (R); eaergy reparations, bE(Ff) and A€(RI), htween R and the forbidden line (F) mi Intercoabination ltne ( I ) ; md resolution, EdA€(RI), raquiraJ to analytc Her! se line ratios. 117 REFERENCES Becker, R.H. , Hol t , S.S. , Smith, 6.W. , white, N.E. , Boldt, € .A . , Mushottky, R.F . , Becker, R.H., 6oldt, L A . , Holt, S.S . , Serlemitsos, P.J., and White, N.E. 1980b. Cul l , S.F. 1975. N.N.R.A.S. - 161, 47. Halt, 5. 1980. "X-Ray Spectra o f Supernova Remnants,' i n X-Ray-Ast?;ompti) and Serleaaitsos, P.3. 1980a. AJ. 3. Letters, - 235, LS. &. - 3. Letters, - 237, 177. Proc. NATO Adv. Study Inst., Erice, S i c i l y (eds. R. Giacconi an * p. 35. Itoh, H. 1977. P.A.S.3. - 29, 813. Kirshner, R.P. , and Chevalier, R.A. 1977. AJ. - - J. 218, 142. k K e e , C.F. 1974. &. - - 3. 188, 335. McKee, C.F. , and Hollenbach, 0.3. Pradhan, A.K. , and Shull , J.fl. 1981. &. - 3. 249, i n press. Pravdo, S.H. , and Smith, 6.W. 1979. AJ. . - 3. Letters - 234, L195. Raymond, J.C. , and Smith, 6.W. 1977. &. - 3. Suppl. - 35, 419. 1979. - - - Ann. Rev. As t r . 9. - 18, 219. Sedov, 1.1. 1959. S im i la r i t y - and Dimensional Methods - i n k c h a n , a (New Yo, Academic Press). k: Sward, F. 1981, pr ivate communication. Shapiro, P. R. , and b o r e , R.T. 1976. &. - - 3. 207, 460. Shull, 3.M. 19818. AJ. - 3. Suppl. - 46, 27. Shull , J.M. 1981b. AJ. - J., t o be submitted. Shull, 3.M. , and Szpkwiak , A. Spitrer, L. 1962. Ph sics of Fu l l y Ionized Gases 7' 2nd Ed. (New York: Wiley- Taylor, G.I . 1950, Proc. ROJ. --- SOC. London, Mol, 159. !F I o t n t ~ ~ ..I.' , S . T . , Canizares, C.R. * Clark, G.W. , Markert, T.H. , Kalata, K. 1981, i n preparation. I ntersc i ence) , + p. -- - and Schnopper, H.W. 1981. &. i. 265, 574. PFZECXDING PAGE BLANK NOT FILMED INTERSTELLAR, INTRACLUSTER AND SUPERCLUSTER GAS Lennox L. Cowie Physics Department, Center for Space Research, and Center for Theoretical Physics Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Talk Presented at: Symposium on X-Ray Astronomy and Spectroscopy Goddard Space Flight Center, October 1981 120 I. INTRODUCTION The topics which this talk has to cover range in size from about 100 Pc to 100 MegaPc but in many respects are remarkably similar. In all cases we are considering hot tenuous plasmas with tebnperatures in the range lo6 + 3.10e OK and densities of lo-' + lo-' ~m'~. The dominant radiation processes are thermal bremstrahlung and collisional line excitation and all except the very hottest objects will have observable X-ray emission lines. How- ever the most important unifying point is the anaular scales involved. For the interstellar gas one may expect structure ranqing ir? size from about 10' (shadowing by IS clouds; Fried e t a l . 1980: to tens of degrees (nearby evolved supernova remnants (Nousek e t a l . 1981). Nearby (distance ~ 1 2 0 M Pc) irregular clusters will have structure on qalaxy scalelengths of a few minutes, which is similar to the core sizes of distant. more regular clusters (e.9. Fvrman and Jones 1982). The cores of very distant clusters at z=l will be resolvable at 30'' -c 1' [ e . g . Henry e t G T Z . 19791. Superclusters will have sizes of degrees [e.g. Murray e t a l . 19781. Thus there are an enormously wide range of problems which an instrument with a spatial resolution of $ * -c 1' and a field of view of degrees could tackle. Though there are problems in these areas which such an instrument could not cover,* I think it is fairly clear that what we need most is an instrument of this general type. *Examples are t!-.e spatial mapping of cooling flows in clusters (Fabian c t , J I . 1981) or the mapping of gas around galaxies in distant irregular clusters. 121 Of the three classes of problems listed in the title the only one on which we have really excellent X-ray data is the clusters [e.g. Forman and Jones 19821. We have a good deal of information on X-ray emission from the interstellar gas both from rocket flights and the HEAO 1 and SAS C experiments [e.g. Fried et aZ. 19801. Finally we have very little solid information of any sort on the superclusters. Despite this diversity in existing understanding, the way to the future in all three areas must be through larger collecting areas.* In the rest of the talk it will be seen that there are many problems where a highly s p e c t r a l resolution [R n- 100-1000] would be useful or even essen- tial. However, we could make a great deal of progress with an instrument of moderate resolution (R \r 10) provided ;it? could obtain the necessary increase in sensitivity. A rather incomplete list of important problems in these fields is as follows: Interstellar Cas: Mapping at moderate ( 1 ' ) spatial Morphology of individual structures in resolution: the IS gas Separation of stellar and gaseous con- tributions to the soft X-ray background Interstellar cloud shadowing Structure o f the galactic halo (High Spectral Resolution Studies : 1 Chemical composition of the galactic halo X-ray absorption line studies of the disk and halo Spectroscopy of individual structures *The necessity for large area applies irrespective of the spectral resolution. 122 Clusters : Mapping at 1' resolution: (High spectral resolution studies: 1 Superclusters: (Figh spectral resolution studies:) Samples of distant clusters/c:uster evolution Outer halos of rich homogeneous clusters Interaction of galaxies and cluster gas in irregular clusters Morphology of gas in clusters Spectroscopy of the centers of radiatively cooling clusters Abundance gradients Primordial material in irregular clusters retection and mapping of hot gas Interaction bekween clusters and superclusters Chemical enrichment in intercluster qas I've split the .'.ist into experiments which could be performed without high spectral resolution and those in which it is essen- ! tial. I think it's worth noting that roughly 504, of the experiments I require hiat spectral resolution as well as the imaging capability. Clearly if both sensitivity and high spectral resolution could be obtained this would he ideal. t f I now want to consider a few of these topics in more detdil. ! t The choice is somewhat arbitrary and I have avoided topics which i 1 are touched on elsewhere (in particular in the talks of Fabian and of Shull). i I I. SUFZRCLUSTER GAS It seems increasingly probable that the general intergalactic gas lies at temperatures less than lo6 O K and probably cannot be detected by its X-ray emission (e.g. Sargent e t a ? . 1979). ks 123 I yet the arguments and observations of this point are in no way compelling but it seems worthwhile to focus our attempts to detect the IGM in the X-rays on those regions where the heat input is most likely to be concentrated. The best location for this is probably in the intercluster regions of the superclusters. Bookbinder st a t . (1980) have argued that galaxies of present visual luminosity L, = L release about lOb1.’ ergs into the intergalactic gas at temperatures around lOgoo K in their early stages of evolution. 0 Such material will not be bound to any cluster of which the galaxy is a member. At this stage the cluster would then possess a hot outflowing wind. * Assuming that the net visual luminosity of a typical super- cluster is around lOI4 Lo, the total energy release would amount to about 2x106’ ergs. If this occurred at z=5, allowing for adiabatic expansion losses, it would heat a gas of density = 9.6~10-~Rh~ ~m’~, (h = Ho/lOO km s-l Mpc-l), filling a radius ”e of 20 Mpc to a temperature of ~2xlO~R”h‘~ OK.** itlass of gas in t h e superc lus ter is 1316Ch2 ;-la while the galaxies will have released about 5 ~ 1 0 ~ ‘ Ma of processed gas or about lOI4 The total *As was first pointed out by Ostriker (1979), this has two very attractive features. Firstly it drives out any primordial gas which would like to fall into the cluster. Secondly, by removing very metal enriched qas from the cluster it can solve various abundance problems of the type discussed by deYoung (1978). * *A complete self consistent calculation would follow the evolution of the blast wave generated by the overlapping cluster winds and calculate the temperature from this (Ostriker and Cowie 1981). The result is very close to the quick estimate given above and the present radius of the blast wave is around 20 Mpc for R=l. I i 124 Mo of metals (Bookbinder et a l . 1980) . The metallicity may therefore be a substantial fraction of cosmic particularly if R is less than 1. Assuming that the emissivity is around n2 ergs cm3 s-’, e a temperature of about 200 ev-1 Kev, as is suggested by the above calculation, the total luminosity of the supercluster would be around lo4’ Q2h4 ergs s-’ and its surface brightness about 5 ~ 1 0 ’ ~ R2h4 ergs/cm’* s-l ster” or a fraction of the diffuse X-ray background at these energies. In general terms and independent of this rpecific model it is clear that the most important requirement for a supercluster search is the ability to exclude member clusters and other X-ray objects from the field and to consider only the intercluster com- ponent. This requires an angular resolution of at least a fraction of a degree. The experiment must also be capable of imaging the whole supercluster which would generally mean a FOV of several degrees for relatively nearby superclusters. For a supercluster of radius 3 O at a distance of 500 Mpc we can estimate that a 30 detection above fluctuations in the back- ground would crudely require a source with 10% of the diffuse X- ray flux in the same region (Levine e t a t . 1977, Schwarz 1 9 8 0 ) . This would imply that a sensitive experiment could reach surface brightness limits of about 2x10’’ ergs cm’2 sec” ster” (Fried e t aZ. 1980) and constrain R < 1 or so for an individual supercluster. Clearly a survey of a large number of superclusters could provide very interesting constraints on the IGM density. 125 A spectroscopic instrument, measuring the relative line strengths, could also constrain the metallicities of the gas. This could be of crucial importance, if and when the superclusters were detected, in distinguishing between cluster heating of the intracluster gas as discussed above and alternative heating mechanisms such as gravitational in-fall. The history of cluster X-ray emission studies should remind us how important such information can be. 111. CLUSTER X-RAY SOURCES I want to say relatively little about this topic, since the problems are so well known to everyone. be found in the reviews of Cowie (1981) and Forman and Jones (1982). Briefly, a large are3 imaging instrument is needed to study distant clusters and to ex:.end the Einstein results of Henry and coworkers (e.g. Henry e t al. 1979). Realistically there is little hope of obtaining cosmological information from this type of measurement but a very large sample of distant clusters may allow us to obtain t h e evolution of the average X-ray luminosity and gas core radius f : r inlividual classes of cluster. Hopefully we may at least determine if there has been any evolution of the potential of the cluster over recent ( z 5 1) times. However, one should contrast the results of Cowie and Perrenod (19791 with those of Perrenod (1977) to see how small the expected dif- ferences are. Discussion can A second direct application of a large area imaging detector would be the study of gas lossage by galaxies in irregular 126 clusters (e.g. Fabian e t al. 1980). Pushing existing results farther back in z would probably help us understand the nature of galaxy stripping in clusters, gas evolution in galaxies and cluster galaxy evolutionary effects such as the color changes described by Butcher and Oemler (1978). Models for the formation of cluster atmospheres such as those by Norman and Silk (1979) or Sarazin (1979) make detailed and testable predictions of the evolution of the gas distribution in clusters which could be checked. There are a number of additional problems which a spectro- scopic instrument could tackle. Perhaps the most interesting would be an extension of the fascinating Einstein SSS and FPCS results (e.g. Canizares e t at. 198U, Mushotzky e t aZ. i981) on the cooling central cores of clusters out to more distant clusters. Refinement of the cluster metallicities which can be obtained in this way could be of primary importance in under- standing galaxy evolution within the clusters and cluster gas evolution. IV . INTERSTELLAR GAS In this section I want to consider two very specific experi- ments, one of which illustrates what could be done with a large area imaging detector and one of which would additionally require high resolving power. a. Cloud Shadowing of the Soft X-Ray Background The great majority of the soft X-ray background appecrs to be truly diffuse, arising primarily from a local hot spot in the 127 galactic disk and from. a hot gaseous galactic halo (Marshall and Clark 1981). About 20-30% at 1 Kev and substantially less at 200 eV arises from dM stars (Rossner e t aZ. 1981) with about 10 stars in each square degree contributing most of the flux. The fractional contributions from Pop 11 stars to the halo emis- sion may still constitute a problem. However if the soft X-ray background could be imaged on a scale of minutes, fluctuation analyses of the type given by Levine e t a l . (1977) at larger scales should allow us to obtain a stringent limit to the stellar contribution. Imaging of the X-ray background at such scales is a demaneing problem however, since the high galactic latitude photon arrival rate in the 200 eV-1 Kev range is only around 5~10'~ ph cm'2 sec" (a')-'. with long exposure and a low background are necessary. Therefore both a large area detector As Fried e t al. (1980) have recently pointed out, typical diffuse interstellar clouds with column densities of 3 ~ 1 0 ~ ' cm* or more are opaque to radiation in the 200 eV energy range. The local hot gas behind such regions and, more importantly, the hot halo gas or halo star contribution will be substantially shielded by such regions. A typical line of sight ai. high latitudes is most likely to pass through exactly one cloud but has a finite possibility of passing through none or more than one (e.9. Spitzer 1977). Since the cloud sizes are large compared to arcmins (at 100 pc a 1 pc radius cloud has an angular radius of 30'1, indi- vidual clouds will appear as large shadowed regions on the diffuse X-ray background at low energy. Since we may also estimate the distance from the amount of missing soft X-rays this type of 128 measurement could determine the size spectrum of interstellar clouds a quantity of ctxial importance to theories of the ISM (McKee and Ostriker 1977). It is worth commenting that this quantity is remarkably difficult to determine by other methods. b. X-Ray Absorption Lines from the Hot ISM The possibility of making X-ray absorption line studies, analogous to the optical and W absorption line studies which have been so important in stvdying the interstellar medium, has only recently begun tn be considered. The rea.;ons for this are clear - the instrument requiremerts are quite severe both as regards effective w e a and spectral resolutlon - but the poten- tial for studying the intergalactic medium (Shapiro and Bahcall 1980) and the hot interstellar meJium and the galactic halo (York and Cowie 1981) are enormous. The reason for this is that the absorption lines directly measure column densities of a given ionization stage and remain sensitive to low density gas in contrast to the emission lines. In Table I, I have summarizec? from York and Cowie the strongest available lines together with the expected equivalent widths €or a 1018=s cm'2 column density of hot gas at the optimal tempera- ture. This value is representative of the hot ISM (e.g. McKee and Ostriker 1977). It can be seen from Table I that a resolution of at least R a 100 is required to discriminate neighboring strong lines. In addition, one may simply estimate on the basis of photon statistics the required instrument parameters. The necessary effective area ! 129 Table 1 (adaptel from York and Cowie 1981). I I ' Species c v c VI 0 IV o v 0 VI1 0 VI11 Si VI11 Line Wavelength eft, 40.27,40.73,41.4 33.7c 22.86 22.52 21.8,21.6 19.0 61.0 'Logarithmic Temp. at Which Species is Dominant Ionization Stage 5.7 6.0 5.4 5.4 e o 6.3 6.0 f (Oscillator Strength) 0.65 0 .42 0.45 0.62 0.69 0.42 1.2 A is given in terms of the sourc :;=tensity I by 3 x 10'' 1.6x10-' I. 6x10' 3 x 10" 3 x io-' 1.6x10-' 10- [p$)lz J (kev/ (cm' s kev) I . A(cm2)Rt(hrs) = 3 ~ 1 0 ~ n2/ 1 1 For a source with I = 5 kev cm'2 s-l kev" at 500 eV, such as the Crab (Charles e t a l . 1979), a 3a detection of a line with WE/E = 3x10"' in an hour's exposure would require A(cm')R = 6000. terms this is a factor of 30 or so over the FPCS detectors aboard Einstein (Giacconi e t a l . 1979). In rough There are a number of problems with this technique, of course, not least that of finding sufficiently string background sources with well defined continua in the rieighborhood of the absorption lines. in s3me cases (e.g. ScoX1) there may be relatively narrow emission lines which will confuse the absorption line studies. Synchrotron sources such as the Crab are ideal, of course. ! 1 130 I I v. SUMMARY Clearly this has been an idiosyncratic apd personal view of a complex range of topics, but I think two pints are very clear. Firstly, a large area imaging instrument could cover a great deal of ground with or without a spectroscopic capability. However, high resolution spectral capability is highly desirable and would greatly enhance the power of such an instrument. Either class of instrument would be highly flexible, capable of dealing with a wide range of problems and of great general utility. ACKNOWLEDGEMENTS I would like to thank J. Grindlay and A.C. Fabian, who pointed out omissions in the original presentation. This work was supported by NASA grants NAGW-208, NSG-7643 and NGL-22-009-638. I 131 i . - References Bookbinder, J . , Cowie, L.L., Krolik, J., Ostriker, J.P. end Rees, M. 1980. AI . ' . * \ . 2 3 7 , 647. Butcher, H. and Oemler, A. 1978. A p . J . 2 2 6 . 559. Canizares, C. c t a l . 1980. In H i g h l i ~ i i t s of AStrOnc.ni3 5 . 657, ed. P.A. Wayman (Dordrecht: Reidel). Charles, P. u t a l . 1979. A p . . ? . 230. L83. Cowie, L.L. and Perrenod, S. 1978. A p . J . 2 1 9 , 354. deloung, D.S. 1978. A p . * l . 223. 47. Fabian, A.C., Hu, E.M., Cowie, L.L. and Grindlay, J . 1981. A p . 1 . 2 4 0 , 47. Fabian, A.C., Schwarz, J . and Forman, W. 1980. MNRAS 192, 135. Forman, W. and Jones, C. 1982. A K A A 20. 000. Fried, P.M., Mousek, J.A. , Sanders, W.T. and Kraushaar, W.L. 1980. A { ) . J . 2 4 2 . 987. Giacconi, R. t * ! a ! . 1979. A p . J . 2 3 0 , 5 4 0 . Henry, J . P . t * t n l . 1979. A p . . r . 2 3 4 . L15. Levine, A . , Rappaport, S., Halpern, J. and Walter F. 1977. Ar.d. 2 1 1 , 215. Marshall, F. and Clark, G. 1981. Private communication. McKee, C.F. and Ostriker, J.P. 1977. A p . J . 2 1 8 , 148. Murray, S.S . , Forman, W., Jones, C., and Giacconi, R. 1978. A ; .,r. 2 1 9 , ~ 8 9 . Mushotzky, R., Holt, S.S., Smith, B.W., Boldt, E.A. and Serlemitsos, P.J. 1981. A I . . . ' . 2 4 4 . 1.47. Norman, C. and Silk, J.P. 1979. A p . d . 2 3 3 . L1. i i 132 Nousek, J.A., Cowie, L.L., Hu, E.M., Lindblad, C.J. and Garmire, G. 1981. Ap.J. 248, 152. Ostriker, J.P. 1979. Private communication. Perrenod, S. 1978. Ay.J. 224. 285. Rossner, R. e t a l . 1981. Ay.J. 240. L5. Sarazin, C. 1979. A~troph!48icu L r ? t tct’tl 20. 93. Sargent, W.L.W., Young, P.J., Boksenberg, A. and Tytlcr, D. 1980. A[’.J. ( S u [ ) p ? . , J 42, 41. Schwarz, D.A. 1980. l ’hysic*a Sc-ripta 2 1 , 644. Shapiro, P.R. and Bahcall, J.N. 1980. A ; - . - ’ . 241. 1. York, D.G. and Cowie, L.L. 1981. A p . 2 . In press. 4 7 :‘1p82‘ 2 6 0 6 1 133 X-ray Emission from Galaxies and the Universe A.C. Fabian Ins t i t u t e of Astronomy Madingley Road Cambridge CB3 O W U.K. INTBODUCTION - The study of X-ray emission from normal galaxies began with the launch of the Einstein Observatory i n 1978. Before tha t t i m e only our own Galaxy and the Magellanic Clouds had been studied i n any d e t a i l (see e.g. Markert -- et al. 1977). and -200 point sources have been resolved i n the nearest ones. sources a re now known i n other galaxies than were known i n our own before 1978. It seem clear that statistical s tudies of bright Galactic sources, which r e l a t e t o the i r evolution, are r e l i an t upon the discovery of sources i n many other galaxies. galaxies) requires a telescope system with be t t e r than a r d n u t e resolution and a sens i t i v i ty be t t e r than IO x tha t of Einstin. Naw many galaxies of a l l Hubble types have been detected More A substant ia l future increase (the Virgo c lus te r L i t t l e is known of the diffuse X-ray emission from galaxies. The s o f t X-ray background i n our Galaxy (Tanaka & Bleeker 1977, Fried e t al. 1981) and evidence fo r a gaseous halo (Savage 6 de Boer 1980) suggest tha t a signif icant f ract ion of the gaseous matter i n galaxies is a t X-ray emitt ing temperatures. shallow that the gas densi t ies are then so low as t o make detection diff:’-.r’t. Searches fo r diffuse sources of l i ne emission are l ike ly t o be the most rewarding. by groups and cluscers of galaxies. is t o be observed on a grand scale. -- Unfortunately the potential. w e l l s of most galaxies are so We can study diffuse gas much be t t e r i n the deeper wells provided Here the evolution and cycling of gas 134 Finally, I would stress that the lumpiness of the Universe on large scales ( a 10 Mpc) can best be determined from X-ray measurements. The investigation of the detailed patchiness of the X-ray background should be an important priority for the future. NORMAL GALAXIES The various components of X-ray emission in our own Galaxy are listed in Table 1. and, below 0.25 keV, by the soft X-ray background. contribution is that due to stars (see e.g. lLosner -- et al. 19811, especially late-type low mass Population I1 stars that may constitute a large fraction of the stellar population. issue, but if the indication of an inverse correlation between X-ray emission and age proves to be correct sources. The emission is dominated by the bright Galactic "Bulge" sources The most uncertain Unfortunately little has been reported on this then they are likely to be only weak X-ray The point source population of a few nearby galaxies is given in Table 2 from the work of Long -- et al. 1980, 1981, Sevard i Mitchell 1981, Van Speybroek et al. 1970, 1980 and Palumbo -- et al. 1981. of M31 and our Galaxy are in marked contrast. 20 gaobular cluster sources in M31 have an X-ray luminosity greater than any in our Galaxy and why M31 has so many more 'bulge' sources. The X-ray luminosity functions Future work muSt decide why -- tong (1980) has pointed out that the X-ray dssion of * 40 g-laxies of all Hubble type appears to be correlated with the optical brightness of the galaxy. Lx !Y IOg3 - IO4 Lvis. Although the sample is small, this does suggest the presence of a camon factor. "he first possibility is the prevalence of Population I1 ('bulge'-type) sources. that globular clusters with X-ray sources fall approximately 011 this correlation. Since the ration of L to the stellar X-ray emission in Population I1 sources TWO possibilities are apparent. One indication that this may be correct is to be found by noting X I- ,-,. , 3 6 8 is Q, 10 -lo4, only X-ray source, independent of Hubble type, i n order to explain the correlation. 'star' i n 5 10 -10 is needed t o be a 'bulge' type I cannot, however, see how the extended emission from M84 (Formaa 1980) or from Cen A (Peigelson et el. 1981) can be eas i ly explained by 'bulge! type sources, which should co l lec t a t the centres of galaxies. poss ib i l i ty is hot gas, from stellar ma88 loss heated by Supernova explosions. This cannot explain why globular c lus te rs or the Magellanic Clounds f i t the correlation. but could be a s iga i f icant , or dominant, f ract ion of the emission from unresolved galaxies. r iva ls tha t of the point sources below 5 1 keV, and a contribution of t h i s magnitt*de seem8 possible i n M31 aa well (Von Speybroeck e t al. 1979). The second Certainly the diffuse emission from our own galaxy Galactic coranae w i l l be d i f f i c u l t t o detect (see e.8. B r e w 1980). 2 The gas temperature should be :j_r%@,'k whered is the internal velocity dis- persian. would be lo s t ; cooler gas would cool fur ther and collapse. Galactic halo observations indicate that a 'ga lac t i r founta in ' (Shapiro 8 Field 1976, Bregman 1980, Songaila 1981) then up t o cooling muat take place. of which w i l l emerge as l i nes below .\t 1 keV. I Thus a temperature of 1, lo6-10 1< is to be expected. Hotter gas I f the IUE is operating lMe yr'l of 40 The routine detection of diffuse This represents .\t 10 e r g 8-l i n s o f t X-rays, mu~h emission from galaxies w i l l probably start when substant ia l high-galactic la t i tude exposures are made with detectors and spectrometers sensi t ive a t 0.1 - 0.2 keV. This w i l l have implications/or gas flows i n galaxies, the stellar death and b i r t h rates and fo r chemical enrichment. Much of the p a l o s t i n winds an8 by s t r ipping i n isolated galaxies must be trapped i n the potent ia l wells of c lus t e r s , which appears t o be continuing a t the present time, and of the galaxies within them r e l i e s heavily on X-ray observations. A vigorous appmach i n the future should enable us t o understand the evolution of these largest known en t i t i e s . The evolution of c lus te rs , 136 The mass dis t r ibu t ion i n c lus t e r s can be determined from detai led observations of the surface brightness p ro f i l e a t two separate energies. that any gas motions are highly subsonic, that the gas is l ike ly to be closely Maxwellian and that the emission is predominantly bremsstrahlung and line radiat ion then accurate solutions are possible. galaxy motions can unravel the expected anisotropies i n t h e i r veloci ty dis t r ibut ion. Since we know Later comparisons with the The formetion of galaxies may be observed through t h e i r X-ray emission. Of interest now is the gro*wth of cent ra l galaxies i n c lus t e r s v ia a cooling flow of in t rac lus te r gas (Fabian it Nulsen 1977, Cowie 6 Binaey 1977). Here the gas from other cluster galaxies settles and presumably forms stars around a central s ta t ionary galaxy. Perseus c lus t e r (Fabian -- e t a l . 1981) and is being observed i n an increasing number of other c lus te rs . phenomena i f a large f rac t ion of the energy produced with the metals is released as X-rays (Bookbinder e t e l . 1980; Sunyaev, Tinsley 6 Meier 1979). Perhaps a l so the non-thermal a c t i v i t y of galaxies is a l so inversely correlated with age, a s for stars. produce X-rays. Isolated extragalact ic HI1 regions observed local ly , i n which bursts of star formation a re taking place, m y give a good picture of the emission from a primeval galaxy. one a t Lx : 0.1 Lvis with the Einstein Observatory, This process occurs around NGC 1275 i n the Young (primeval) galaxies may be r i ch i n X-ray -- The diss ipat ion of chaotic motions may ind i rec t ly We (Stewart -- e t al . 1981) have detected The ac t iv i ty of galactic nuclei is discussed elsewhore, but does allow E. Boldt has pointed out t o galaxies to be me the poss ib i l i ty that X-ray spectral observations can t e s t whether Seyferts and quasars a re two separate c lasses or not. index of Seyferts i s close t o 0.7, a t least from % 3 - 100 keV, and an extra- polation of non-evolving Seyferts out t o 2 % 1 gives the MeV bump i n the hard ident i f ied a t large redshif ts . Basically, the energy spectral 137 X-ray background (see e.g. Boldt 1980). Seyfert8 does not evolve and predicts that many active galaxies with 44 L 5 IO erg s- 2 % 1. otherwise they must be re la t ive ly steep. This sug&ats tha t the class of should be observable with spectral indices of % 0.7 out t o Quasars must have a much f l a t t e r spectrum i f they compose the background, THE LUMPINESS OF THE X-RAY BACKGROUND The usefulness of X-ray observations i n constraining matter f luctuat ions on the scale of 100 - 1000 Mpc is discussed i n Fabian, Warwick & Pye 1978, Rees 1979 and Fabian 1981. L i m i t s on 6o/p of a few percent are obtained from the c'irrent limit of 1 percent on excess f luctuat ions on the scale of ?, 5 degrees (Fabian b Bees 1978, Schwartz 1979). Hopefully these w i l l be improved when the W - I A2 analysis is complete (Shafer 1381). The observed P(D) fluctu- a t ions due to unresolved sources that a r e not ckmped provide a basic l i m i t - at ion. down t o 2, 0.01 UPU and scale of 0:s would allow a residual f luctuat ion of 61 - 2 30 n-* percent t o be measurable where n is the number of regions surveyed. I For % 100 f ie lds , the clumping of galaxies (and of quasars) a t the 20 Mpc end of the galaxy covariance function (Peebles 1979) should begin t o be a t ta inable i f the source density is high enough. An imaging detector (2 - 6 keV) that enables sources t o be eliminated Large-scale anisotropies i n the X-ray background are expected due t o the galactic background, our motion re la t ive t o and t o shear and other e f f ec t s on the X-ray background. where the galactic e f f ec t s are hopefully small, and where the steepening of 6 1 the background spectrum enhances I via the Compton Getting e f fec t . The s i tua t ion may be optianun a t high energies (> 30 keV) I thank E.A. Boldt and h i s colleagues a t GSFC fo r supporL and hospi ta l i ty . 138 BBFBBBNCES Boldt, E.A., C m . on Astrophys., - 9, 97. Bookbinder, J., Cowie, L.L., Krolik, J.H., Ostriker, J.P. 8 Rees, M.J. 1980, Astrophys.J., 237, 647. Bregamn, J.N. 1980a, Astrophys.J., 237, 681. Bregman, J.N. 1980b, Astrophys.J., 236, 577. C d e , L.L. 6 Binney, J. 1977, Astrophys.J., 3, 723. Fabian, A.C., 1981, Ann.N.Y.Acad.Sci. in press. Fabian, A.C. & Rees, M.J. 1978, M.N.R.A.S., - 185, 69. Fabian, A.C. & Nulsen, P.E.J. 1977, M.N.R.A.S. - 180, 479. Fabian, A.C., Warwick, R.S. & Pye, J.P. 1980, Phys.Scripta, 3, Fabian, A.C., Cawie, L.L., Hu, E. 6 Grindlap, J. Feigelson, E. et al. preprint. Forman,W. 1980. Prc . N.A.T.O. A.S.I. on X-ray Astronomy. Fried, P.M., Nousek, J.A., Sanders, W.T. & Kraushaar, W.L. 1980, Astrophys.J., 1. 1981, Acatrophys.J., 248, 47. -' 242 987. Long, K.S. 1980, report of talk at 156th meeting of AAS. Long, K.S., Helfand, D.J. & Grabelsky, D.A. 1981, Astrophys.J., _248, 925. Long, K.S., d'Odorico, S., Charles, P.A. & Dopita, M.A., Astrophys.J., 246, L61. Markert, T.H., Canizares, C.R., Clark, G.W., Hearn, D.R., Li, F.K., Sprott, G.F. & Winkler, P.F., 1977, Astrophys.J., E, 801. Palumbo, G.G.C., Maccacaro, T., Panagia, N., Vettolad, 6 Samorani, G. 1981, Astrophps.J., 261, 484. Peebles, P.J.E. 1974, Astrophys.J., 32, 197. Rees, M.J. 1980, IAU Symp. No. 92, ed. G.O. Abell & P.J.E. Peebles, Reidel (Dordrecht) . Roener, 8. -- et al. preprint Savage, B.D. & de Boer, K.S. 1979, Astrophys.J., E, LI/. 139 Schwa&, D.A. 1980, Phys.Scripta, - 21, 644. Seward, F.D. & Mitchell, M. 1981, Astrophys.J., 243, 736. Shafer, R. 1981, in preparation. Shapiro, P.R. 6 Field, G.B. 1976, Astrophys.J., - 205, 762. Songaila, A. 1981, Astrophys.J., 248, 945. Sunyaev, R.A., Tinsley, B. B Meier, D. Tanaka, Y. & Bleeker, J.A.M. 1977, S.S.R., 20, 815. Van Speybroeck, L., Epstein, A., Forman, W., Giacconi, R., Jones, C., Liller, W. 1978, Corn. on Astrophys., l, 183. U Smarr, L. 1979, Astrophys.J., 236, L45. Van Speybroeck, :.& Bechtold, J. 1980. Proc. A.A.S. Meeting on X-ray Astronomy, ed. R. Giacconi. 140 - TABLE !. X-RAY EMITTING CONSTITUENTS OF OUR GALAXY Source type Early type binaries L~ > 5 1036 Pop I1 binaries(?) Low luminosity binaries Galactic nucleus Cataclysmic variables 0-stars Main sequence stars Halo M-dwarfs Supernova remnants Galactic (diffuse) background E < 0.25 keV E > 2 keV Number % IO % 20 2 100 5 % 10 7 % S X 10 3 1o1O < loll ? ? % lo3 37 6 x 10 < 35 5 x 10 38 2 x 10 < 3 ? 1 o~~ nX (local) erg pc-3s-1 % 24 < 3 X l O % 2 X l O 28 1 O2(j %or 38 3 x 10 39 1.5 x 10 < 7 5 x 10 35 37 36 38 38 38 < IO 5 x 10 2 x 10 < 3 x IO !O 14 1 TABLE 2. Galaxy LMC SMC M3 1 M33 M I 0 1 MI00 D M 55 kpc 10" MQ 9 65 kpc 1.5.10 Ma 1 1 750 kpc 3.10 Ma 10 750 kpc 4.10 Mo I 1 6 Mpc 10 Me 15 Mpc 1Ol1 Me Binaries Pop I % 8 1 + 2 Pop I1 20 Glob 19 Bulgc 1 1 1 1 j 20 15 47 $ 9 $ 5 $ 2 IW 143 e1p82* 2 6 0 6 2 f X-RAYS PROEI QUASARS AlOD ACTNB GALAXIES* Alan P. Lightman Barvard-Smitbonian Center for Astrophysics I. INTRODUCPION There is evident ly a great deel of a c t i v i t y and commo- t i o n i n deep space, unrevealed by t h e naked eye or the deli- cate twinkling of t he stars. As far as w e can teli, quasars (QSOS) and .active galactic nuclei ' (AGN) have r e l a t iwe ly enormous power outputs produced i n wery small volumes. A typical quasar can produce a hundred t o a thousand timen the I luminosity of a normal galaxy from a region one hundred thousand times smaller i n size. Roughly speaking, i f the c i t y of Boston were a galaxy i n terms of its power output and size, then a quasar would have the power of the e n t i r e United State3 produced i n a region the s i z e of a baseball. A t var ious tines, it has been suggested t h a t new kinds of physical laws OK phenomena mus t be r q u i r e d t o expla in these objects. But more and more evidence has accumulated t h a t shows a continous range of ene rge t i c a c t i v i t y , s t a r t i n g w i t h normal ga lax ies , going through Seyfe r t ga l ax ie s and o ther AGN, and jo in ing wi th the less luminous quasars. Noreover, t h e hypothesized model of gas f a l l i n g through a deep g r a v i t a t i o n a l w e l l , probably caused by a massive black hole, together wi th a n e t angular momentum i n t h e bottom of t h e w e l l , seems capable of explaining, a t least q u a l i t a t i v e - l y , t b e f u l l range of observed phenOmeM, from t h e ene rge t i c s t o the s t t i k i n g , long-lived "jets" of a a t t e r observed t o ema- * Based on a lecture given a t the Goddard Workshop on X-ray Astronomy (October 1981 ). 144 n a t e f r o a t h e c e n t e r s of these objects. I n any case, t h e r e does seem t o be good ev idence f o r some u n i v e r s a l i t y i n t h e mechanisms and p h y s i c a l cond i t ions . Among t h e most impor tan t i s s u e s i n unders tanding QSOs and AGN are (1) t h e n a t u r e of t h e power source r (2) t h e radi- a t i o n p rocesses , and (3) t h e mechanism f o r format ion and col- l i m a t i o n of t h e jets. We w i l l d i s c u s s t h e s e i s s u e s in t u r n , g i v i n g a brief, model-independent sketch of some of t h e im- p o r t a n t t h e o r e t i c a l ideas and o b s e r v a t i o n s ( w i t h more em- p h a s i s on our own i n t e r e s t s ) , and a u g g e s t i r q f u t u r e work. I t is possible t h a t no new o b s e r v a t i m s w i t h i n t h e f o r e s e e a b l e f u t u r e w i l l s a t i s f a c t o r i l y p i n down the above i s sues . W e w i l l be p a r t i c u l a r l y concerned w i t h phenomena t h a t produce X-rays. The clear a s s o c i a t i o n of s t r o n g X-rays w i t h QSOs and AGN and t h e r a p i d time v a r i a b i l i t y seen i n X-rays i n d i c a t e t h a t t h i s region of t h e spectrum may c o n t a i n much in fo rma t ion about t h e c o n d i t i o n s near t h e c e n t r a l r eg ion of t h e o b j e c t s . Of cour se , s imul taneous o b s e r v a t i o n s of a l l re- g i o n s of t h e spectrum may provide impor tan t clues t o t h e me- chanisms a t work. For some recent reviews of t h i s subjec t , see e.g. Rees (1971,1978,19801, Fabian and Rees (19391, and Bradt (1980). 11. NATURE OF THE POWER SOURCE: ACCRETION ONTO A MASSIVE BLACK HOLE A. General C o n s i d e r a t i o n s We w i l l t e n t a t i v e l y adopt g a s a c c r e t i o n on to a massive black hole as t h e " s t anda rd model" f o r t h e power source. The 145 l e n g t h scale is t h e n set by t h e Schwarzschild r a d i u s of t h e black hole, 5 =2Gm/C2, where M is t h e mass of t h e hole. We w i l l use t h e n o t a t i o n f l ~ Z (rho &. Unless t h e hole is rotat- i n g nea r maximum a n g u l a r v e l o c i t y , we can expec t t h e ene rgy p roduc t ion t o peak i n t h e r e g i o n lr- 10% . The l u m i n o s i t y , 8 L, can be w r i t t e n as L= G Itlc2> ( 1 ) where d is t h e e f f i c i e n c y parameter and is t h e mass ac- c r e t i o n rate. The v i r i a l t empera tu re , Tv e set by t h e pro- t o n rest mass energy , is The period of a c i r c u l a r o rb i t , P, also denoted by , i n t h e Newtonian approximat ion , is The l i g h t t r a v e l t i m e across t h i s r e g i o n is The t o t a l mass accreted, mac$ I i n t h e a c t i v e l i f e t i m e r Both t h e combinat ion of i n f e r r e d a c t i v e l ifetimes and ob- se rved l u m i n o s i t i e s , and t h e s i z e s and v e l o c i t y d i s p e r s i o n s o b t a i n e d from opt ica l s t u d i e s , i n d i c a t e an ~ C L C ~ and a n M i n t h e range We d e n o t e t he r a t i o of rad ia l ve l - o c i t y , \/I , t o Kepler v e l o c i t y , l/k , by /06-h>9flo . f3 z V+K . cs4 Tbe rad ia l i n f a l l time, tc. , is t h e n t, = t K / B , Note t h a t is related t o t h e v i s c o s i t y parameter of the 146 ORIGINAL F&GZ 15 OF POOR QUALITY "aL-model" d i s k s (Shakura and Sunyaev 1973) by where h is t h e d i s k t h i c k n e s s a t radius r . In " t h i c k accre- t i o n d i s k s " w e might expec t h a r , so t h a t . The con- where w e have normalized L i n terms of t h e Eddinqton luminos- We w i l l deno te t h e mass of t h e p ro ton and e l e c t r o n by mp and m, r e s p e c t i v e l y , and is t h e Thornson cross s e c t i o n . From e q u a t i o n (6) w e o b t a i n t h e " i o n s c a t t e r i n g depth" The Thomson s c a t t e r i n g depth is wh2re L a n d n+ are t h e e l e c t r o n and p o s i t r o n number dens i - ties. Although 1, might be expected t o approach u n i t y i n s o u r c e s r a d i a t i n g a t near t h e Eddington l i m i t , equation (71, t h e l a r g e o p t i c a l p o l a r i z a t i o n seen i n some QSOs and BL Lac objects ( S t e i n , O ' D e l l , and S t r i t t m a t t e r 1976; Angel 19781, t o g e t h e r w i t h t h e as y e t unobserved evidence of s t r o n g Comp- t o n i z a t i o n , sugges ts ?& 4 I . We t h e n o b t a i n a n upper l i m i t for t h e mass i n t h e c e n t r a l , e m i t t i n g region, M e w Since Men: 44. , most of t h e mass has a l r e a d y co l - l apsed w i t h i n t h e e m i t t i n g reg ion , a rgu ing a g a i n s t some ! 147 i non-black-hole models for t h e cen t r a l power source. Of course, t h e majority of QSOs and AGN do not show strong opt i - c a l polar izat ion. B. Gas flow models The pr inc ipa l requirements fo r e f f i c i e n t conversion of grav i t a t iona l energy i n t o radiat ion, G 2 0.1 , a r e t h a t (a ) t h e viscous d i s s ipa t ion time scale be comparable t o or short- e r than t h e r ad ia l i n f a l l time s c a l e and t h a t (b) t h e cooling time of t h e gas be shor te r than t h e r ad ia l i n f a l l t i m e scale. When angular momentum is present, an accre t ion d i s k forms; most accre t ion d i s k models proposed s a t i s f y t h e above re- quirements. Even when t h e i n f a l l i n g gas does not have much n e t angular momentum, a d i s k may form o u t t o a radius r,..IO fs due t o t h e "dragging of i n e r t i a l frames" by a rapid- l y ro ta t ing black hole (Bardeen and Pet terson 1974). Spherical accret ion models require rapid d i s s ipa t ion , since t h e i n f a l l i n g matter is not delayed by angular momentum. z Por a review of t h e t y p e s of &-disk models, see Eardley et. a l . (1978). These models d i f f e r i n whether t h e y a r e gas or rad ia t ion pressure dominated, o p t i c a l l y t h i c k or t h i n , equal or unequal temperatures for e l ec t rons and ions, and i n t h e source of photons; they have i n common t h e assumption of a s p a t i a l l y constant value for t h e v i scos i ty parameter OG and t h e assumption of a t h i n disk ht der ives from equation (Sa), I f w e assume t h a t 6 is a constant ( a s g i v e n by a nunbet of the @-d i sk models), then equation (Sa) gives t h e logarithmic r-'dr/d$ = p . s p i r a l We mention t h a t simple ana ly t ic so lu t ions may a l s o be obta- 153 ined when 8 is a general power law in radius, rather than a constant. The above forms for 6 are only applicable several Schwarzschild radii away from the black hole: similar gener- al-relativistic expressions could be obtained closer to the hole. Further theoretical work needs to be done on the time variability to be expected by matter inhomogepaiLies neat the black hole. Accretion instabilites (Pringle, Rees, Pacholc- zyk 1973; Lightman and Eardley 1974; Shakura and Sunyaev 1976) may contribute to the oberved variability. None of these possibilites has yet been modeled with sufficient deta- il, in the nonlinear regime, to make quantitative predic- tions. D. Gas Supply A clue to the environment of the power source might be gotten from study of the varicira sources of "fuel", the re- quired conditions for these sources, and the different impli- cations for luminosity evolution. Some of the models that have been proposed for the source of gas are (a) stellar col- lisions in a dense star system (e.g. Spitzer and Saslaw 19661, (b) tidal stripping of stars in a dense stellar system by a central massive black hole (Hills 19751, (c) infall of intergalactic gas (Gunn 19791, (d) galaxy mergers (e.9. Roos 19811, (e) intragalactic gas released by normal stellar evo- lutionaiy processes. One difticulty with models in which gas originates at large radii from the center is the necessity to dissipate a large amount of angular momentum. If the gas is provided by a dense stellar system, and if fl*>l and 154 *S l.3 10 the stellar system must be dominated by physical atellar colli- sions (Shields and Wheeler 1978; McMillan, Lightman, and Cohn 1981). S" , then it can be shown that the dynamics of It seems clear that substantial luminosity evolution oc- curs in QSOs (e.g. Turner 1979); AGN may be the late time phase of the same evolution. In principle, the gas supply mechanism should have something to do with luminosity evolu- tion. The various mechanisms mentioned above, through their different dependences on the external environment of the OS0 or AGN (in (c) and (d)), and on the internal parmeters of the black hole and dense stellar system (in (a) and (b)), predict different functional forms for the evolving gas sup- ply rate m ( t r , and hence for the evolving luminc : t y L(t). A comparison of theoretical models for the gas supply mechan- ism with a large data set of received fluxes and redshifts might be able to rule out or partly confirm some of the mo- dels and clarify the relevant physical conditions. For exam- ' \ ple, models (a) and (b), with evolution of the stellar-system parameters included, yield late-time asynptotic laws of the form (McMillan, Lightman and Cohn 1981) where p=2 for mechanism (a) and p=l for mechanism (b). The test of any theory of luminosity evolution is a sta- tistical problem, requiring a value for the cosmological par- ameter ?,,, some information about the distribution of ini- tial conditions or luminosities (the luminosity function), and some information about the distribution of "turn-on I 155 tilucs" of t h e objects -- in addition to an evolutionary law of the form of equation (14) for a single object. If addi- tional assumptions can be made about these distributions, they may not need to be completely specified a For example, Turner (1979) assumes the luminosity function main- tains its shape in time, which, in our context, would allow determination of a value for p for every assumed value of p. Unfortunately, Turner's assumption has little physical justification and seems to require some external clock that determines the physical conditions of each new quasar as it is born.In any case, a very dense X-ray Eubble diagram, with the fluxes and redshifts from many QSOs, will be necessary to solve the essentially statistical problem of luminosity evolucion. X-rays seem particularly useful here, since QSOs may be readily identified by their X-ray emission, and this emission may constitute the majority of the total luninosity. 1 good determination of the local luminosity function would be quite useful in the above project. Figure 2 below shows a luminosity function calculated for a number of r,earby object s. prior i . 156 ORIGINAL PAGE K3 OF POOR gUAlm o X-ray hmtrssity f m c t i o n of SeMfert 3nLzztes. e Loml, o p t i c a l luminosity funccion 0:- 2Si7'8. Space d e n s i t y of Seyi ' sr t nvclei ' s p t i c a L ! . + Radio t m i u o s i t y f u n c t i o n oJ. &+so cores ( 5 GHr). Rzdio Z:uninosi?y ?tnc*ion of E+S3 3alaricts (1.4 tiHal. . - . . Figure 2: Luminosity func t ion of QSOs and AGN. The dashed l i n e is f i t from model (d) and a d d i t i o n a l assumptions. From Roos (1981). The u n i v e r s a l i t y of t h e forms i n Figure 2 , e s p e c i a l l y above about L = z X ( O I suggests some universa l type of process t h a t es tabl ishes the range of condi t ions f o r QSOs and AGN. y.3 111. RADIATION PROCESSES A. Types of Mechanisms I t should be stated a t t h e beg inn ing t h a t , w h i l e much of t he en i s s ion i n t h e rad io is e a s i l y diagnosed a s synchrotron r ad ia t ion , t h e r e is l i t t l e agreement on how the X-rays and gamma rays a r e produced. I t is almost c e r t a i n t h a t t h e con- d i t i o n s i n t h e X-ray emi t t ing region a r e f a r from simple. A good f r a c t i o n of t h e p a r t i c l e s , eftpecially those t h a t u l t i - mately form t h e observed je t s , may be re la t iv i s t ic : t h e r e a r e acc re t ion i n s t a b i l i t i e s ; shock waves and magnetic f i e l d s 157 abound: t h e emission may be part thermal and part nonther- mal. The r ad ia t ion mechanisms w e d i s c u s s can be divided i n t o two categories: 1. Power-law electron models and 2. Non-power-law e l e c t r o n models, The second category can be fu r the r subdivided i n t o thetmal (random motions of e l e c t r o n s dominate eocission) and nontbermal ( b u l k motions of eletrons dominate emission) processes. As w i l l be shown b e l D w , t h e observed X-ray emission is w e l l f i t by a power law of t h e form I, oc v-s I where I p is proportional t o t h e energy per u n i t frequency 9. 1. Power-law e l e c t r o n models If t h e X-ray emission is synchrotron r a d i a t i o n (requir- ing e l e c t r o n Lorentz factors X m 10 for e q u i p a r t i t i o n mag- netic f i e l d s of e-10 G I or r e l a t i v i s t i c inverse Compton, rc 't then a spectrum of t h e form of equat ion (15) can be achieved by a power l a w d i s t r i b u t i o n of electrons of form dn-/dy oc X" CI 6) where r = 2s+I (1bb) For t h e observed values of t h e power-law index, S d0.7 ,see below, w e require . \ r -Z."t . A number of groups (Axford, Leer and Skadron 1977, B e l l 1978, Blandford and Ost r iker 1978) have inves t iga ted f i r s t r rder Fermi mechanisms, whereby p a r t i c l e s a r e acce lera ted i n t o a power law d i s t r i b u t i o n by t h e converging flows across a shock f ron t . These mechanisms yield values V-2-3, and have been appl ied t o cosmic rays. I t is also q u i t e possible t h a t s u c h processes a c c e l e r a t e 158 e l e c t r o n s i n t h e central teqiona of AGN and QSOs, where s t rong shocks may be present , ani3 ev ident ly produce an elec- t r o n index i n t h e appropr ia te range fo r synchrotron or in- verse Comgton radiation, If t h i s is t h e n a t u r e of much of t h e r a d i a t i o n produced, then h igh X-ray polar iea2ion might be expected. F u t u r e X-ray plarization expe r imen t s may be q u i t e important -- i n c l a r i f y i n g t h i s p o s s i b i l i t y . 2. Non-power law e l e c t r o n models A power law spectrum can also be produced by t h e s c a t t e r i n g of ssit photons t ransmi t ted through a f i n i t e medi- um of hot e l ec t rons , even when these e l e c t r o n do not them- se lves ~ P V C 8. power-law d i s t r i b u t i o n . In t h i s case, a power law r e s u l t s Tzom t h e absence of an enerqy scale over a l a r g e range of energy. When t h e e l ec t ron6 have a thermal, nonrela- t i v i s t i c d i s t r i b u t i o n , t h e n s may be r e l a t e d t o t h e Thomson depth t + h and temperature T (Shapiro, Liqhtman and Eardley 1976, Katz 1976) ( r+h? I) where -fa 2 kT/mcz 0 [ 10) When t h e e l e c t r o n s have a thermal, r e l a t i v i s t i c d i s t r i b u t i o n , t h e n t h e corresponding r e s u l t is (Pozdnyakov, Sobol and Suny- We mention t h a t power laws may a l s o be produced by t h e above process f o r non-thermal electrons, as long as t h e i r d i s t r i b u - t i o n is sharp ly peaked a t some "effective temperature." Taka- hara (19801, Takahara, Tsuruta and Ichimaru (1981) and Maras- c h i , Roasio and Treves (1981) have r e c e n t l y cons idered models f 159 using t h e above r a d i a t i o n mechanism. The s o f t photons for t h i s p rocess could be gene ra t ed by c y c l o t r o n r a d i a t i o n (Eard- l e y and Lightman 1976; Takahara, T s u r u t a and Ichimaru 19811, produced a t perhaps /O'O Hz and absorbed u p t o about t h e 100 th harmonic. Soft photons could a lso be pro- duced by c o l d dense c louds i n p r e s s u r e e q u i l i b r i u m w i t h t h e h o t phase of t h e plasma. A prDblem w i t h t h e Comptonization mechanisms above is t h a t t h e spectral index depends on t h e parameters and 1 , which might be expec ted t o va ry from one source t o ano the r , i n c o n t r a d i c t i o n w i t h t h e narrow range of s a c t u a l l y observed (see D. below). For a s c a t t e r i n g model t o work i n t h i s c o n t e x t , a "characteristic" geometry or s t r u c t u r e is re- quired. For example, i f t h e s o f t photons are e x t e r n a l l y i n - c i d e n t on a r eg ion of hot , thermal e l e c t r o n s w i t h a l a r g e s c a t t e r i n g depth , t hen t h e reflected X-rays have a more univ- ersal spectrum. Lightnan and Rybicki (1979a) have i n v e s t i - gated t h i s p rocess and o b t a i n t h e u n i v e r s a l quasi-power law . where vo is t h e t y p i c a l f requency of t h e s o f t photon input . T h i s power l 3 W ex tends up t o photon e n e r g i e s b - k T and i n t o t h e r e l a t i v i s t i c domain i f t h e e l e c t r o n s have r e l a t i v i s t i c r andom motiutis. I n t h e above s c a t t e r i n g mechanisms, it is t h e gandom mo- t i o n of t h e e l e c t r o n s t h a t dominates t h e emission. A c l o s e l y r e l a t e d mechanism t h a t depends on t h e b u l k e l e c t r o r . motion is I i I 160 t h e s c a t t e r i n g of i n t i a l l y sof t photons by the converging flows a c r o s s a shock f r o n t . T h i s process ha8 been i n v e s t i - ga t ed by Blandford and Payne (1981a, 1981b) and g i v e s a h igh energy asymptot ic result s = ( pi'- c M ~ + G ) (M ~ . I ) ~ ~ CZ A t where M is t h e Mach number of t h e shock ( n o t t o be confused w i t h a mass). T h i s power law ex tends up t o f r e q u e n c i e s such t h a t t h e photon momentum e q u a l s t h a t of t h e e l e c t r o n . Thus, t h e b u l k motions must be r e l a t i v i s t i c t o produce photons of e n e r g i e s b\v>mCL @, . Since characteristic bulk v e l o c i t i e s are on ly of t h e order v ~ 0 . 3 ~ a t F-IOrs , t h i s s c a t t e r i n g process may have d i f f i c u l t y i n producing r e l a t i v i s t i c pho- tons . I n any case, Equat ion (20) g i v e s a v a l u e of s a l i t t l e too s m , i l l and e q u a t i o n (21) , foL a s t r o n g shock, M)'r 1, g i v e s a va lue of s a l i t t l e t o o l a r g e . It is p o s s i b l e t h a t r e f i n e - ments of these models might be more s a t i s f a c t o r y . Some g e n e r a l s t a t e m e n t s may be made. Any thermal pro- cess must r e s u l t i n a characteristic thermal ( exponen t i a l ) t u rnove r i n t h e spectrum, and p o s s i b l y a Wien hump, a t hd kT. A shock mechanism should have a c u t o f f i n t h e spectrum a t e n e r g i e s h u d m c ( h ) 6 a I n almost a l l cases, no t u r n o v e r s of any k i n d have y e t been seen. I t is impor tan t t o extend X-ray o b s e r v a t i o n s u p t o h ighe r energies, i n search of s u c h a tu rnover . I n any case, it is c l e a r t h a t some tu rnove r mus t occur , s i n c e t h e observed v a l u e of s -0 .7 indicates an i n f i n i t e t o t a l energy i f t h i s power law c o n t i n u e s t o arbi- t r a r i l y h igh f r equenc ie s . Changes of t h e spectrum dur ing luminos i ty f l uc tua t ions I I . 1 i may a l s o provide some information about t h e emission mechan- i s m , I n any scattering model i n which s o f t photori are scat- t e r ed up t o high energ ies , one expec ts t h e spectrum t o harden during a f l u c t u a t i o n i f t h e temperature remains constant. Rybicki and Lightman (1979b) ar?d Payne (1980) have i n v e s t i - gated t h e time-dependent ve r s ions of Comptonization by ther - mal e l e c t r o n s wi th in t h i o context. Some a t tempts have been made t o model t h e observed (or absence o f ) s p e c t r a l evolu t ion i n o u t b u r s t s by such processes (e.g. Tennant et . al. 1981). If t h e Compton cool ing t i m e is much shorter than t h e energy input t i m e , it may be t h e s o f t photon source rather than t h e temperature t h a t remains cons tan t during f l u c t u a - t ions . G u i l b e r t , Ross and Fabian (1981) have inves t iga ted t h i s p o s s i b i l i t y and f i n d t h a t t h e spec t rum f i r s t hardens and t h e n sof tens as t h e temperature decres.!aes. Over a range of parameters w i t h ~ " 1 , t h e time-averaged spectrum has a quas i power-law shape, w i t h O < s < l , and a thermal cu tof f ( w i t h no Wien hump) a t t h e i n i t i a l e l e c t r o n temperature, It is poss ib le t h a t i n many cases, s teady s ta te s p e c t r a a r e n o t re- levant and we should always be consider ing t i m e averages of nonsteady processes. Better spectral resolution during luminosity f l u c t u a t i o n s w i l l be he lpfu l here. There a r e var ious pocesses t h a t may be involved i n pro- ducing t h e e l e c t r o n d i s t r i b u t i o n , i f it is n o t thermal. I n add i t ion t o t h e shock a c c e l e r a t i o n already mentioned, l a r g e s c a l e magnetic and e lectr ic f i e l d s may be present (Blandford 1976, Lovelace 1976, Blandford and Znajek 1977). Caval iere and Morrison (1980) have pointed out t h a t t h e e l e c t r o n s m u s t be reaccelerated in crossing the emission region. Defining a parameter q by q=( probability ) x(fractional energy loss of scattering per scatter ing = ( q f i g r ) x ( YI'h*/llrnc4 - Here ng is the photon density so that In any source where q esceeds unity, reacceleration is neces- sary, Note that q is about equal to the Kepler time divided by the Compton cooling time, B. Thermalization Time Scales A principal uncertainty is whether any of the observed X-ray emission from QSOs and AGN is thermal. Such emission requires that the electrons be able to thermalize, via two-body collisions or perhaps collective plasma motions, on a time scale shorter than energy is deposited or radiated and shorter than pair creation when the latter is important. We will consider two-body thermalization below, keeping in mind that collective effects may always be faster. First we consider the ion thermalization time. The bulk of the released gravitational energy is probably intially de- posited into the ions. Taking a value of 25 for the Coulomb logarithm, the ion thermalization time, ti; , via ion-ion Substituting equation (6 ) into equation (25a1, and taking T= ORIGINAL PAGE IS OF POOR QUalW IO'* K, we obtain 163 and using equations ( 3 ) and (Sb), we obtain ( Z S C ) Here an6 elsewhere in this section, all quantities will be evaluated at Y=lOV, for simplicity, unless otherwise stated. Since energy is deposited into the ions on a timescale kr , the last ratio must be less than unity for ion-ion collisions to thermalize the ions. Ions may ~ l s o thermalize by scatter- ing with the electrons. If we denote this timescale by where 7; and %denote the ion and electron temperature, res- pectively. (If the particles are not thermal, then tempera- tures should be replaced by mean energies.) For * Id2K and T'= pc\cz= 6x10 K, equation (26) gives ki;/t;e .v 50 $-tO+)/N, Thus, a relatively low electron mean energy and the presence of many electron positron pairs may allow the i o n s to ther- malize. 9 Next we consider electron thermalization via electron-electron scattering. For relativistic electrons, T.71 , this timescale, tQe I is I 2 The electron cooling time, troel , may be evsluated in a model-independent way: 164 We t h e n obta in t h e r a t io which must be less than un i ty f o r t he electrons t o thermal- ize. Equation (29) sugges ts t h a t i n some cases w e might ex- pect thermal e l e c t r o n s and i n some cases not. As long as t h e e l e c t r o n s are only marginally r e l a t i v i s t i c , t h e e l e c t r o n s should be able t o thermalize u n l e s s t h e luminosity is very much below t h e Eddington l i m i t and there are few e l e c t r o n po- s i t r o n pa i r s . It seems l i k e l y , however, t h a t a f r a c t i o n of t h e e l e c t r o n s w i l l be accelerated u p t o very r e l a t i v i s t i c en- e r g i e s by some of t h e processes mentioned above. I n t h i s case w e would not expect t h e e l e c t r o n s t o he able t o s u s t a i n a thermal d i s t r i b u t i o n . I n any event, as is discussed below, t h e copious production of e%- p a i r s prevents t h e temperature of an o p t i c a l l y t h i n thermal medium from becoming very rela- t i v i s t i c . The t i m e scale for ions, t e i , sa t i s f ies Since e l e c t r o n s can cool they receive energy from e l e c t r o n s t o rece ive energy from n +n-) bJ r e l a t i v e l y e f f i c i e n t l y , fas ter t h a n t h e ions , w e may expect t h e e l e c t r o n mean energy t o be much lower than t h a t of t h e ions, a s sug- gested by Shapiro, Lightman and Eardley (1976). The conclusion is tha t t h e r e may well be two populations of e lec t rons : a marginally r e l a t i v i s t i c , thermal population and a highly r e l a t i v i s t i c , nonthermal popu 1 a t ion . Unfortunately, it is d i f f i c u l t t o calculate t h e r e l a t i v e pro- po r t ions of these two popt. lations a t t h e present time. ORLGfi~erc hi';€ IS OF POOR QUALITY 165 ORIGINAL PAGE 13 OF POOR QUALITY C. Pair Ef fec ts With t h e h igh v i r i a l temperatures avai lable , cf . equation (21, much exceeding t h e e lec t ron rest mass, it seems l i k e l y t h a t some s izeable f r a c t i o n of t h e e lec t rons w i l l be a t l e a s t marginally r e l a t i v i s t i c , and most detailed models i n the l i t e r a t u r e (e.g. Shapiro, Lightman and Eardley 1976; Maraschi, Roasio and Treves i981; Takahara, Tsuruta and Ichimaru 1981) do indeed have t h i s character. En such a si- tuat ion, the e f f e c t s of electron-positron pa i r production m u s t be incrJded. When p a i r s a r e present, t he d i s t r i b u t i o n s of rad ia t ion and p a i r s m u s t be solved fo r self consis tent ly , since photons produce pa i rs , p a i r s produce photons; and a l l t h e c ross sec t ions a re , i n general , energy dependent. Some recent work i n these processes has been done for steady plas- mas by Stoeger (19771, Liang (19791, Lightman and Band (19811, Lightman (1981;, and Svensson (19811, and i n t h e con- t e x t of i! time-dependent cooling " f i r e b a l l " by Cavallo and Rees (19781. As BI! example, we consider pro5uction of pai rs by t h e react i o n e' ? e + e + e , which may dominate when i: l a rge photon d e n s i t y is present and the e lec t rons a r e energetic. T h i s reaction gives a positron production r a t e ii+ e-qa c rT af ng (n, +n-) ( 3 4 (where d~ is t h e f i n e s t ruc tu re only logarithmi- c a l l y ser,ait ive t o t h e p a r t i c l e and photon d i s t r ibu t ions , as long as t h e threshold conai t ion 166 is s t r o n g l y s a t i s f i e d . The pair product ion time scale f o r where we have used equa t ions (231, (5b) and (31, and t h e L refers t o t h e luminos i ty i n those p h o t m s above threshold. I n sou rces t h a t are no t t o o f a r below t h e Eddington l i m i t , w e t h e r e f o r e expect t h i s process t o have ample time t o produce pairs. Since pairs produce more pairs, there may be a non- s t eady behavior i n which, f o r a s h o r t per iod of t i m e (and w i t h A x v a r y i n g r a p i d l y perhaps) , an overabundance of pairs is produced. It is a l s o of i n t e r e s t t o compare t h e time scala, , f o r t h e pair a n n i h i l a t i o n p rocess t a , e++e-* 2 Y t o t h e cool ing t i m e . ( I n a s t eady state t h e c r e a t i o n t i m e would equal t h e a n n i h i l a t i o n time.) If pairs coo l be fo re they a n n i h i l a t e , t hen w e m i s h t expec t t o see a f e a t u r e i n t h e spectrum a t t h e e l e c t r o n rest mass energy, as has been seen ( r edsh i f t ed by 20%) i n t h e March 1979 gamma ray bu r s t event. For QSOs a t r e d s h i f t z, t h i s f e a t x e would be a t the energy 511 k e v / ( l + z ) . For e l e c t r o n s of Lorentz f a c t o r 1 , I n addi 'on t o comparing equa t ion (32) t o equat ion (28) we can assume a specific coo l ing mechanism. For example, t h e time scale f o r synchro t ron cool ing , , is = 6 7 t m ~ / C i f i 3 ' ~ ~ ) . ( 3 3 ) Then. using t h e expres s ion f o r t h e g a s and magnetic pres- 167 s u r e s , p6 , and ?B , r e s p e c t i v e l y , we can o b t a i n t h e r a t i o C 3 d For e q u i p a r t i t i o n magnetic f i e l d s , p~ a PG , pairs w i l l ev- i d e n t l y cool b e f o r e t h e y a n n i h i l a t e as long as y z z . The absence of an observed a n n i h i l a t i o n l i n e may i n d i c a t e t h e magnetic f i e l d s are below the i r e q u i p a r t i o n value. When re- l a t i v i s t i c p a i r s a n n i h i l a t e before s u b s t a n t i a l coo l ing , t h e photons produced w i l l a t most y i e l d a broad f e a t u r e a t h 2 - p L'wCf , and even t h i s may be dominated by Comptonized bremsstrahlung r a d i a t i o n (Lightman and Band 1981) . Recent work (Lightman 19813 Svensson 1981) has shown t h a t t h e r e is a maximum temperature t h a t can be achieved by a thermal , o p t i c a l l y t h i n plasma i n equi l ibr ium, dependent on t h e va lue of t h e i o n s c a t t e r i n g depth, e q u a t i o n ( 7 ) . A maxi- mum tempera ture comes about because t h e p a i r a n n i h i l a t i o n r a t e decreases w i t h i n c r e a s i n g tempera ture and t h e p a i r pro- d u c t i o n r a t e i n c r e a s e s as t h e photon d e n s i t y i n c r e a s e s (which i n c r e a s e s wi th & 1, T h i s maximum (or, e q u i v a l e n t l y , t h e max- i m u m r~ p o s s i b l e f o r each va lue of TI is shown i n Figure 3. Furthermore, t h e r e is g e n e r a l l y an enormous range i n luminos- i t y over which t h e tempera ture remains i n t h e narrow range 0.1 ST* < I , w i t h copious pa i r product ion a t t h e h igh end and t h r e s h o l d effects a t t h e low end s e r v i n g as a the rmos ta t . 168 lo-' lo-2 - - - 1 I I IO 20 30 40 1~i~L 0 kT/mc2 Figure 3: Maximum ion sca t t e r ing depth, Thomson depth , and photon t o e lec t ron r a t i o t h a t can be achieved a t zach temperature i n an op t i ca l ly t h i n , steady thermal plasma. From Lightman (1981). Ear l ie r work by Bisnovatyi-Kogan, Zeldovich, and Sunyaev (19711, which d i d not i n c l u d e f i n i t e s i z e e f f e c t s and ne- glected photon processes, obtained t h e maximum temperature kT=41mc2 The magnitude of t h i s maximum is s e t by t h e rec i - procal of the f i n e s t r u c t u r e constant. I n t h e ca l cu la t ions r e s u l t i n g i n Figure 3, only in t e rna l sources of photons were included (bremsstrahlung. double Compton, and ann ih i l a t ion ) . Any addi t ional source of photons, e.g. a Soft photon source, m u s t lower the maximum T a t each value of c,q, since photons produce bu t d?r n o t d e s t r o y p a i r s i n an o p t i c a l l y t h i n plasma. Thus, t h e maxima shown i n F i g u r e 3 are upper l i m i t s . Observed s p e c t r a w i t h power laws extending above h$ 7 LTmnrc i n d i c a t e t h a t ei ther t h e plasma is nonthermal or nonsteady. Another c o n s t r a i n t on t h e mean energy and number d e n s i t y of t h e pairs a r i ses from r e q u i r i n g t h a t t h e t o t a l system of i o n s and pairs m u s t be g r a v i t a t i o n a l l y conf ined . T h i s leads t o t h e i n e q u a l i t y c 3 7 ) When equa t ion (37) is v i o l a t e d , magnetic confinement requires magnetic f i e l d s above e q u i p a r t i t i o n s t rength. Equat ion (37) m u s t be s a t i s f i cd on average, b u t could be v i o l a t e d for s h o r t times or over small reg ions . We f i n a l l y mention t h a t photon-photon p a i r p roduc t ion - &-Y --3 e++: may l i m i t t h e p resence of nard gamma rays . I f hi's is t h e energy of a s o f t photon and hvh is t h e energy of a hard pho- ton , t hen t h e t h r e s h o l d c o n d i t i o n f o r p a i r p roduc t ion is h-& h 3 h c,) c z) 3 The s c a t t e r i n g dep th t o t h e above r e a c t i o n is where f i s is t h e number d e n s i t y of " f i e l d " photons i n a bandwidth 2 7 ~ " around ?! . Some c o n s t r a i n t s may be p laced on source8 f o r which s t r o n g gamma emiss ion is seen , by r e q u i r i n g t h a t t h e number d e n s i t y f15 be s u f f i c i e n t l y small t o ach ieve rrr41 I ( l i e r t e r i c h 1974) . For example, Fabian and Rees (19793 have shown t h a t t h e observed gamma and X-ray emission 170 from 3C 273 ( s e e Figure 4 ) requite t h a t t h e gamma rays be em- i t t e d i n a region i" 710 cm , much l a r g e r than t h e X-ray emit- t i n g region. We mention, however, t h a t t h e above c o n s t r a i n t may be v io l a t ed i f t he produced p a i r s themselves produce more gamma rays t h a t a r e not s u b s t a n t i a l l y downTraded i n energy. Since t h e var ious r e a c t i o n s t h a t would r ed i s t r ibu te t h e spec- t rum, e.g. Compton s c a t t e r i n g , a r e energy dependent, it is n o t c l e a r how large rrt can be before t h e gamma rays would c e r t a i n l y be cut off. T h i s is an area i n which more research is needed. D. Observed Spectra 80 The composite spectrum of t h ? QSO 3C 273 is shown i n F i g u r e 4. + Ariel V 1/76 + COS B 6/76 ++ + HEAO-A4 6/78 + 3C273 Figure 4: Spectrum of 3C 273, From Bradt (1980) The portion of t h i s spectrum frola 13 to 120 kev is w e l l f i t by a p u c r law with s=0.67, This qec trum is a c t u a l l y f a i r l y charac ter i s t i c of m a ~ y AGN and QSOs. In Figure 5, w e see a histogram of spectral m d i c i e s t h a t describe power laws seen i n Seyfer t ga lax ie s . (The index d, i n ?igure 5 is the same as the s given i;. z+ation (15) . I ! 172 I I I I I 1. I LUMlNOSiTY : L SPECTRAL INDEX: Q Z r-i SEYFERT GALAXIES (HEAO-I A 2 1 43ROAD LINES - I : XXNARROW LINE< ' 0 I I I . I I I I I I I I X - X I I I I I . I . 1 . I - I . I I I I I I * I - . - i e l I . I I I * I , , I I - 45 44 -J W 0 -J 43 42 Figure 5: Bistoqrara of spec t r a l ind ic ies observed for Seyfer t galaxies. CG is t h e energy index. From Mushotzky (1981) The da ta , over an observed range 1-100 kev, a r e well f i t v i t h an s a 0.69, and a srsll s c a t t e r about t h i s mean C r - 0 ~ 6 . Th.s r e s u l t has been reported by Mushotzky (1981) and by Mac- cacaro, Perola and Elvis (1981). The universa l i ty of these spectis is s t r i k i n g and suggests a universal type of emission mechanism, a s discussed above. Emission above 1 Mev has a l s o been seen i n a L~,, other ob jec ts , e.q. NGC 4151 (Schonfelder 1978). I t is c ruc ia l t o obtain spectra of as many QSOs 9 s pssible , t o confirm t h e s i m i l a r i t y of t h e i r spectra t c those - 173 of AGN. For t' 's purpose l a rge a rea de t ec to r s l i k e LAHAR should be u s e f u l . From a l l t h e ---. above considerations, t he re is a clear need fo r observations a t higher energies, going i n t o t h e gamma rays. There may be a wonderland of i n t e r e s t i n g physics wait- i n g t o be learned a t h7JZmCz . The proposed s a t e l l i t e GRO should have such a capab i l i t y . To summarize the needs fo r s u c h observations: (1) With a spec t r a l index of ~ ~ 0 . 7 , most of t h e t o t a l l u - minosity has y e t t o be observed! Look back a t a l l t h e go-or-no-go formulae t h a t contain an L. Ontil t h e observa- t i o n s a r e pushed t o s u f f i c i e n t l y h i g h energies t o see a t u r - nover i n t h e spectrum, w e only have lower l i m i t s on t h a t L. (2 ) When a break, or turnover, i n t h e spectrum is seen a t h igh energies, w e w i l l have a much b e t t e r idea of whether t h e emission is thermal or nonthermal . ( 3 ) As sugges ted by t h e discussion i n C. above, a number of in te res t ing effects associated w i t h electron-positron p a i r s may be seen a t energies L\V2Vscz. These f ea tu re s and the physical effects they represent may be completely hidden a t lower energies. Cbservations of t h e spec t ra a t low energies, especial ly near t h e s o f t X-ray absorption cu tof f , h v $ l kev, a r e a l s o important. Lawrencz and Elvis (1981) have recently found good evidence for a cor re l a t ion between t o t a l X-ray luminosi- t y and t h e r a t i o of hard ( hv= 2-10 kev) t o s o f t ( k J p O . 5 - 4 . 5 kev) components. They i n t e r p r e t t h i s r e s u l t t o ind ica te t h a t the covering factor of the broad l i n e emission i 174 region over the continuum X-ray source is a monotonically de- creasing function of X-ray luminosity. Two possible explana- tions are that brighter sources, either through radiation pressure or hydrodynamic effects, are able to "blow awayn the surrounding absorbing gas, or that the accumulation of ab- sorbing gas is associated with ageing (e.g. through stellar evolutionary processes) and older objects are in the declin- ing phase of their activity (see luminosity evolution in I1 D. above!. I V . BEAMS AND JETS It is impossible to consider the subject of QSOs and AGN without mentioning the dramatic, well collimated streams of matter seen emanating from the centers of these objects. These "jets' have been detected in a variety of different ob- jects and in several different wavelengths. Recently, they have been seen in the X-ray band also (e.g. Schreier et. al. 1379, F'eigelson et. al. 1981). The opening angle of a jet is typically 8 % /oO, the velocity of the matter 3 0 0 ~ ~ s ~ ' < V t C , and the jet liEetime 2 10 V . See Rees (1980) and Begelmar., Blandford and Rees (1981) for recent re- veiws of the physics of jets. 6 Since jets are ubiquitous in QSOs and AGN, whatever pro- duces a jet should be a commonly occuring phenomenon, not re- quiring special conditions. Furthermore, the jet nrernembersn its directionality for a million years or more, so the funda- mental beaming mechanim must be associated with a longlived and stcble axis of symmetry. Most researchers feel, that net I 175 angu la r momentum is a key i n g r e d i e n t i n d e f i n i n g t h e r e q u i r e d axis, b u t t h e detailed mechanism for a c c e l e r a t i n g matter a long t h i s a x i s is n o t y e t understood. One of t h e ear l ies t s u g g e s t i o n s was t h e 'twin-exhaust" hydrodynamic model of Blandford and Rees (19741, in which a heat source a t t h e center of a r o t a t i n g gas cloud, i n an e x t e r n a l g r a v i t a t i n g p o t e n t i a l , creates a low d e n s i t y channel of gas t h a t burrows a t u n n e l a long t h e path of least resis- t ance , t h e r o t a t i o n axib. The f i r s t detailed hydrodynamical c a l c u l a t i o n s t o test t h e v a l i d i t y of t h i s sugges t ion have re- c e n t l y been completed by Norman et. al. (19811, and some of their results are shown i n F igu re 6. Figure 6a: Dertsity c o n t o u r s of Figure 6b: The v e l o c i t y t h e c loud j e t s t r u c t u r e a t t= vec to r f i e l d for t h e j e t 1.6. The s o l i d l i n e is t h e j e t i n 6a. Note t h e c e n t z a l wall. The r o t a t i o i l a x i s i s ver- shock. From Norman e t . t i ca l . U n i t s g lven i n t h e text. 81. (1981). From Norman et . a l . (1961). 176 The u n i t of l e n g t h here is G M k , where M is t h e mass of t h e c o n f i n i n g c l o u d and e is t h e i n t e r n a l ene rgy per u n i t mass of t h e hot, l i g h t g a s i n t r o d u c e d a t the c e n t e r . The u n i t of t i m e is GN/'? . The c e n t r a l c a v i t y is a t a r a d i u s R=O.lS. The gas accelerates smoothly t o s u p e r s o n i c v e l o c i t y , termi- n a t i n g i n a shock f r o n t a t R=O.l , a t which p o i n t c o l l i m a t i o n begins . These r e s u l t s i n d i c a t e a c a v i t y - n o z z l e s t r u c t u r e more compact t h a n t h a t s u g g e s t e d by Blandfo rd and Rees (1974) . The c a l c u l a t i o n s of Norman et. al. (1981) assume A f l a t bottomed g r a v i t a t i o n a l p o t e n t i a l , rather t h a n a p o i n t source p o t e n t i a l t h a t would be g e n e r a t e d by a ceqtral black hole . F u t u r e such c a l c u l a t i o n s , p e r h a p s mor2 applicable t o t h e AGN and QSO c o n t e x t , d i l l y i e l d one of t h e f i r s t reliable estimates for t h e amount of c o l l i m t t i o n t h a t may be a c h i e v e d by t h i s mechanism. hod-.er idea, s u g g e s t e d by Lynden-Bell (19781, is t h a t r a d i a t i o n p r e s s u r e may accelerate matter a l o n g t h e i n t e r i o r f u n n e l formed by a t h i c k a c c r e t i o n d i s k (e.g. J a r o s z y n s k i , Abramowicz and Paczynski 1980) . Some r e c e n t k i n e m a t i c a l s t u - Cies by Abramowicz and P i r a n (1980) s u g g e s t t h a t t h e d i s k f u n n e l must be ex t r eme ly narrow f o r t h i s method t o y i e l d ade- q u a t e collimation, w i t h a r a t i o of o u t e r t o i n n e r d i s k r a d i i of a t least 10' . Po c a l c u l a t i o n s have y e t been done t o self c o n s i s t e n t l y de t e rmine t h e p a r a m e t e r s of t h e f u n n e l , or t o t e s t whether such a narrow f u n n e l is stable in t h e pres- e n c e of r a d i a t i o n and maLter. I t is q u i t e p o s s i b l e t h a t none of t h e above mechanisms are able t o a c h i e v e t h e observed c o l l i m a t i o n , and t h a t much of t h e co l l imat ion is a c t u a l l y achieved f a r from t h e black hole, perhaps by l a r g e scale magnetic fielas. Blandford and Payne ( 1 9 8 1 ~ ) have r ecen t ly inves t iga ted such a model, i n which angular momentum is remoyed from an accretion d i s k by magnetic f i e l d s t h a t extend t o l a r g e radii. An outflow of matter along t h e r o t a t i o n a x i s is then PciSSible. A t l a r g e d i s t ances from the d i s k , the toroidal component of t h e mag- n e t i c f i e l d may co l l ima te t h e matter. V. CONCLUSIONS We conclude by recapping some of t h e important f u t u r e ooservat ions t h a t may c l a r i f y t h e physics of QSOs and AGN. 1. Temporal Resolu t ion . (a) The var ious theoretical c o n s t r a i n t s OF L, A L, a n d O t , when pushed t o the i r l i m i t s , may provide our best handles on t h e efficiencies and emission-region s i zes . (b) Spectral changes during f l u c t u a - t i o n s may he lp spec i fy t h e r a d i a t i o n mechanism. (c) Siqna- tures of s p i r a l l i n g matter or a c c r e t i o n i n s t a b i l i t i e s may be revealed by h igh time resol tuion. 2. Large Sample of QSO Spectra, I n t e n s i t i e s , and Red- s h i f t s . (a) I t is important t o l e a r n whether a s i n g l e univ- e r s a l spec t rum desc r ibes OSOS a s well as AGN. (b) Wi th a l a rge X-ray Hubble diagram fo r QSOs, w e can begin t o tes t lu- minosity evolu t ion aga ins t t h e o r e t i c a l models of t h e gas sup- p ly mechanism. 3. X-ray Polar iza t ion . Synchrotron r a d i a t i o n t y 2 i c a l l y produces large polar iza t ion . Most o ther mechanisms produce a small po lar iza t ion . 178 4. High Energy Measurements hV- \-lo fiv . (a) With an observed spectral index s-0.7 tor AGN, we now have only a lower bound on t h e t o t a l luminosity of most of t hese objects. (b) The shape of t h e turnover i n t h e spectrum, a t high ener- g i e s , should t e l l u s about t h e emission mechanism, ( c ) A good deal of physics assoc ia ted w i t h p a i r production (maximum temperatures of thermal, s teady plasmas; a n n i h i l a t i o n fea- tures) can only be observed a t energ ies .5 MeV. To end wi th a note of caution: Most of t h e ideas have Seen presented as i f there were one universa l framework f o r a l l QCOs and AGN, one basic scena r io of gas a c c r e t i o n onto a massive black hole, one dominant r a d i a t i o n process, one me- chanism f o r producing and col l imat ing t h e jets. I think sometimes, e spec ia l ly i n astronomy, we tend t o use Occam's razor t o o much. W e should be r ecsp t ive t c t h e p o s s i b i l i t y t h a t a number of d i f f e r e n t proczsses and mechanism may be operat ing i n d i f f e r e n t objects, or perhaps going on s imul- taneously i n t h e same object -- even though t h e net: r e s d t s of l u m i n o s i t i e s , spec t r a , and v a r i a b i l i t i e s can be squeezed i n t o a common framework. Ac know1 edgments Ye thank 0. Eardley, M. E lv is , P . Gorenstein, 3. Grindlay, 0. Payne, G. dybicki, and H. Tananbaum a t the Harvard-Smi thsonian Center for Astrophysics for useful comnents. This work was sup- ported i n part by NASA g;.ant NAGW 246. I 179 REFERENCES Abramowicz, M.A. and P i r an , T. 1980, Ap. J. L e t t . , 241, 17. Angel, R. 1978, i n Proc. P i t t s b u r g Conference on BL Lac Objec t s , ed. A.M. Wolfe Axford, W.I . , Leer, E., and Skadron, G. 1977, Proc. 1 5 t h I n t . 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Culhane Mullard Space Scier.ce Laboratory Department of Physics and Astronomy Unive r s i ty Col lege London ABSTMCT A n X-ray Spectroscopy, Timing and V a r i a b i l i t y mission s t u d i e d by the Eurpoean Space b e n c y Cs descr ibed. Five in s t rumen t s (Brags Spectrometer, Large Area Propor t iona l and Sc?nt:l lation Counter De tec to r s , Wide F i e l d X-ray Cameras and a Gamma-Ray Burst Monitor) are d i scussed and estimates of t h e i r performance are given. The i r s c i e n t i f i c aims are summarised and sample observing programmes are discussed. 185 11. THE PAYLOAD, THE SPACECRAFT AND TtiE HISSION.1 A possible payload €or the X-80 misslon is Indicated in t a b l e I. Table I. S c i e n t i f i c Payload f o r the X-80 Mission I 1-- 1 I Ins t rme n t 1 Photon Energy Range I I 1 (keV) I Aims I I I I I 71- I I I I I I I I I I I I I I I I I I I 1 I I I I I I I Large area pro- porttonal counter (LAPC) , ( s e n s i t Lve area 2500 cn2) I Wide f i e l d (WFC) cameras (4 un i t s , FDV ,. 1.4 s te r ) crystal spectro- I meter I (projected area 6 panels t yp ica l ly l io00 c m 2 each) I IPhoswLch de tec tor system ( sens i t i ve larea 650 cm2) I 1 Gamma-ray burs t ldetector I I I I I I I I I I I I I I I I I I I I I I I I I I 1.2 - 20 2.0 - 20 (imaging) 2.0 - 50 ( t iming) 4 i n t e r v a l s In t he 0.5 - 10 keV energy range ( 0 , st , s, Fe) 15 - 200 40 - 130 Source variability (< 1 ms I to days). Moderate resolu- I t i o n spectroscopy. b n i t o r I for c:ystal spectrometers I I Source v a r i a b i l i t y (seconds I t o years). A l l sky monitor I (days t o years). Loca l i sa t ion and study of t r a n s i e n t s , X-ray I and weak gama-ray bur s t s I I Spectra 1 var i a b i l l t y . I sources. I 1 I Source va r i ab€ l€ ty and 1 scopy. Cyclotron l i n e I spectroscopy . I 1 Local isat ion of intense gamma-l ray burs t s and hard X-ray I t r a n s i e n t s I High reso lu t ion spectroscopy. I Spect ra l mapping of extended I moderate resolutLon spectro- I 1. Further detai ls may be found i n the ESA assessment s tudy document No. SCI(8O)S Paris, June 1980. Science team members were R. Berthelsdorf , A. Brinkman, N. Lund, C. Reppin, R. Rocchia, A. Scheepmaker, H. Schnopper, G. Spada, R. Staubert, M. Turner. B. G. Taylor. ESA s t a f f involved were R. Laine, H. O l t h o f , 186 While t h e payload h a s t h e a b i l i t y to s t u d y s o u r c e v a r i a b i l i t y o v e r a wide ene rgy range by mesns o f the large area p r o p o r t i o n a l c o u n t e r s (WAC) and t h e phoswich, i t can a lso moni tor i n s t a n t a n e o u s l y a f i e l d o f 1.4 s t e r a d i a n w i t h t h e a id o f t h e wide f i e l d cdmerds (WFC). The f r e e s t a n d i n g Bragg c r y s t a l spectrometers allow h i g h r e s o l u t i o n s t u d i e s o f s o u r c e s p e c t r a o v e r s e l e c t e d wavelength ranges. compliment t h e Bragg ins t rumen t by c o v e r i n g a larger photon ene rgy range , a lbei t a t lower r e s o l u t i o n , and by p e r m i t t i n g t h e s l o p e and i n t e n s i t y o f t h e c o n t i n u o u s spectrum to be determined. I n a d d i t i o n t h e two broad band sys tems (LAPC and Phoswich) While m i s s i o n s l i k e t h e E i n s t e i n o b s e r v a t o r y and t h e proposed Advanced X-ray Astronomy F a c i l i t y (AXAF) are g e n e r a l purpose ana logues o f t h e Palomar 200" o p t i c a l t e l e s c o p e , i t is clear t h a t t h e huge cost o f such sys tems means t h a t t h e i r number w i l l i n e v i t a b l y be r e s t r i c t e d . llowever some enormously e x c i t i n g p h y s i c s c a n be under taken by t h e s t u d y of t h e 300-400 b r i g h t e s t s o u r c e s w i t h a m i s s i o n of t h e k ind d e s c r i b e d here . This p o s s l b i l i t i y h a s been recognized by s e v c r a l d i f f e r e n t n a t i o n s and s p a c e a g e n c i e s . Thus, i n a time o € €isca l s t r i n g e n c y , i t would seem a p p r o p r i a t e t h a t t h e p o s s i b i l i t i e s of j o i n t a c t i o n i n t h i s area be examined c a r e f u l l y . Fol lowing t h e Uhuru, Ariel V and HEAO-1 su rvey m i s s i o n s and t h e o u t s t a n d i n g r e s u l t s t h a t have been o b t a i n e d w i t h t h e E i n s t e i n o b s r v a t o r y , i t has become a p p a r e n t t h a t o b s e r v a t i o n s a t X-ray wavelengths are o f fundamental importance i n astronomy. by t h e i n t e r a c t i o n s of e n e r g e t i c charged p a r t i c l e s w i t h magnet ic f i e l d s or lower ene rgy photons. In o r d e r to unders tand t h e p h y s i c s involved , i t is necessa ry t o proceed w i t h t h e d e t a i l e d s t u d y of t he b r i g h t e r X-ray sources . This work should i n c l u d e a complete i n v e s t i g a t i o n o f s o u r c e v a r i a b i l i t y o v e r a wide epectral range t o g e t h e r w i t h h igh r e s o l u t i o n s p e c t r o s c o p i c o b s e r v a t i o n s o f X-rays are g e n e r a t e d i n h igh t empera tu re (T > 106 K) plasmas or 187 the high temperature plasaras t h a t give rise t o X-ray emission i n many galactic and ex t ra -ga lac t ic sources. Furthermore, i t is the combination of spectroscopic and photometric observat ions i n a single mission that can lead t o major advances i n our understanding of the physics of these sources. dlscusslon of the individual instruments later a f t e r a descr ip t ion of the spacecraf t and s o m e preliminary ideas on mission operat ions have been presented. We w i l l r e turn to a The p r inc ipa l parameters of the mission are given i n t a b l e 11. The space- craft is compatible with ha l f of the Ariane launcher capacity. costs can be reduced i f a s u i t a b l e par tner can be found. hi, the mission l i f e t ime w i l l be a t least 3.5 years. f o r an "end of l i f e " c a p a b i l i t y of 385 watts. Thus launching A t a height of 600 The solar a r r a y is designed Since the most demanding angular Table I1 Details of the X-80 Mission ORBIT: LAUNCHER : - Circular , 600 km, 0" i n c l i n a t i o n ESA Ariane nAss : - DIMENSIONS: AlTITUDE CONTROL: Instruments 500 Kg Spacecraft 400 Kg Height - 3.2 Dim. - 2.3 m max. 3 Axis i n e r t i a wheel Target acqu i s i t i on to f 5' A t t i tude determinat ion I) S ta r sensors t o f 0.5' ii) Coarse + f i n e sun sensors iii) Gyros Orientat ion - anyuhe:e within 2 30" of plane perpindlcular t o s o l a r vector. POWER REQUIREMENT: ,. 200 watts t o t a l DATA SYSTEM: Mean rate - 44 K b i t s / s Peak rate - 250 K b i t s / s Downlink rate - 700 K b i t s / s Mass WUIOCY - 250 M b i t Data dump - 1 per o r b i t a t Kourou Ground l i n k - 8 K b i t s / s to ESOC, Darmstadt r e s o l u t i o n requirement is about 5 ' a6 set by t h e Brau spectroiacter and t h e WFC, the a t t i t u d e c o n t r o l and measurement system parameters i n d i c a t e d i n the t a b l e are more than adequate. I n e r t i a wheel a n g u l a r momentum is inloaded by a c o l d Nitrogen gas system. rates of 250 K b i t s l s to be s u s t a i n e d from i n d i v i d u a l i n s t rumen t s for p e r i o d s of up to 20 minutes. Use of a large bubble memory can allow peak d a t a The proposed 600 km e q u a t o r i a l o r b i t w i l l provide a n environament of l o w and s t a b l e particle background. r a d i a t i o n belts or t h e South A t l a n t i c magnetic anomaly. Since t h e s p a c e c r a f t The spacecraft w i l l noc pass through t h e i s i n nea r -ea r th o r b i t , s i g n i f i c a n t regions of t h e sky are obscured from view a t any given time. However t h e l a r g e slew c a p a b i l i t y w i l l allow t h e po ln t ing a x i s t o be moved r a p i d l y between sources. Ground s t a t i o n s w i t h i n 10" of the equa to r can be used. While both Malindi i n Kenya and Kourou i n French Guyana are a v a i l a b l e , t h e mission has been planned on t h e b a s i s of 6 minute c o n t a c t s with Kourou o c c u r r i n g once per o r b i t . Although the solar array is a f i x e d s t r u c t u r e , t he sun-pointing f a c e c a n be t i l t e d by up to f 30' o f f t he sun-lfne, and s t i l l provide adequate power. With a 3-axis s t a b i l i s e d s p a c e c r a f t , which is free t o r o l l about t h e solar vec- tor, which is 60' wide by 360". t he narrow f i e l d instruments can view anywhere w i t h i n a band on t h e sky, The viewing band r o t a t e s around t h e sky with t h e Ea r th ' s motion and the whole c e l e s t i a l sphe re 1s a c c e s s i b l e w i t h i n a s i x month season. Thus, any source can be observed f o r up t o 2 months a t a time, every half year, and observing is then only t l l t e r rup ted f o r a period i n the v r b i t when t h e e a r t h obscures t h e instrument f i e l d of view. The o p t i c a l axes of t he narrow f i e l d in s t rumen t s are perpendtcular t o t h e p l ane c o n t a i n i n g t h e WFC and t h e sun. The o p t i c a l axes of t he cameras are 60 degrees wide. f 30' s o l a r off-set i n t h e plane of t h e camras allows u s wi th t h e proposed lay- o u t t o view t h e whole sky except f o r f 30' around the sun. p r i n c i p l e be reduced f u r t h e r , by s h i f t i n g t h e cameras nearer t o t h e sun but i n vlew of thermal problems, r e l a t e d to t h e solar i l l u m i n a t i o n of t h e mask, t h e present des ign is cons t r a ined t o f 30'. The f u l l width ze ro response (FWZR) of t he UFC is f o u r tlmes 34 degrees squared i.e. 11% of t h e celestial sphere. With one a t t i t u d e change per o r b i t , 9 o r b i t s are needed t o scan the w h o l e sky. T h e f i e l d s o f t h e 4 cameras are a d j a c e n t to one another . The fils cone can i n As t h e sa te l l i t e is i n a low e a r t h o r b l t , large reg ions of t h e celestial sphere are obscured by the e a r t h a t any i n s t a n t . When a narrow f i e l d instrument t a r g e t is occu l t ed by t h e e a r t h , i t would be u s e f u l t o swi t ch t o ano the r one to maximise o b s e r v a t i o n time. However such manoeuvres, t y p i c a l l y 1-2 r a d i a n s , should be accomplished i n a f e u minutes. It should be noted however t h a t t h e proposed l ay -ou t of f o u r wide f i e l d cameras h a s t h e advantage t h a t i n no s i t u a t i o n are a l l t h e cameras obscured by t h e e a r t h . The o r g a n i z a t i o n of the ground o p e r a t i o n s is i l l u s t r a t e d i n Figure I . During s p a c e c r a f t c o n t a c t s with Kourou. d a t a from t h e on-board bubble memory are recorded a t t h e s t a t i o n while commands for s p a c e c r a f t and instrument r e c o n f i g u r a t i o n and a t t t t u d e manoeuvring are uplinked. After vdch pass , d a t a are t r a n s m i t t e d to the ESA c o n t r o l c e n t r e (ESOC) a t Darmstadt by iaeans of a permanent 43 k bLt/s l i nk . However tliis l i n k has y e t t o be implemented. The p resen t 8 k b i t / s l i n k would i n f a c t be e n t i r e l y adel;z:.rte. look analysis , d a t a from the i n d i v i d u a l i n s t rumen t s w i l l be s e n t t o t h e a p p r o p r i a t e i n s t i t u t e s by telephone l i n e s . A t ESOC, af ter qu ick I I 1 190 SCIENTIFIC DATA FLOW KOU R OU A A STORAGE R€C. AOCS RECONST DATA FORMAT Figure 1. Data flow on the ground after its reception from the Space- craft. 19 1 23 ORIGINAL P A a a OF POOR QUALrn ~ c r u u r ' Cluster 1 - 111. THE BRAGG SPECTROMETERS High r e s o l u t i o n spectroscopy pe rmi t s us t o s tudy the physics of e n e r g e t i c co rona l and photoionised plasmas which are known t o be p re sen t i n many sources both i n s i d 2 and o u t s i d e t h e galaxy. Measurements of emission l i n e i n t e n s i t i e s i r r t h e s e plasmas w i l l a l l ow estimates of gas temperature d e n s i t y and i o n i s a t i o n s ta te , element abundance and gas v e l o c i t y t o be made f o r a v a r i e t y of o b j e c t s . The d e t e c t J o n of abso rp t ion f e a t u r e s can provide similar information i n cases where I . i tense compact X-ray sources are imbedded i n extended gas clouds. The proposed instrument inc ludes f o u r s e p a r a t e crystal specLrometers s e n s i t i v e i n t h e energy range 0.5 t o 10.0 keV to s p e c t r a l f e a t u r e s a r i s i n g from cosmical ly abundant elements t h a t are ion i sed i n lo6 K t o lo8 K plasmas. The range of temperatures covered by t h e spectrometers is shown i n Figure 2. The r o l e of t he broad band d e t e c t o r s is a l s o ind ica t ed . It is clear t h a t t h e s e d e t e c t o r s complement the spectrometer by t h e i r s e n s i t i v i t y t o high temperature , 192 The c r y s t a l panels are s p h e r i c a l l y curved and focus on to two dimensional p o s i t l o n s e n s i t i v e d e t e c t o r s . Curvature i n the plane of d i s p e r s i o n e s t a b l i s h e s a wavelength scale on one dimension of t h e d e t e c t o r . plane permits t he mapping of extended sources and minimises t h e d e t e c t o r a c t i v e Curvature i n t h e o t h e r volume f a r each wavelength r e s o l u t i o n element. Focussing spectrometers have been discussed by Dasgupta (1961) i n the neutron d i f f r a c t i o n con tex t and by Schnopper (1966). presented by Woodgate (1373). allowed t h e development of p a l a l l e l e n t r y sys t ems A d e t a i l e d d e s c r i p t i o n of t h e c o n i c a l focuss ing geometry was The advent of p o s i t i o n s e n s i t i v e d e t e c t o r s (Rapley - e t al., 1977) which e l i m i n a t e the need f o r scanning i n wavelength. more rel iable data a n a l y s i s and i n t e r p r e t a t i o n , p a r t i c u l a r l y f o r r a p i d l y va ry ing Th i s provides f o r much sources. The des ign of s p h e r i c a l l y curved Spectrometers is d i scussed by G r i f f i t h s e lsewhere i n these proceedings. We w i l l t h e r e f o r e simply l i s t , i n t a b l e 111 t h e parameters of t he sys tem chosen f o r t he X-80 mission. The wavelength Table I11 SDectrometer Parameters l - v I I I I System I I ( A ) I Range I I I I I I 1 1 I I I I Line I Crystal I 2d I Wavelength I Resolut ion I I ron I L I F (22011 2.85 I 1.7-2.0 A I 2.3 mA I Sulphur I PET 1 I 8.73 I 4.8-5.5 A I 4.0 m A I S i l i c o n I PET 2 I 8.73 I 6.1-6.8 A I 6.1 mA I Oxmen I T1 AP 125.76 I 18.-22. A I 92 mA _ - I I I I I - I Pro jec t ed I I Area I I I I I I 1440 em2 I I 930 cm2 I I 695 cm2 I I 1470 em2 I I I r e s o l - J t i o n is set by c r y s t a l properties on t h e assumption that a spa t ia l resol- u t i o n of better than 1 mm can be achieved i n t h e d e t e c t o r s . A number of important t r a n s i t i o n s are l isted i n table I V . A spectrometer of t h i s kind can be OR131NAL PAGE IS OF POOR QUALm Table IV Important Transitions 1 I ---7 I I Transitions and Wavelengths I I Crystal I I I --I I I I 1~2-1~2plp I is2-i82p3~ I 182-1828% I is-2~2~ i I I I I I I l V - l - - V - 1 LiF I PET I PET I I I I I I I 1 I I I I I I I Fe XXV (1.852A) s xv ( 5.039A) si XI11 (6.650~) 0 V I 1 (2 1.602A) I I 1 1 I I I I I I I Fe XXV (1.857A) s xv (5.067 A) si XI11 (6.690A) 0 V I 1 ( 2 1.804A) I Fe XXV I Fe XXVL I (1.867A) I (1.790A) I I I s xv I s XVI I (5.O99A) I (4.729A) I I I si XI11 I si XIV I I I 0 VI1 I 0 VI11 I (22.100 A) I (16.969A) I (6.737A) I (6.182A) I 1 I I I I 1 I I I 1 I I I I I I - Figure 3. An Fe absorption edge in the spectrum of 4U 0900-40. Data from the GSYC OSO-8 spectrometer (Bccker et a1.t 197t3) lo-' 1 n 1 VELA X1 (4UO900-40) (0%-8) 194 used fo r a la rge number of as t rophys ica l ly important obuervations. shows a spectrum of the source bU 0900-40 (Vela X I ) obtained with the OSO-8 proport ional counter spectrometer (Becker e t al., 1978 ). edge is apparent. the energy of the f ea tu re with s u f f i c i e n t prec is ion t o allow the Fe ion i sa t ion s tage t o be deduced. from the compact source with the surrounding gas may give rise t o a range of ion isa t ion stages the dynamic behaviour of which could supply important c l u e s t o the geometry and d i s t r i b u t i o n of the accreting gas. tha t precise determination of f luorescent emission l i n e wavelengths can provide information of similar value though i n t h i s case, broadening due t o Compton sca t t e r ing o r mass motions could render the observat ion more d i f f i c u l t fo r t h e crystal spectrometers t o undertake. Figure 3 A deep i ron absorpt ion However the proport ional counter (E < 10) can not e s t a b l i s h Ais Al te rna t ive ly photoionisat ion in t e rac t ions by X-rays It is clear a l s o A s i m i l a r l y exc i t i ng p o s s i b i l i t y may e x i s t €or the study of the gas d i s t r ibu ted around the nuc le i of Seyfer ts and o ther a c t i v e galaxies . fo r the presence of var iab le i ron emission f ea tu res i n the spectra of two ac t ive ga lax ies (Hayes e t al., 1980, Hall e t aLr1981) has been obtained r i t h proport ional counter detectors . Models f o r the production of t h i s emission involve fluorescence emission from the gas i n the broad l i n e region (Hayes e t al.) o r recombination r ad ia t ion from a hypothet ical s h e l l of f u l l y ionised gas tha t e x i s t s very c lose t o the nucleus (Fabian and Ross, 1981). Ir. the former case, an increase i n the luminosity of the nucleus (Lx) l eads t o an increase of the f luorescent l i n e in tens i ty . For the la t ter model, a reduct ion i n Lx is required t o permi t the i ron nuc le i t o recombine with e lec t rons . In e i t h e r case Iden t i f i ca t ion of the appropriate model and understanding the in t e rac t ion of the a c t i v e nucleus with its surroundings requi res t h a t ind iv idua l l ines be resolved and the wavelengths iden t i f i ed . Evidence The r o l e of X-ray emission l i n e observat lons i n the spec t ra of extended cluster sources is too well known t o requi re much discussion here. However the re is a tendancy t o dismiss observations of t he in t r a -c lus t e r medium because d i f f i c u l t i e s with understanding the evolut ion of the c l u s t e r s and t h e i r media render the use of these ob jec t s fo r cosmological t e s t i n g much less simple than was once supposed. While t h i s is undoubtedly t r u e , i t merely emphasises the tendancy of a l l cosmological t e s t i n g techiques t o become more complicated as they become b e t t e r understood. formation and evolut ion of c l u s t e r s and ga lax ies would be ava i l ab le from the study of dens i ty and temperature va r i a t ions i n in t r a -c l a s t e r media i f the X-ray observat ions of adequate wavelength reso lu t ion and s e n s i t i v i t y could be ca r r i ed out. Meanwhile a wealth of information on the F ina l ly i t is obvious t h a t the X-ray l i n e eaission from supernova remnants can provide a wealth of diagnost ic information on the temperature, dens i ty , i on i sa t ion s ta te and element abundances of the hot plasma. has been much discussion of the Latter two points. Given the present ly ava i l ab le low reso lu t ion Fe emission da ta from proport ional counters , it is not possible t o e s t a b l i s h unambiguously the r e l a t i v e i n t e n s i t i e s Fe I C V and Fe W.VI f ea tu res which, together with temperature information, would allow departures from ionisa t ion balance t o be recognised. Work a t longer wavelengths with the Eins te in FPCS clearly ind ica tes the p o s s i b i l i t i e s (Winkler e t al. , 1981) but emphasises the need f o r good wavelength resolut ion. In pa r t i cu la r there The po ten t i a l of the spectrometers can be judged from the data presented i n Figures 4a and b where minimum detec tab le l i n e f luxes (at 5 u) a r e p lo t ted aga ins t continuum f lux with observation time as a parameter. cluster sources are t r ea t ed as both compact (i.e. 5' by 5 ' ) and extended targets. The broad l i n e case for Cas A assumes expansion a t the cu r ren t shock veloci ty . In Figure 4a, i I 196 The bensitivities of all four systems are more than sufficient to achieve the aims outlined in previous paragraphs, (Figure 4b) is 100 times more sensitive than the Einstein FPCS. In particular the Oxygen line spectrometer The scientific role of the Bragg Spectrombters may be summarlsed in the f o 1 lowing manner. Study: High temperature (106 - 108K) and photo-ionised plasmas ir. emission and absorption (estimate temperature, density, ionisation stale, abundance, gas velocities, redshifts). Deduce: Intra-cluster gas origl.., gas motion around active nculei, binary system gas dynamics, ISM density and compositton, SN element pro- duction, stellar corona and active star gas dynamics, extra galactic distances , Observe: QSO's and other active galaxies, Clusters of galaxies, X-ray binaries, Supernova remnants, Active stars and stellar coronae, X-ray bursters and transients Figure 4. Minimum detectable line s continuum flux for two of trength plotted against the four spectrometers I b) a) Iron line instrument. b) Qxygen line instrument 197 IV. “pe WIDE FIELD CAMERAS (WFC) The W i d e Field Crmrera is a Dicke or transform camera (Dicke, 19683 comprising a aask wick- a r a n b a r ray of holes located above a pos i t i on sensitive detector. X-ray sources w i l l cast shadows of the m z s k onto the Cetector. decontrolution ana lys i s is used to reconstruct an image of the sky f i e l d . A n ideal mask p a t t e r n w i l l h a w e a f l a t spatial frequency response o u t to the freqaency represented by t he ‘uni t ’ hole i n the mask. Deconvolution ;rf t he image w i l l then reconstruct the sky br ightness d i s t r ibu t ion . Camera is therefore able to provide high s e n s i t i v i t y over a w i d e field of view due to the l a rge ‘open’ area of t h e mask, combined w i t h good antr~lar reso lu t ion . Correlat ion or The Wide Field Four cameras are arranged so as to allow waximum sky access. Rotation of the spacecraft around the Sun axis, d i n e d w i t h the -- + 30° allowable of f -se t , makes nearly t h e w h o l e sky available to the WFC a r ray 2t any one t i m e , except for a cone of 30° radius around the Sun. is 1.4 ster. The characteristics of the camera a r e given i n Table V. The mask is 60 c m x 60 cm and contains a pa t t e rn of 0.5 nun square holes based on a Pseudo- Random-Noise sequence, repeated once i n each dimension. I t is mounted a d is tance of 48.6 cm from the pos i t ion s e n s i t i v e detector w h i u h has a kWHM pos i t i on resolut ion o f 0 . 5 mm i n both dimensions over an ac t ive area 30 cm x 3d cm. For a pe r fec t pos i t ion sens i t i ve Cetectcr the angular re-solution would be set by the mask hole size and the detector-mask s * : a ~ a t i o n , y ie ld ing an angular reso lu t ion of 3.4 arc minutes. equal giving an optimised angular reso lu t ion of % 5 arc minutes. The f i e l d c f view of t h e HFC system I n the present case the reso lu t ion of mask and de tec tor are The reso lu t ion degrades s l i g h t l y o f f axis due mainly to the dee-per pene- t r a t i o n of energe t ic photons. i n the spectra of mast sources t h e e f f e c t is s l i g h t l y ( reso lu t ion - 5 arc minutes). Vue to the predominance of lw energy photons The above configurat ion, with a repeated mask pa t t e rn and a collirtator I . I . I i 1 I t i i ! ! ! , i ! I ! gives i n p r inc ip l e per fec t coding of the image, even fo r sources at the edge of 198 Table V. Ch.racterietlcr of Wide F i e l d Cameras I r i Configurat ion I I Fie ld of view I I Sol id a w l e I I Angular r e so lu t ion I I Error box area I I hergy racye: imaging I I t h i n g I I &\ergy r e s o l u t i o n I I & n s i t i v e area I I Background ( d i f f u s e ) I I Hask area I I Detector area I I Gss f i l l i n g I I Uindow material I I camera alignment I I Total mass I I I I I I I I I I I i I I I I I I I I I I I I I I I I I I I I 4 wide f i e l d cameras 34' x 34" FWZR per camera 0.35 stet per camera 5 a r c min I I l(atc d n ) 2 l imi t ed by camera i n t e r n a l alignment I 2.0 - 10 keV f u l l f i e l d 2.0 - 20 keV c e n t r e of f i e l d 2.0 - 5 0 keV 20% a t 6 keV I 320 cm2 per camera (mask t r a n m i e s l o n , 50%) I I 150 ctslsec per canera I I I I I I X e I C l b at 2 atm, I I 60 CD x 60 cm 30 cm x 30 cm 125 micron beryl l ium 6 0 O f 1" between camerae 126 kg the f ie ld of view (Proc tor , 1978). "he mask can be cons t ruc t ed by e t ch ing t h e p a t t e r n i n t o a s t a i n l e s s steal f o i l , or by e l ec t ro -depos i t i on o f n i cke l . p o a i t i o n s e n s i t i v e detector measures 4 0 cm x 40 cm 7.5 cm and ie sealed wi th a 125 micron Be window. photons. The d e t e c t i o n e f f i c i e n c y exceed8 100 over the range 2 to 50 keV and spectra can be obta ined throughout. Pos i t ion ing is l i m i t e d to the rangem 2 to 10 keV due to photon p e n e t r a t i o n problano a t the edge o f the f i e l d of viewi i n the c e n t e r of theme f i e l d s of view hawever a p o d ' i o n r e s o l u t i o n of a few arc minutes can be achived over the energy range 2 .o 3 0 keV. The The Xe/CN4 f i l l i n g g a s .hits pene t r a t ion of high energy ORIGINAL F'AGE 1s POOR QUALITY The sensitivity of instruments of this type has been discu8sdl by many authors (Dicke 1968, Palmieri 1974, Gunson 1976). and observing time. dominated by the diffuse X-ray emission at high galactic latitude and by the contribution of strong sources in the galactic plane. component can be reduced to an insignificant level. source strength at a 5 sigma confidence level is shown in Figure 5 a8 a function of the observation time for a high galactic latitude and the galactic center region. Xt depends on area, background Because of the large field of view the background is The charged particle The miniwm detectable ?he instruments ability to observe intensity changes in the oource is shown in Figure 6a. Figure 5. Wide Ficld Camera Sensitivity plotted against integration time I I I 1 WiOE FIELO CAMERA 10 - High Gdoctff \atihrdcs 1.0 - Integroti ti- in seconds 1 orbit I 199 LOO 5[ J i 1 v i m P i c t lo'. 0 Figure 6 . Time taken to detect a s p e c i f i c fract ional change i n source intens i ty a) wide Field Camera, b) Large area proportional counter and c ) Phoswich. , ! - PAGE IS OF-wAUry However the performance of the instrument is bes t i l lustrated by simulation. Deconvolved images of the galact ic centre region and a reigon of high galact ic lat i tude (Coma) are shown in Figure 7 . 20 1 Simulated performance of the WFC(for a) a 100s observation of the gala.=tic centre region and b) 5oooS observation of the Coma region. Source intensities (MiliCrab) are shown in brackets . Since the s c i e n t i f i c r o l e of a wide f i e l d camera system is r a the r easy t o understand we w i l l simply sumarise the appropr ia te s c i e n t i f l c aims. Study: Galactic and ex t r aga lac t i c X-ray v a r i a b i l i t y , long term behaviour of X-ray b inar ies , X-ray t r a n s i e n t and burs t a c t i v i t y . Deduce: Proper t ies of ac t ive g a l a c t i c nuc le i (size, energy release), X-ray masses, neutron star, white dwarf , or black hole presence, acc re t ion d i s k proper t ies , t r ans i en t and burs t source mechanisms. Observe: QSO’s and o ther a c t i v e galaxies, X-ray b ina r i e s , X-ray t r a n s i e n t s , X-ray bu r s t e r s , Galactic bulge sources, Actlve stars. V. THE U R G E AREA PROPORTIONAL COUNTER (LAPC) The instrument that covers the energy range 1.2 t o 20 keV is a l a rge area array (0.25 n2) of sealed proport ional counters. proportional counters Is moderate (19% a t 6 keV) compared with o ther medium energy de tec to r s (gas s c l n t l l l a t o r 10% a t 6 keV, s o l i d state decectors 3% a t 6 keV) however, they have a lower u l t i m a t e background noise , can be produced with t h i n windows, are simpler and cheaper t o bul ld , and have by far the lowest mass pe r u n i t s e n s i t i v e area. demonstrated t o be adeqrrate fo r broad band spectroscopy. The energy r e so lu t ion of In addi t ion t h e i r energy reso lu t ion has been The c h a r a c t e r i s t i c s of the LAPC are given i n Table V I . The modules are mounted, v i a two p ivots and a t i l t mechanism, t o the spacecraf t . This enables a f 2’ o f f s e t from the source fo r background determination. a gas depth of 4 cm. The 37 micron beryll ium window is supported aga ins t the in t e rna l pressure of 2 atmospheres by a co l l imator formed from Hexcell mater ia l giving a circular f i e l d of view of 45’ diameter (c’wHM). malnly Argon f o r e f f i c i e n t background r e j ec t ion but w i l l contain s u f f i c i e n t Xenon t o give 10% e f f i c i ency out t o 20 keV. Two anode layers w i l l be used, The de tec to r s have The gas used w i l l be ORIGINAL PGGE IS OF POOR QUALITY t h e s e w i l l i nco rpora t e guard wires, end guard cathodes (Bailey 1978) and when used i n a n t i c o i n c i d e n c e w i l l g i v e 5 s i d e d p r o t e c t i o n to t h e upper layer f o r low background obse rva t ions . employed for e f f i c i e n t background r e j e c t i o n . In a d d i t i o n risetime d i s c r i m i n a t i o n w i l l be For b r i g h t sou rces the layers w i l l o p e r a t e t n series t o g i v e P wider energy range. Table I V . C h a r a c t e r i s t i c s of t h e Large Area Propor t iona l Counter Array 1--- ---- I I I made up of 4 d e t e c t o r s I I Con€ igura t ion I 3 modules, 2 made up of 2 d e t e c t o r s and 1 I I 1 I S e n s i t i v e a r e a I 0.25 m2 1 I Fie ld of view I I Energy range I I Rate from cyg X-1 I I 45' x 45' FWHM I I I I I I 1.2 - 20 keV a t > 10% e f f i c i e n c y - lo4 coun t s per second I Background I < 10 coun t s pe r second I I 40 cm x 20 cm x 35 cm 37 micron beryl l ium I Detector dimensions I I I I Window I I I I I I Tota l mass I 79 kg I I I I I cas f i l l i n g I ArgonIXenonlquench a t 2 atm. -- - The coun te r bodies w i l l be o f s t a i n l e s s s tee l and s u f f i c i e n t mass and volume h a s been allowed t o p e r n i t inexpensive c o n s t r u c t i o n methods f o r d e t e c t o r s and co l l ima to r s . This is p a r t t c u l a r l y important with a n instrument of t h i s s i z e . P r c 3 m p l i f i e r s w i l l be mounted on t h e back of each d e t e c t o r , t o g e t h e r w i t h high vo l t age gene ra to r s . The pu l ses from each anode l a y e r w i l l be d i g i t i s e d with a 128 channel ADC and a s s o c i a t e d with a coun te r i d e n t i f i c a t i o n code, and time b i t s s u f f i c i e n t t o time the photon a r r i v a l t o 2 8 microseconds. Depending I ? i i I ' on t h e t o t a l count rate, s e l e c t i o n s from t h i s information w i l l be presented t o the s p a c e c r a f t d a t a handling sys tem € o r s t o r a g e and subsequent t ransmission. To accomodate the occas iona l very high d a t a rate (- 251) kbps) experienced du r ing o b s e r v a t i o n s of t h e b r i g h t e s t sou rces , t h e experiment w i l l c o n t a i n a h igh I I c a p a c i t y bu f fe r memory. 1 This w i l l feed t h e d a t a , c o l l e c t e d over s h o r t pe r iods t o t h e s p a c e c r a f t d a t a handl ing sys tem a t a compatible rate. For most obse rva t ions the d a t a rates w i l l vary between 1 and 20 kbps. I I The time r e s o l u t t o n and s e n s i t i v i t y of t h e LAPC are shown i n Figure 6b and 8 r e spec t ive ly . V I . THE PHOSWICH DETECTOR SYSTEM For the energy range 15 - 200 keV, a n ino rgan ic s c i n t i l l a t i o n d e t e c t o r sys tem is employed. The t o t a l s e n s i t i v e area (- 0.8 x geometric area due t o c o l l i m a t o r e f f i c i e n c y ) w i l l be ,oO cm2. c r y s t a l s , c a l l e d a Phoswich, provide 2 t p r o t e c t i o n a g a i n s t background e v e n t s i n a r e l a t i v e l y low c o s t per u n i t area d e t e c t o r . X-ray induced s c i n t i l l a t i o n s i n t h e ' f r o n t ' c r y s t a l (NaI ( T l ) ) are d i s t i n g u i s h e d from background induced s c i n t i l l a i t o n s i n the 'rear' crystal (CsI (Na)) by means of t he d i f f e r e n t f l u o r e s c e n t decay times of t h e two s c i n t i l l a t o r s . A CsI (Na) s c i n t i l l a t o r s h i e l d surrounds tbe Phoswich and c o l l i m a t o r so as t o i n c r e a s e d i s c r i m i n a t i o n a g a i n s t charged particles and gamma rays. The bottom p a r t of t h e d e t e c t o r i s covered by a pass ive graded s h i e l d (Pb-Sn-Cu) t o reduce t h e background count ing rate i n t h e CsI (Ne) element of t h e Phoswich. Two d i f f e r e n t 'sandwiched' s c i n t i l l a t i o n A Phoswich i n combination with a n inexpensive plastic s c i n t i l l a t o r s h i e l d gave background rates of a few x coun t s cm'zsec'lkeV-l ( independent of d e t e c t o r a r e a ) i n bal loon f l i g h t s (Scheepmaket 1976, Kendziorra 1977). The 205 ORIGINAL PAGE IS OF POOR QUALlW high energy experiment on the HEAO-a satellite ( A - 4 ) consisLing of Phoswfch detectors wfth an active CsI shield gave similar background rates (Priminl 1979). The background rate in the proposed instrument will be lower than the A-4 experiment due to the equatorial orbit. of the instrument and Figures 6c and 9 show the time resolution and sensitivity, respect lvely. Table V I 1 gives the main characteristics LARGE AREA PROPORTIONAL COUNmR bsoocm' dhcbro,boc*gmund 1 0 ~ ~ ) Source Cmkcim Limit c i a i 0.1 1 1 10 - 1 IO 1.0 Figure 8. LAPC sensitivity plotted against observing time. Figure 9. PHOSWICH sensitivity plotted against observing time. 206 Table VII. C h a r a c t e r i s t i c s of Phoswich Detector System I Configurat ion I One u n i t comprising 4 Phoswich detectors I I I I I F i e l d of view I I Energy range I I I I I I I I I I I I I I Energy r e s o l u t i o n Geometric area Enve lope To ta l mass Background I I I I I I I I I I I I I 1.9' x 1.9' FWHM 15 - 200 keV 30% a t 20 keV, 19% a t 50 keV, 12% a t 200 keV I I 800 cm2 100 cm x 6 5 cm x 65 cm 125 kg 15-30 keV 3.1 x 10'4cts/cm2 sec keV I 30-40 keV 2.3 x 10-4cts/cm2 sec keV I 40-80 keV 1.7 x 10'4cts/cm2 sec keV I 80-200 keV 1.1 x 1Om4cts/cm2 sec keV I I Each Phoswich d e t e c t o r c o n s i s t s of a 14 cm x 14 cm x 0.3 cm t h i c k NaI ( T l ) c r y s t a l , viewed by a 5 i nch p h o t o n u l i t i p l l e r through a CsI ( l a ) crystal 5 cm thick. The Phoswich is herme t i ca l ly s e a l e d i n an enc losu re , with a 1 mm Be X- ray e n t r a n c e window and a q u a r t z e x i t window. Four of t hese d e t e c t o r s , each with a geometric c r y s t a l area of - 200 cm2 w i l l make up t h e instrument. To d i s t i n g u i s h X-ray e v e n t s i n t h e NaL ( T l ) c rys t a l ( - 25 ~ r s decay time), information from both a pu l se shape d i s c r i m i n a t o r (PSD) and a p u l s e he igh t a n a l y s e r (PHA) is used. To ensure t h a t t he d i s t i n c t i o n between NsI ( T l ) and CsZ (Na) e v e n t s is maintained, t he temperatures of the Phoswiches have t o be maintained a t 20 f 10'C. 'l'his p recau t ion is important i n view of t h e s t r o n g temperature dependence of t h e f l u o r e s c e n t decay time of NaI (TI ) . To d e t e c t a weak X-ray source and determine i t s spectrum it is necessary t o observe the source and background f o r equa l pe r iods of time. This is achieved by rocking the c o l l i m a t o r some degrees from t h e c e n t r e p o s i t i o n every few minutes. 207 The r o l e s of the two broad band de tec to r s may be sumarised as follows. Study: Continuous plasma and non-thermal spectra, hard X-ray spectra, g a l a c t i c and extragalactic source v a r i a b i l i t y (millisec t o years), source periods, cyclotron fea tures . Deduce: Proper t ies of a c t i v e nuc le i (size, energe t ics ) , non-thermal c l u s t e r proper t ies , nature of X-ray b ina r i e s (Black holes , neutron stare, white dwarfs), accre t ion d i s c proper t ies , t r ans i en t and burs t mechanisms, neutron star magnetic f i e l d s , supernova and ISM proper t ies , nature of X-ray background. Observe: QSO's and o the r a c t i v e ga lax ies , Clus te rs of galaxies, X-ray b inar ies , Supernova remnants, X-ray bu r s t e r s and transients, Galact ic bulge sources, Active stars and stellar coronae. i VII. THE GAMMA BURST MONITOR The de tec tor is based on a modification of the r o t a t i o n modulation col l imator (RMC) pr inc ip le . The X-ray photons are detected i n a s c i n t i l l a t o r mosaic cons i s t ing of a l t e r n a t i n g s t r i p s of C s I (Na) and NaI (Tl). The s t r i p s a r e 5 mm wide and 2mm thick. a s ing le 75 mm phototube. tungsten s t r ip s 0.5 mm t h i ck separated by 5 mm gaps is placed above the scin- t i l l a t o r mosaic. The mosaic is 80 mm i n diameter and is viewed by A l a rge c i r c u l a r shadow mask consieing of 5 mm wide By means of a small motor the s c i n t i l l a t o r mosaic and t h e shadow mast are kept revolving around the symmetry ax is a t a sp in rate of 60 revolut ions per minute . During the sp in , the tung6Len s t r i p s w i l l a l t e r n a t i v e l y obscure the NaI - and the C s I - s t r i p s as seen from a source pos i t ion i n the sky. obscurat ion (modulation) sequence is c h a r a c t e r i s t i c fo r each possible source pos i t ion within the 120" f i e l d of view. The 1 1 i I 208 mosaic, and the conventional RMC design employing a single scintillator and two parallel shadow grids is that time fluctuations in the source flux, which is a well established characteristic of the gamma-burst sources, are not confused with the instrument modulation of the flux. / The signals from the two kinds of scintillator crystals can be separated at the output of the photomultiplier tube due to the different decay time characteristics of the light emission of the crystals. Computer simulations of the detector performance have Indicated that tie burst sources can be located to better than 20 arc minutes for bursts with - 500 detected photons in the 30-130 keV interval (see Figure 10). This assunes that the burst duration is not much less than half the spin period. The number of ‘iela-type bursts of this magnitude is roughly 25 per year within the field of view of the detector. The characteristics of the Gamma Burst Monitor are given in Table VIII. Transient X-ray sources with hard spectra can also be positioned with this Cnstrument. The expected background counting rate in the equatorial orbit is rougly 5 cps. A source of the same strength as the Crab can be Localised within one orbit of observation. The instrument rill include a microprocessor with 1 memory in which data can be stored at high rates during a burst. perform a deconvolution of the RMC patterns and determine source positions on line. transient source with the narrow field instrument is thus facilitated and such a decision could be taken as early as the next ground station pass. The microprocessor will also Any decision to reorient the satellite for the purpose of studying a I 209 Table VIII. C h a r a c t e r i s t i c s of t h e Gamma Burst Monitor , 1 I i Field of view i 120' (FWHP) 132' (FWZR) I I I So l id a n g l e I I I 3.1 s t e r a d .20 arc minutes I Angular r e s o l u t i o n I I I I Energy range 1 30 - 130 keV I I I I I I I I I I I I I I Geometric area 1 50 cm2 I I Detector s i z e I 25 cm x 25 cm x 25 cm I I T o t a l mass I 6 kg , Figure 10. Angular r e s o l u t i o n of the G a m a B u r s t Monitor as a func t ion of b u r s t s t r e n g t h . The r o l e of rhe gamma b u r s t monitor may be o u t l i n e d i n t h e fol lowing way. Study: Hard X-ray t r a n s i e n t s and gamma r a y ~ ~ r s t s . I 210 Deduce: Sky d i s t r i b u t i o n o f gamma r a y b u r s t sources . P o s i t i o n s o f X-ray t r a n s i e n t s a l lowing in-depth s t u d i e s by t h e narrow f i e l d i n s t r u n e n t s . Observe: All sky monitoring. As t h e purpose of t h e gamma b u r s t monitor is t o d e t e c t new, unknown sourceR ove r a very wide f i e l d of view, i t does no t c o n s t r a i n t h e p o i n t i n g s t r a t e g y of t h e s p a c e c r a f t . V I 1 1 INSTRUMENT DATA HANDLING In a mission such as X80 where t h e need t o a c q u i r e d a t a a t high rates and with good t iming is d i c t a t e d by t h e s c i e n t i f i c requirements, i t is necessary t o permit some f l e x i b i l i t y i n a s s i g n i n g t e l eme t ry t o t h e va r ious instruments . B i t a l l o c a t i o n s and t h e expected count ing rates fram t h e f i v e in s t rumen t s are given i n Table IX. Time r e s o l u t i o n t o 0.1 seconds is a v a i l a b l e from s p a c e c r a f t t iming and t z l eme t ry synchronisat ion. Higher time r e s o l u t i o n is der ived from w i t h i n t h e instrument . The t o t a l mean b i t rate a v a i l a b l e is 44 k b i t s l s e c o n d wi th peak rates up t o - 220 kb i t s l s econd . f o r example when t h e LAPC obse rves Cyg X-1, o r t h e WFC monitors t he g a l a c t i c c e n t r e . However t h e s e o b s e r v a t i o n s w i l l occur i n f r e q u e n t l y and never s imultaneously. of t h e d a t a handl ing system and w i l l f i l l t h e bubble memory i n less than one o r b i t , t h e obse rva t ion then being h a l t e d t i l l d a t a are t e l eme te t ed t o ground. Since t h e longes t obse rva t ion allowed by e a r t h o c c u l t a t i o n w i l l r a r e l y exceed 60 minutes t h e a v a i l a b l e d a t a c a p a c i t y w i l l i n g e n e r a l be s u f f i c i e n t . For a few very b r i g h t sou rces , t h e LAPC will only observe f o r about 20 minutes, which is adequate t o realise t h e s c i s n t i f i c o b j e c t i v e s . In a l l o t h e r cases the in s t rumen t s w i l l o p e r a t e toge the r and t h e total d a t a c a p a c i t y w i l l not be exceeded. The LAPC and WFC can i n d i v i d u a l l y exceed t h i s mean rate, For such a n obse rva t ion one i n s t r u n e n t w i l l havc e x c l u s i v e use 211 Table. I X Instrument Data Bates I bunt ing I btaRate I I Rate (c/s) I (kb/s) I lkagg Spec trometer I I I i I I I I I I )Detector ID 2 b i t s Ix-Y position !8 b i t s IDetector energy 6 b i t s lluse time 6 b i t s I 1"" event I ILAPC 1- I energy 7 b i t s I Cyg X-1 I 104 I 240 I [Detector I D 4 b i t s I m;C 4151 I 200 I 4.8 1 I T h i n g 12 b i t s I Background I 10 I 0.3 I 1 b i t I I I I I I I I 24 b i t s / I I I I I event I I I I I I I I 32 b i t s l I Spare I Total i -1- I I I I envisaged 1 8 0 I 2.5 I I flux on a l l I I I I spectrometers I I I I I I I I I I I I I I I I I I I i I I I I I I Energy IDetector I D I P U ~ ~ shape I Time iVeto I TOTAL I I I I t 7 b i t s I Crab I 2 b i t s I NCC 4154 I 7 b i t s I Backgroune I 7 b i t s I I 1 b i t I I I I 24 b i t s / I I event I 1 I 1 I I 290 I 8 I 30 I I I I I I I 7 I 0.2 I 0.7 I I I I I I I I wc I- I Posi t ion 20 b i t s I Energy 4 b i t s ICamera Ident. 2 b i t s I Spare 2 b i t s I 28 b i t s / event I I I I I I I I I I I I 1 1 I i I I 49 I I I D i f f u s e back- I I I P a r t icle I I I ba Ykground I 60 I 1.7 I ( 4 cameras) I I I Crab (1 camera) I 590 I 16.5 I Galactic cen t r e (1 camera) ground (4 cameras) I 600 I 16.8 I I 1750 I I I I I I ! I - I I ICaoma Burst Hon. I I I 0.1 I I I I I I f I i I I I ! i ! ! 1 ! , I 1 Xbased on a wide pulse shape window; fu r the r optimisat*on of s i g n a l t o noise ratio can be done on t he ground. 2 11' IX. S W L E OBSERVATIONS AND A POSSIBLE MISSION PLAN In o r d e r to deeronstrate t h e c a p a b i l i t i e s of t h e v a r i o u s instruments, i t is i n s t r u c t i v e to l ist t h e observing times requ i r ed to ach ieve 8 number of alms in t h e s tudy of t h r e e sources. of t y p i c a l i n t e n s i t y 3 Uhuru f l u x u n t t s (UFU). The instrument c a p a b i l i t i e s are The first of t5ese is NCC 4151, a S e y f e r t galaxy l i s t e d f o r an o v e r a l l observing time of 5 d8yr. ( i ) Detect NCC 4151 (11) Register 10% f l u change ( i i i ) Detect Fe f e a t u r e B. Phoswich ( i ) Detect NCC 4151 ( i i ) Register 10% f l u change C. Bragg Spectrometer (1) Detect Fe l i n e s ( t i ) Measure A's to - h A (iii) Search for Fe edge D. WFC - ( i ) Detect NCC 4151 ( i t ) Register l00L f l u x change - 10s - 300s - 3000s - 3000s - 10% - 3. 104s - 3.105s - 3.104s - 3000s - 60006 In t h e case of t he Perseus C lun te t which is an extended source of i n t e n s i t y 18 UFU, 8 one d a i obse rva t ion would permit t h e fol lowing t o be achieved. - A. - B r a s Spectrometer -- ( i ) Lktect Fe l ines from t he Persua C l u s t e r - 1000s ( i t ) Hap ( 5 ' x 5 ' ) i n l t n e e - 3.104s ( l t i ) Detect S, Si, 0 lines (NCC 1275) - 1000s . .. 213 B. Phoswich ( i ) Detect t h c Perseus C l u s t e r 4000% ( i i ) E s t a b l i s h h a r t spectrum - 9x10 s c. LPAC - ( i ) Detect t h e Perseus C l u s t e r Is ( i i ) E s t a b l i s h medium spectrum - 308 ( i i i ) Detect Fe l i n e - 10s (1) Nonitor NCC1275 f o r v a r i a b i l i t y , ( i i ) Monitor o t h e r c l u s t s r galaxies 100% change - 6000s (IC310, MCC1265) - 9x104s Fina l ly f o r t h e X-ray binary 4U 0900 - 40 (Vela XI), a source of t y p i c a l i n t e n s i t y 250 UFU, t h e fol lowing o b s e r v a t i o n s are p o s s i b l e i n a 10 d e y observing per io d . A. LAPC I_ (1) Detect 10% f l u x change in 4U 0900-40 - l000S ( i i ) Register Fe emission/absorpt ib.1 f e a t u r e s - 100/ 1000s B. Phoswich -- ( i ) Detect 100% f l u x change in 4U 0900-40 - 1000s ( i t ) Search f o r c y c l o t r o n f e a t u r e s - 3 x 1 0 s ~ C. Bragg Spectrometer (1) Detest Fe t i n e s / e d g e s ( i l l Measure A's t o M A Pointed instruments monitor for binary phase, r e l a t e d e v e n t s and fo r pu l se phase f e a t u r e s . D. W F C - ( i ) Detect 4U 0900-40 ( i i ) Regi s t e r 100% f l u x change - 103/105s - 103/ 105s - Is - 3s I t is a l s o i n s t r u c t i v e t o o u t l t n e a n observing programme for a t h r e e year mission in o r d e r t o demonstrate how t h e instruments can work together . Table X shows such a programme. 2 14 Table X. Three Year Mission Observing Prograrm~c I I 1. SO QSO's and a c t i v e g a l a x i e s 400 days 2. SO c l u s t e r s of g a l a x i e s 200 days 3. SO X-ray b i n a r i e s 200 days 4. SO a c t i v e stars and coronae 100 days 5 . 20 scoernova r e e r n a n t s SO days 6. Sky-survey with WFC ( 3 days every 10 days) 300 days 7. Galac t ic c e n t r e region wi th WFC 50 days It should be noted t h a t many wide f i e l d camera aims can be s a t i s f i e d by r o t a t i n g f 30' around t h e poin t ing d i r e c t i o n determined by t h e narrow f i e l d instruments. Furthermore, i t w i l l be possible to conduct s h o r t observa t ions of some sources with narrow f i e l d ins t ruments dur ing t h e wide f i e l d camera a l l sky surveys. The WFC and t h e Gamma Burst Monitor will cont inuously monitor a l a r g e f r a c t i o n of t h e sky f o r t h e occurrence of b u r s t s and t r a n s i e n t s . X. CONCLUSIONS The X-ray sky survey d s s i o n s undertaken by spacecraft such as Uhuru, Ariel V and HEAO-1 and t h e high r e s o l u t i o n observa t ions c a r r i e d o u t wi th t h e Eins te in Observatory (HEAO-2) a t longer wavelengths have e s t a b l i s h e d X-ray astronomy as one of t h e most e x c i t i n g and product ive branches of as t rophys ics . The European mission, WOSAT, to be undertaken in 1981 through 1983 is expected t o make a n ex tens ive c o n t r i b u t i o n in e n l a r g i n g t h e scope of t h e subjec t . Appropriately instrumented follou-on missions must now be prepared for d e t a i l e d s t u d i e s of energy s p e c t r a and time v a r i a b i l i t y , t o develop our understanding of t h e sources which include such d i v e r s e and phys ica l ly i n t e r e s t i n g o b j e c t s as c l u s t e t e of g a l a x i e s , quasars , neutron stars and black holee. Although o t h e r missions (e.8. ASTRO-C, XTE) have been s t u d i e d , the mission 215 proposed here is unique in combining spectroscopic , timing and variability observations. hensive study of X-ray sources from a single free-flying satellite with the goal of obtaining: - a determination of mass, dimensions, magnetic fields, and other basic informa- It is designed to perform an integrated, systematic and compre- tion on matter under extreme conditions as found, for example, in neutron stars and degenerate white dwarfs. characteristic signatures of black holes and confirmation of their existence. an understanding of accretion processes on compact objects. detailed information on the conditions prevailing in high temperature plasmas, e.g. in stellar coronae, supernova remnants, galactic nuclei and clusters of galaxies, thereby gaining insight on evolutionary processes. - - - - a description of the photoionised gas that exists in binary systems and in the broad line regions aroud active galactic nuclei. - an understanding of the innermost regions of active galaxies. Five instruments, consistent with these goals are described, namely Bragg Crystal Spectrometers, Large Area Proportional Counters, Crystal Scintillators (Phoswich) , Wide Field Cameras and a Gamma Burst Monitor. m e case for high resolution observations with crystal spectrometers is now overwhelming in view of the strong iron emission and absorption features detected by the proportional counters on Ariel V, OSO-8 and HEAO-1 and of oxygen, iron silicon and sulphur emission by the SSS and FPCS instruments on the Einstein Observatory. "he Bragg Spectrometer has sufficient sensitivity to study the detailed structure of these features for a large number of X-ray sources with observation times of typically 10 seconds. The Large Area Proportional Counter will have sufficient sensitivity to perform sub-millisecond timing on bright sources and to study time and spectral variability in faint sources. It also has the broad spectral range to provide a monitor function for the crystal 4 ! i 216 . . . - . .. . , spectrometers. The Phoswich detector will extend the spectral and variability meaawements from a few keV to some 200 keV and will permit the detailed study of the recently discovered cyclotron features. image the sky to detect, locate and meausre X-ray transient events, as well as X- and gamma-ray bursts. all X-ray sources on tiaiesclaes of days to years Included to provide wide field observations at higher energies. The mission can be readily undertaken on a free-flying satellite with modest The Wile Field Cameras will Furthermore it provides an efficient means of monitoring A gamma ray burst monitor is performance requirements on attitude control (% 5 5 arc min), and measurement ( 0 . S ' ) power (% 350 watts) and mass ('L 900 kg) . A l o w altitude (600 Jan) circular orbit at low inclination (approx. 0 deg.) is optimum and can easily be achieved with the Ariane vehicle launched from Rourou. The Spectroscopy, Timing and Variability mission must be seen as a logical follow-on in X-ray astronomy for the mid-1980'8, to capitalise on the non incon- siderable financial and manpower resources already invested and to consolidate the expertise and experience which have been firmly established in Europe. in times of financial stringency on both sides of the Atlantic, the possibilities for collaboration should be examined carefully by both ESA and NASA. However AcKN(JwLED(;EMENps I am grateful for the considerable effort put into the mission phase A study by the members of the x-80 study team. in preparing this presentation. The efforts of R. A. Lahe Future Projects Office), €I. Olthof (ESA €iQ, AWG Secretary), A. Peacock and B.G. Taylor ( - 8 SSD) in bringing the study to a auccessful conclusion are also acknowledged with gratitude. I have used their report extensively i 217 REFERENCES - Bailey, T.A., 1978, Nuclear I n s t r . Meth., g, 177. Becker, R.H., Rotschi ld , R.E., Boldt, E.A., B o l t , S.S., Pravdo, S.H., Ser lemi tsos , Das Gupta, K., 1961, Rev. Sci. In s t . , 32, 602. Fabian, A.C., Ross, R. , 198 , Mon. Not. R. astr. SOC., Gunson, J., Polychronopulous, B., 1976, Mon. N o t . R. astr. SOC., 177, 485. H a l l , R. , R i c k e t t s , M.J . , Page, C.G., Pounds, K.A., 1981, I n press, Space Science Hayes, M., Culhane, J.L., B l i s s e t t , R . J . , B a r r , P., B e l l Burnel l , S . J . , 1980, Kendziorra, E., e t al . , 1977, Astrophys. J., 217, L93. Pa lmier i , T.M., 1974, A s t r . Sp. Sc., - 20, 4'.31. Primini , F.A., e t a l . , 1979, Nature, - 278, 234. Proc tor , J., e t a l . , 1978, Mon. N o t . R. astr. Soc., 185, 745. Rapley, C.G., Culhane, J.L., Acton, L.W., C a t u r a , R.C., Joki, E.G., Bakke, J.C., Schnopper, H., e t al., 1966, Paper presented to 133rd AAS meeting, Washington, Dc. Scheepmaker, A., e t a l . , 1976, Astrophys. J., 205, L65. Winkler, P.F., Canizares, C.R., Clark, G.W., Markert, T.H., P e t r e , R., 1981, P.J., Swank, J.H., 1978, Astrophys. J., - 221, 912. - Reviews. Mon. N o t . R. astro. SOC., 193, 15P. - 1977, Rev. Sci . I n s t . , 48, 1123. - Astrophys. J. , 245, 574. X-Ray Astronomy Program i n Japan Yasuo Tanaka Ins t i t u te o f Space and Astronautical Science, Komaba, Meguro, Tokyo 153 Japan 1. Introduction One o f the great achievements i n the short h istory o f X-ray astronomy i s t o have established the real existence o f neutron star rather than j u s t a theoretical conception. undoubtedly continue t o produce important science through t h i s decade and beyond. amusing as well as amazing t o f i n d that t h i s 96 kg s a t e l l i t e w i h i l imi ted capabil i- t i e s gives us plenty o f interesting resul ts and signi f icant questions concerning neutron stars. These questions i n fact constitute the basis fo r our planning of the future progran, I would, therefore, l i k e to i l l u s t r a t e some o f the problems that emerged from the Hakucho observations and are t:, Le understood by future investigations . Pursuing t h i s extreme astronomical body and i t s environment w i l l It i s We have been operating the Hakucho s a t e l l i t e f o r over two years. 2. X-ray pulsars o f Vela X-1. Vela X-1 i s a slow pulsator with the approximate pulsation period o f 283 sec and the binary period o f 8.97 days. Fig. 1 shows the history o f the pulsation period variation. It i s clear that the neutron star had been spinning up u n t i l 1979, consistent with the picture o f acceleration by the accretion torque (e.9. Rappaport and Joss 1981 1. features. continues (Nagase e t a l . 1981). The average i ) / P 2 3.5 x 1979 - 1981. the order o f the orb i ta l period i s s igni f icant ly resolved fo r the f i r s t time (Nagase 1981). 6 / P o f t h i s f luctuation ranges over (-3 % 4) x yr-', an order o f magnitude larger than that for the long-term variation. The observed long-term slowing down i s not readi ly explained. There has been no signi f icant Among many X-ray pulsars, we have conducted so f a r most extended observation Hakucho observations revealed two remarkable i) The spin period started to slow down since 1979 and t h i s trend yr-' f o r the period ii) An up-down f luctuation of the spin period with a time scale o f i 220 ORIGINAL PAGE IS OF POOR QUALITY change i n the average X-ray luminosity through past several years. Would t h i s be due t o the change o f the d i rect ion o f accretion f low wi th respect t o the spin vector o f the neutron s tar? A r en t i re l y d i f f e ren t idea suggested by Makishima i s tha t the secular var ia t ion o f the pulsation period might due t o the Doppler e f fec t by the o rb i ta l motion o f the Vela X-1 binary around an unseen t h i r d body with a period o f the order Vela X-1 I I 1 1 I 1 t Q s3* CP A5 53 c0 08 BA 0 +oe COPefnlCS Arid-5 sns-3 COS-0 050-8 Balloon bkUCh0 t -I I 1 I I I 1 I 75 76 77 78 79 80 81 282.7L Fig.1 Histroy o f the pulsat ion period o f Vela X-1 10 years. hence implying a giant black hole. o f the neutron s tar revolut ion i s also observed f o r GX 301-2 (Kelley, Rappaport and Petre , 1980). The short-term up-down f luc tua t inn o f the pulsat ion per iod poses an in t r igu ing question. If the f luc tua t ion i s o f an external or ig in , the d i rect ion o f the accre- t i o n flow may change rather quickly. due t o the coupling and decoupling between the crust and the superf lu id i n t e r i o r such as proposed by Lamb, Pines and Shaham (1978). outside neutron stars by accurately monitoring the behavior o f X-ray pulsars. I f i t were so, the mass of the t h i r d body would then be roughly 100 Me, It i s worth not ing tha t a s imi lar slowing down I f i t i s o f an in ternal or ig in, t h i s may be The example o f Vela X-1 demonstrates tha t we can learn a l o t about inside and 3. X-ray bursters As a matter o f fact, one o f the main emphasis o f Hakucho was placed on the obser- vations o f X-ray bursts. D is t r ibu t ion o f burst sou-:es including those we found i n the general d i rect ion o f the galact ic center i s shown i n Fig, 2. A d i s t i n c t c luster ing towards the galact ic center i s evident. This d i s t r i bu t i on by i t s e l f suggests the Pop. I 1 nature o f burst sources, which i s strongly enhanced by the fac t tha t nearly h a l f as many burst sources are associated w i th globular clusters. radius of the neutron star. Owing much t o the SAS-3 group (Lewin and Joss 19811, it i s by now established tha t the "black-body" radius i s conserved during the burst decay and o f the order 10 km. We estimated the "black-body" r a d i i f o r several X-ray burst i s another outstanding phenomenon tha t neutron s tar exhibi ts. O f par t i cu la r importance o f bursts i s tha t one can i n pr inc ip le measure the I 221 I 1743: 1743-29 (P BURST SCURCES I Y GLOBULAR CLUSTERS 0 OTHEFC BURST SOURCES . - . . .. . . Fig.2 D is t r ibu t ion o f burst sources around the galact ic center burst sources, including three globular c luster sources, tha t are most probably near the galact ic center. They a l l come close t o 10 km w i th in 20%, implying tha t they are about the same size. I must caution, however, tha t i t i s too ear ly t o take t h i s number as the actual neutron s tar radius f o r t h i s requires correct i n te r - pretat ion o f the burst spectrum which i s yet the theme o f fu r ther investigations. Nevertheless, i t i s very important tha t we have means t o measure the neutron s ta r radius i n our hands. While, X-ray pulsars enable us t o measure the mass of neutron stars. We have sofar learned tha t every indiv idual burst source can produce a var ie ty o f bursts i n the time p ro f i l e , peak luminosity and t o t a l energy. The thermonuclear f lash model provided a persuasive p ic ture o f the burst phenomenon. There are, however, several observational facts that challenge theoret ical interpretations. i ) Successive bursts a t a very short in te rva l We observed three such events; 10 min. f o r 1608-52 (Murakami e t a l . 19801, 8 min. f o r Terzan 5 source (see Fig. 31, and 8 min. f o r MXB 1636-53. l h e SAS-3 group observed three bursts i n such short succession from a galactic-center source (Lewin e t a t , 1976). Such an in terva l i s de f in i te ly too short t o replenish the required nuclear fue l by the mass accretion. The nuclear fue l ought t o be hidden somewhere on the neutron s tar surface, but where? and how? are the b i g questions. 222 ORIGINAL PAdE 1s OF POOR QUALln ii) Possible supercri t ical luminosity A puzzling resu l t was obtained from examination o f the burst peak luminosities (Inoue e t a l . 1981). The peak 1 uminos i t y d is t r ibut ions for f i v e burst sources near the galactic center are shown i n Fig. 4. If the distance t o the galactic center i s taken t o be 10 kpc, the largest peak luminosity ob- served f o r each source large- l y exceeds the Eddington 1 i m i t f o r a 1.4 Me neutron star. This conclusior remains va l id unless the galactic center distance i s much smaller - m 1Q) e-nluv 2 6 6 8 w n Is di T I M E ( Y I N ) Fig.3 X-ray bursts a t 8 min. in terva l from - T e r s n 5. than 7 kpc. The same can be concluded f o r a Terzan 2 ourst (Grindlay e t a l . 1980) and f o r the bursts from NGC 6624 (Clark e t a l . 1976), the d is tawe t o the l a t t e r being p re t t y well estimated. can be exceeded by blowing o f f the neutron s tar atmosphere. However, the observational resu l t indicates tha t the "black-body" radius remains constant from the burst peak through the decay, excluding the expanding photosphere. Strong magnetic f i e l d can reduce the Thomson cross section, thereby increase the c r i t i c a l luminosity. This e f fec t i s s ign i f i can t only i f the f i e l d i s o f the order o r greater than 10'' Gauss. Why then does the d i s t i n c t difference i n the behavior and the spectral hardness between X-ray pulsars and X-ray bursters ex is t? We assess a great significance upon this problem o f possible supercr i t ica l 1 uminosi t y . There may very well be physics i n t r i n s i c t o the superdense photons. One might think tha t the c r i t i c a l luminosity Fig.4 Peak Flux ( e r g c d s d ) Dist r ibut ions o f burst peak luminosities f o r f i v e sources near the galactic center. I i 223 The rapid bursts from MXB 1730-335 are interpreted by Lewin e t a1 . (1976) as due t o the :hopped accretion f low caused by a cer ta in i ns tab i l i t y . Fortunately, we met i t s remarkable benavior i n August 1979 (Inoue e t al. 1980). By then, the rap id burster produced a long t r a i n o f bursts with trapezoidal shape. The length o f the f l a t top o f these trapezoidal bursts ranged from h a l f a minutes t o over 10 minutes. The famous l inear re la t i on between the burst size and the time t o the next burst s t i l l holds f o r these bursts, implying a reservoir o f the capacity i n excess of lo2' g o f matter. a trapezoidal burst, the luminosity as w e l l as temperature are kept constant. Hence, the emit t ing area i s also constant. This seems t o indicate that the trape- zoidat burst i t s e l f represents a stat ionary state. Another stationary state exists, which corresponds t o the low level ( 6 ~ 1/10 the burst peak) persistent emission tha t appears between two bursts. We, therefore, revise the p ic tu re o f the rap id burster tha t i t undergoes rap id switchings between the high and low states rather than a s ingle shot o f matter onto the neutron star. However, the mechanisms o f the reservoir, switching and cont ro l l ing the mass f low are key questions t o be answered by fu turo study. I n two o f the trapezoidal bursts, c lear osc i l la t ions were discovered. As shown i n Fig. 6, the power spectrum f o r each exhib i ts a prominent spike a t about 0.5 sec. The osc i l l a t i on does not appear as due t o the neutron s tar revolution, since the observed spikes show a structure and are s l i g h t l y but s ign i f i can t l y d i f f e ren t from each other, The o r i g i n o f t h i s o s c i l l a t i o n i s unknown. As can be seen i n Fig. 5, an important resu l t i s that, during t h f l a t top of Fig.5 Example o f tho trape- zoidal burst from the rap id burster. FREQUENCY (HER21 Fig.6 Power spectra f o r two trapezoidal bursts, f o r which a prominent spike i s seen. i ri I ? i d 1 i ! 1 1 i ! 1 t I ! I : t 224 ORIGINAL PAOE IS OF POOR QUALll'Y 4. Future sate1 1 i te program vations have put forward concerning neutron star and i t s system. Following Nakucho, we have two more approved X-ray astronomy missions, ASTRO-B and ASTRO-C. The task o f these missions i s t o bet ter understand the above-described problems and t o explore fur ther . vehicle a t ISAS has been steadi ly increasing. The present ISAS launcher, M-3S, i s capable o f launching a Q 250 kg s a t e l l i t e i n to a 500 km a l t i t ude c i r cu la r o rb i t . New development e f f o r t has started t o upgrade the launch power. l h e f i r s t step i s M-3S I 1 which can carry a payload w e r 400 kg f o r a 500 km orb i t . The second step, M-3S 111, would achieve substai1,ial increase of the launch capabt l i ty over tha t of M-3S 11. So far, I have b r i e f l y covered the c ruc ia l problems tha t the Hakucho obser- Since the time o f the Hakucho launch, the capabi l i ty o f the s a t e l l i t e launch Fig. 7 i l l u s t r a t e s the current mission plan a t ISAS. are approved misr.ions t o date, whereas those wi th dashed c i r c l e s are yet t o be approved. Mzin features o f ASTRO-B and ?qTRn-C are l i s t e d i n Table I , i n comparison with those of Hakucho. ware i s presently under fabr icat ion. The main instrument o f ASTRO-B i s gas sc in t i - 2 l l a t i o n proportional counters with the t o t a l area o f about 9dOO cm . Major increase Those wi th so l i d c i r c les ASTRO-8 weighing 220 kg i s due for launch i n ear ly 1983, and the f l i g h t hard- ' Fiscal Year (Apr.4 1979 I1980 ' 1981 I 1982 1 1983 I 1984 I 1985 119% 1 1 9 7 EXOS-Series I STP-Plasma I ' I ' 1 I 'X-Ray" :- I '- Test s a t e l l i t e s CVGT L -ge S a t e l l i t e s OPEN-J / \ A ,\\"X-Rayn I - 1 -Lb(- \ I V \ I -. - ._ .- - x- -- ---- - A SOP-2 i'\ Tether 0 SE PAC f"? I In ternat lonal 1 Col labotat ion SEPAC SL- 1 ', ? I RTJ Other SPACELAB ?xperiments i Fig.7 Current mission plan a t ISAS for 10 years. The ser ia l number o f s c i e n t i f i c s a t e l l i t e , miss% name and launch vehicle are ind i - cated fo r each. PAGE 1s w p o o f t v 225 as coatpared to Hakucho i s made i n the sens i t i v i t y as w e l l as i n the spectral reso- lution. ASTRO-B w i l l consequently provide us with much bet ter temporal and spectro- scopic infomation about the galactic sources than we are having from Hakucho. As a s ign i f icant feature, the energy resolut ion c f the gas s c i n t i l l a t i o n proportional counter which i s twice that of the conventional proportional counter would enable us to study the spectral de ta i l and in par t i cu la r w i s s i o n l i nes o r absorption features. ASfRo-C i s the Coltow-on X-ray astronomy s a t e l l i t e scheduled f o r launch i n 1986-87. f f 2 spacecraft w i l l be approximately 400 kg i n weight and 3-axis stabi- lized. A t present, we are i n the biginning of the prototype design phase. The slain instrument w i l l be low-background proportional counters w i t h the to ta l area 2 of about 5000 an . Gas s c i n t i l l a t i o n proportional counters may well be a pa r t o f it. An al l-sky monitor w i l l also be included. The primary mission objective of ASTRO-C i s the study of the time var iab i l i t y . b i n euqhasis here i s not only accurate timing analysis o f the galactic sources but the xrutiny o f the time v a r i a b i l i t y o f act ive galactic nuclei. Exoticism o f act ive galactic nuclei i s inwense but yet l i t t l e i s explored. We consider that study o f the time va r iab i l i t y i s o f cruc ia l importance o f the physics involved i n the power house problem. We hope that the ASTRO-series missions with the improved M-launcher w i l l Table I . Main features o f Hakucho, ASTRO-B and ASTRO-C. HAI(u;Ho Asm-B ASTRO-C ~ - ~~~ ~ ~ ~~~ ~ UUKH 1979 1983 1906-7 - _I WEIW (kg) % 220 .\r 400 MII ImTw. PROP. C W E R G.S.P.C. P.C. rmr1-cyr 1 EFFECTIVE 6" x 6" 80 cmz 3" x 3" 1000 c192 z 1' x 3" z5OOo 02 2 AREA 170 17" m tll SC IERTIFIC L. HIDL F.O.V. RElc 0 SPECTRAL All) 0 TINE bARIATION OBJECTIVES BURST SURVEY TEMPOPAL VARIATIOIl OF EKTRAGALACTIC 6 - IIDI1ITORING SEVE- OF W T I C SOURCES U U A C T I C SOURCES RAL SOURCES AT A O W E = 10 I AT 6 keV 0 0.5 UJy SOURCES T Int POWERFUL FOR I N 10 HIN. > 300 GMAXIES 1m L I N S z 0 SOURCE LOCATION c#c 0.5" CYCLOTRON L I R S 0 S I Z E OF ACTIVE FMC 0.05" 1-50 keV I N GALACTIC W L E I 128 CH / 0.5 S 32 CH / 0.125 S STRUCTURE OF MQIIERIITE STARS --Awl IL IART 0 XMCIING P.C. 0 1-DIM. HADAWAW) TEL. ' ALL SKY MWIITOR > 100 4 y - STERADIrn -I_-_-- --_ t 1. 70 c d x 2 2 I m m m s 35m n 2 1.7' x 37' zoo x 20" Q WR-Cey X - M Y 0 smrffi P.C. 1co C J x 2 ~ c o ~ n ~ ~ 20- 1 2 #wc I TOR 80 CUI2 x 2 P 0 I -DIM. REFLECTOR 2 226 continue through 1990's. Besides th is stream, we also envisage t o carry out a larger X-ray astronomy mission than the planned capabil i ty of the !+launcher. This mission was or ig inal ly named the Cosmic X-ray and --ray Telescope (CXGT). Over past years, thc concept o f CXGT has evolved. As X-ray astrophysics advances, more and more sensi t iv i ty and spectroscopic resolution are inevitably needed. It i s therefore logical t o consider a focusing optics f o r CXGT. Finally, I would l i k e t o mention that the government level agreement on the U.S.-Japan col laboration programs i n space science including X-ray astronomy was made. We very much hope t o enhance th t i e through possible collaborations in th is f ie ld . References Clark, G.W., Jernigan, 6.. Bradt, H., Canizares, C., Lewin, W.H.G., Li, F.K., Mayer, W. and McClir.tock, J.E. 1976, Ap.J. ( letters), 207, - L195. Grindlay, J.E., Marshall, H.L., Hertz, P., Soltan, A., Ueisskopf, M.C., Elsner, R.F., Ghosh, P., Darbro, W. and Sutherland, P.G. 1980, Ap.3. (Letters), - 240, 1121, Inoue, H., Koyama, K., Makishima, K. e t al. 1980, Nature, - 283, 358. !noue, H., Koyama, K., Makishima, K. e t al. 1981, Ap.3. (Letters), i n press. Kelley, R., Rappaport, S. and Petre, R. 1980, Ap.J., - 238, 699. Lamb, F.K., Pines, D. and Shaham, 3. 1978, Ap.J., 224, %9. Lewin, W.H.G., Hoffman, J.A. and Doty, 3. e t a l . 1976, H.N.R.A.S., - 177, 83. Lewin, W.H.G. and Joss, P.C. 1981, Space Science Rev., 28, 3. Wjrakami, T., Inoue, H., Koyama, K. e t al. 1980, Publ. Astron. Soc. Japan, - 32, 543. Nagase, F., Hayakawa, S., Kunieda, H. e t a l . 1981, Nature, - 290, 572. Nagase, F. 1981, ta l k presented a t 15th ESLAB Symposium on X-Ray Astronaqy, Rappaport, S. and Joss, P.C. 1981, 'X-Ray Astronomy", ed. R. Giacconi, Proceedings June 1981, k te rdam. o f the HEAO-AAS Meeting, January 1980, Cambridge. THE LAMAR: A HIGH THROUGHPUT X-RAY ASTRONOMY FACILITY FOR A MODERATE COST MISSION Paul Gorenstein and Daniel Schwartz Harvard/Smithsonian Center for Astrophysics Cambridge, MA 02138 Presented at t h e Workshop on X-ray Astronomy Goddard Space Flight Center 5-7 October 1981 ! I 1 . 0 INTRODUCTION Large co l l ec t ing power is e s s e n t i a l for many of t h e object ives or' f u t u r e programs of X-ray astronomy. I n order t o carry out as t rophysical inves t iga t ions d i r e c t l y i n X-rays, a m i n i m u m number of photons must be obtained. I t is evident t h a t good angular resolut ion is needed t o recognize and image d i f f u s e sources, obtain precise posi t ions, reduce background, and avoid confusion. The s p e c i f i c ro l e of t h e h igh throughput instrument is t o c o l l e c t a s u f f i c i e n t number of photons from sources, both yoint-l ike and extended, t o permit an in-depth study of individual ob jec ts and group proper t ies t o be accomplished w i t h i n a reasonable time without t h e l imi t a t ions of background, confusion, and shot noise. The s t u d i e s include: measurements of f l u x e s and luminosity functions, s t u d i e s of morphlogy and imaging of low surface brightness fea tures , temporal var ia t ions , and spec t r a l 2ropert ies . To achieve a good measurement of these parameters, e.g. t o charac te r ize a temporal va r i a t ion i n quan t i t a t ive terms or measure a temperature gradient w i t h precision, t h e measurements mus t of n e c e s s i t y be made a t a h igh level of s ignif icance, t yp ica l ly t e n s of 3, which is well beyond t h a t merely required t o es t ab l i sh t h e existence of a source. A value for effective area t h a t represents a reasonable goal fo r fu tu re h igh throughput imaging instruments is about l o 4 cm2. T h i s is approximately 50 times t h a t of t h e E i n s t e i n Observatory and 10 times AXAF. A reasonable goal for t h e angular resolut ion is about a minute of a r c (diameter of c i r c l e containing 50% of t h e power). With t h i s combination of la rge area and angular resolut ion, t h i s instrument would be u n i q u e i n .. -e-- 229 capabi l i ty when compared t o a l l other mission concepts under s t u d y . The Large Area Modular Array of Reflectors (LAMAR) is a prac t i ca l approach for achieving t h i s desired combination of large e f f e c t i v e area and good anqular resolution. For imaging purposes, t h e s e n s i t i v i t y of an array of i den t i ca l modules (imaging telescopes and de tec tors ) is equivalent i n every respect t o t h a t of a s i n g l e long telescope w i t h t h e same t o t a l aper ture and angular resolution. Given t h e appropriate manufacturing technology fo r mass production, t h e array of modules a r e much eas i e r and l e s s cos t ly t o f ab r i ca t e than a monolithic telescope. There is no need for precise co-alignment of t h e modules. Furthermore, t h e modular approach r e s u l t s i n more eff ic ient use of the avai lable volume on any spacecraf t because t h e viewing aperture can be the la rge area s ide of t h e spacecraft . I n t h e discussion of s c i e n t i f i c object ives and estimated performance, the name LAMAR is used i n a g e n e r i c s e n s e . I t covers a number of technical approaches for both t h e mirrors and de tec tors , i n c l u d i n g several described a t t h i s workshop, t h a t a r e capable of be ing made i n t o an array of lo4 cm2. Different choices f o r the mirror and detector technology providz varying degrees of e f f ec t ive area, angular resolgt ion, f i e l d of view, short wavelength response and energy resolut ion No one mirror-detector system is optimum i n a l l respects. The key t o low cos t , namely remaining w i t h i n t h e l i m i t s of a moderate cost Explorer :nission, is adopting a s ing le set of technologies for t h e mirrors, de tec tors , and d i s p e r s i v e elements, i n pa r t i cu la r technologies which a re amenable t o mass production methods a t low u n i t cost. Also, the technique should not require so special ized 2 30 sZills or t oo l s t h a t t he re would be d i f f i c u l t y , i r respec t ive of cost, of producing t h e required number of modules i n a reasonable in t e rva l of time. A moderate cos t approach is discussed i n Sect. 4. I n t h e discussion t h a t appears i n Sect. 2 , a h i g h throughput X-ray imaging instrument w i t h energy resolut ion is shown t o be an important t oo l for s t u d y i n g key problems i n ga l ac t i c and ex t raga lac t ic astronomy. In so doing, t h e basic parameters assumed fo r the system are: SOX E i n s t e i n 1 O X AXAF 4 2 effective area (mirror): 10 c 5 a t 2 k e V 2 x 10 cn? a t 5 keV resolut ion (SO% f l u x diameter): 1 arcminute or be t t e r on ax i s f i e l d oL view: 1 degree For object ives involving moderate resolut ion X-ray l i n e spectroscopy (E /AE 2 100) t h e use of object ive grat ings is assumed, i n pa r t l cu la r , r e f l ec t ion gra t ings forward of t h e mirrors a s discussed i n Sect. 3. LAMAR is un ique w i t h respect t o any other mission under discussion. Its combination of l a rge co l lec t ing a rea and good angular resolut ion is not para l le led by any past mission nor any fu ture f a c i l i t y t h a t is being planned. I t co l lec t ing power represents an improvement over t h a t of the E i n s t e i n Observatory, as well a s t h a t of EXOSAT and ROSAT, t h e n e x t two telescope missions i n X-ray astronomy, by about two orders of magnitude. W i t h respect t o fu tu re missions, it has an order of magnitude la rger co l lec t ing power t h a n AXAF, as well a s f a c i l i t i e s t h a t a r e being discussed by the European Space Agency and Japan. LAMAR 1 can address scient i f ic objec t ives i n many areas of both g a l a c t i c and ex t raga lac t ic astronomy. It is not r e s t r i c t e d t o t h e s t u d y of one aspect of X-ray sources nor t o a s i n g l e c l a s s of objects. I t can obtain r e s u l t s on many objec ts i n a time t h a t is shor t compared t o any other f a c i l i t y . Being ab le t o obtain r e s u l t s i n a reasonably shor t time is essential because many s tud ie s a r e simply impractical i f t h e observer requires more than about two years t o carry out h i d h e r program. Thus, LAMAR can serve t h e s c i e n t i f i c community i n a very general way and can accommodate t h e needs of a very la rge number of users. 2 .0 EXPECTED PERFORMANCE OF LAMAR We consider t h e performance of a LAMAR i n several hypothetical observations relevant t o t h e fundamental problem areas of astronomy and astrophysics t h a t were described i n t h e theore t ica l presentat ions during t h e f i r s t day of t h i s workshop. These areas are: (1) Cosmology, t h e X-ray Background, and Large Scale S t ruc ture of t h ? fh ivczse (2 ) Clusters of Galaxies and Their Evolution ( 3 ) Quasars and Other Active Galact ic Nuc le i ( 4 ) Compact O b j e c t s i n Our Galaxy ( 5 ) S t e l l a r Coronae (6 ) Energy Input t o t h e I n t e r s t e l l a r Medium Examples a r e considered i n each of these subject areas. 232 2.1 Cosmology LAMAR can s t u d y the overa l l isotropy of the s k y i n t h e redshi f t range z 1 t o 3 by measuring t h e 2-4 keV background. By s tudy ing apparent volume d e n s i t y vs . z for several c l a s ses of ex t raga lac t ic X-ray source, LAMAR may allow an independent estimate of 9,. Studies of evolution can be car r ied out by obtaining complete X-ray selected samples ( w i t h de ta i led X-ray information) of Seyfer ts , quasars, BL Lac objec ts and c l u s t e r s of galaxies t o r e d s h i f t s z 2 0.S t o 3. Features i n t h e X-ray background may reveal la rge sca le s t ruc tu res , and e v e n t s associated w i t h t h e i n i t i a l formation of c l u s t e r s or of galaxies i n c lus t e r s . The X-ray background above a few k e V is f r e e of g a l a c t i c e f f e c t s and offers t he best chance t o measure t h e isotropy of t h e universe over the e n t i r e s k y on sca les between 1 de92 and 1 2 hours, and i n t h e redshi f t i n t e rva l of z " L t o 3 from which t h e b u l k of t h e X-ray background probably or iginates . As pointed out by Fabian a t t h i s Workshop, t h e X-ray background is sens i t i ve t o the s t ruc tu re of the u n i v e r s e on a s ca l e of 10-100 Mpc. The fundamental l i m i t t o measuring t h e isotropy a r i s e s from source confusion noise due t o t h e X-ray source counts N O SI = 2.7 x 0.3 t o 3.5 k e V . Focusing instruments a r e e s sen t i a l i n order t o eliminate individual d i s c r e t e sources t o a s low a level a s possible while integrat ing the d i f fuse background f l u x i n t h e remainder of the f i e l d of view. I n an observation of 2000 seconds, LAMAR can el iminate sources dawn t o a leve l of 10-14 erg/cm2 s. Scaling from measured background r a t e s i n t he S-3/2 s t e r - l , where S - ergs/cm2 s 233 E i n s t e i n Obse rva to ry , we estimate t h a t i n t h i s time, about 6000 c o u n t s from d i f f u s e X-ray background (2-4 keV) and 4 x 104 c o u n t s a t most of non-x-ray background accumula te i n 1 deg2. Source c o n f u s i o n n o i s e ove r t h e 1 deg2 is expec ted t o be 350 coun t s . Therefore, t h e t o t a l u n c e r t a i n t y due t o s ta t i s t ica l f l u c t u a t i o n is 7%. S y s t e m a t i c e r r o r s i n t h e non-X-ray background c a n probably be reduced t o l%, therefore, t h e practical l i m i t 1s about 10% f o r a s i n g l e 1 deg2 f iel?. FGC t h o s e o b j e c t i v e s r e q u i r i n g smaller u n c e r t a i n t y , t h e o b s e r v a t i o n of many f i e l d s w i l l reduce this u n c e r t a i n t y c o n s i d e r a b l y . independent f i e l d s would b r i n g it down t o 0.1%. T h i s would be feasible i n s e v e r a l y e a r s of LAMAR o p e r a t i o n , and would occur n a t u r a l l y i n t h e c o u r s e of normal obse rv ing . Such p r e c i s i o n is e s s e n t i a l l y u n o b t a i n a b l e w i t h an i n s t r u m e n t having l ess a r e a . Measurement of - l o 4 Based upon L A . - a ' s h igh throughput and i t s a b i l i t y t o s t u d y many sourcesI it is p o s s i b l e t o d e f i n e v a r i o u s cosmolog ica l tests. For example, it has been p o i n t e d out t h a t t h e a p p a r e n t volume d e n s i t y VS. r e d s h i f t depends on 4, more s t r o n g l y t h a n t h e classical tests of a p p a r e n t o p t i c a l magni tude o r a p p a r e n t s i z e vs . r edsh i f t . 1 A t t h a t time on ly c l u s t e r s of g a l a x i e s were known t o be u b i q u i t o u s e x t r a g a l a c t i c s o u r c e s . Now, w e might a p p l y t h e test independen t ly t o S e y f e r t s or BL Lac o b j e c t s . A l l of these classes have l u m i n o s i t y f u n c t i o n s of roughly Mpce3 f o r L > 1 0 4 4 e r g / s . Although r e d s h i f t s would have t o be o b t a i n e d by op t ica l meansI t h e power of LAMAR is t h a t t h e samples can probably be d e f i n e d comple t e ly based p u r e l y on X-ray p r o p e r t i e s , e.g., f i n i t e spat ia l e x t e n t f o r c lus te rs of g a l a x i e s , a power law spectrum of index 0.5 t o 0.; f o r S e y f e r t s ( c f . 234 Mushotzky e t a l . l980),2 and a s o f t X-ray excess and time v a r i a b i l i t y for BL Lac objects. To detect "100 objec ts i n a range of Az = 0.1 a t z = 1 requires 1000 f i e l d s of 1 deg2 observed for 2000 sec each. The LAMAR s e n s i t i v i t y w i l l be t o erg/cm2 s, or L = erg/s , i n t h i s time. 6f course, any such measurement of cosmological parameters m u s t simultaneously address t h e p o s s i b i l i t y of source evolution. Use Of t h e c l a s s i c a l tes t w i l l prevent u s from being fooled, even i f t h e test were inconclusive. However, i n X-rays w e have a u n i q u e p o s s i b i l i t y of deconvolvinq t h e evolution because of the rigorous constraint of not exceeding t h e X-ray background. 2.2 Clusters of Galaxies and Their Evolution The r e s u l t s of t h e E i n s t e i n Observatory have confirmed our expectations based upon previous r e s u l t s t h a t X-ray observations a r e an important source of new information corxerning c l u s t e r s of galaxies. Clusters of galaxies a r e e a s i l y i d e n t i f i e d i n X-rays a s a d i f f u s e source w i t h kT above 2 keV. The X-ray p i c tu re of c l u s t e r s may e x h i b i t considerable s t ruc ture . I n f a c t , t h e X-ray s ignature of a c lus t e r of galaxies is so c h a r a c t e r i s t i c t h a t new c l u s t e r s of yalaxies w i l l probably be found much more eas i ly i n t h e fu ture by X-ray measurements than by opt ica l . The E i n s t e i n Observatory was able t o study r e l a t ive ly nearby c lus t e r s . I t es tabl ished t h a t there e x i s t s a d ive r s i ty of morphological types ( c f . , Forman and Jones, A.R.A.A. surpris ing preponderance of c l u s t e r s w i t h complex s t r u c t u r e and asymmetry was found i n comparison t o c l u s t e r s w i t h more smoothly d is t r ibu ted gas, such as t h e Coma Cluster. Subsequent op t ica l 1982) . 3 A 235 measurements have tended t o show t h a t t h e d i s t r i b u t i o n of galaxies is indeed cor re la ted w i t h t h e increases i n X-ray surface brightness a s i n Abel l 2O6go4 A s explained below, X-ray measurements a r e po ten t i a l ly a good method for mapping t h e d i s t r ibu t ion of mass w i t h i n a c l u s t e r , a s well as for determining t h e t o t a l mass of c lus te rs . However, t h e E i n s t e i n Observatory d i d not have the throughput required t o map t h e temperature and temperature gradient of t h e c l u s t e r gas. Thus, it could not provide much d i r e c t information per ta ining t o t h e measurement of c lus t e r masses and t h e i r d i s t r i b u t i o n insi ' fe t h e c lus t e r . While t h e E i n s t e i n Observatory produced important r e s u l t s on the morphology of several dozen individual c l u s t e r s , i ts sample of d i s t a n t c lus t e r s was t o o small for r e s u l t s on c lus t e r formation and evolution t o be conclusive. The E i n s t e i n morphological s t u d i e s reveal c l u s t e r s of d i f f e r e n t ages, a t t h e current epoch (c f . Forman and Jones 19821.3 We would l i k e t o ,.udy c l u s t e r s of d i f f e r e n t age a t a var ie ty of cosmological epochs back t.,, z = 1. It is not obvious a p r i o r i what is required t o perform the morphological s tudy . From an examination of t h e E i n s t e i n X-ray images, it seems t h a t c l u s t e r s w i t h 5 x 5 p ixe l s above the 3 a contour and w i t h a peak contour of a t l e a s t 100 signif icance can be q u a l i t a t i v e l y c l a s s i f i ed . With an estimated 3/4 t o 1 arcmin resolut ion, a 104 of a c lus t e r t o r edsh i f t s of z = 0.2 (L, 2 3 1043 e r g / s ) , z 0.5 (L, 2 1044 e r g i s ) and z - 1.0 (Lx 2 3 x 1044 erg/s. second LAMAR observation w i l l g i v e a morphological p i c tu re LAMAR is t h e key i n s t r u m e n t i n t h i s regard because c l u s t e r s 2 36 of galaxies a r e extended X-ray sources and moderate angular resolut ion is adequate for carrying out many of t h e objectives. Figure 1 is an X-ray contour p lo t of Abell 2069 a s observed by t h e Imaging Proportional Counter of t h e E i n s t e i n I n addi t ion t o t h e t h ree pr inc ipa l condensations, LAMAR would see gas of much lower surface brightness. Superimposed upon t h i s diagram is a 1' x 1' gr id representing c e l l s i n which temperature could be measared. observing time t h e temperature could be measured i n each of those c e l l s t o an accuracy of be t t e r t h a n 13%. A2069 is a t z = 0.12. A t 2 or 3 times t h i s d is tance, according t o ce r t a in models of c l u s t e r evolution, we should be able t o detect systematic differences i n luminosity, morphology and temperattire d i s t r i b u t i o n between those objec ts and the r e l a t i v e l y nearby c l u s t e r s , and t h u s detect t h e manifestations of c lus t e r evolution. O b s e r ~ a t o r y . ~ Simulations ind ica te t h a t i n 1 0 4 seconds of Mass of Galaxies and Clusters of Galaxies Under conditions of hydrostat ic equilibrium .lot gas w i l l be d i s t r ibu ted i n a g rav i t a t ion po ten t i a l according t o the expr e s s ion : Thus , measurements of loca l temperature, t h e temperature gradient , and the d e n s i t y gradient w i l l provide a good determination of M(r), the t o t a l mass i n t e r i o r t o r. The r e s u l t is ra ther unambiguous i f the condition of isothermal equilibrium can be es tab l i shed . Galaxies i n c l u s t e r s , pa r t i cu la r ly i f they 237 238 are r e l a t i v e l y s t a t i o n a r y such as #87 i n Vi rgo , are i n a p o s i t i o n t o have gaseous halos and c a n be expec ted t o be i n a c o n d i t i o n of h y d r o s t a t i c e q u i l i b r i l m . With t h e E i n s t e i n Obse rva to ry , it was possible t o measure a mass of ~ i 8 7 . ~ A LCWAR would have SOX e f f e c t i v e area of t h e E i n s t e i n Obse rva to ry (even more above 3 keV) and t h u s much better c a p a b i l i t y for measuring t e m p e r a t u r e and t e n p e r a t u r e g r a d i e n t s . Consequent ly , it w i l l be able t o de te rmine masses for many more g a l a x i e s . With these r e s u l t s , w e can s t u d y t h e r e l a t i o n s between dark and luminous matter for g a l a x i e s of v a r i o u s types and i n v a r i o u s environments . t h e By t h e same method, t h e t o t a l masses of c l u s t e r s of g a l a x i e s and groups of g a l a x i e s c a n be s t u d i e d w i t h LAHAR. T h i s is beyond t h e c a p a b i l i t y of t h e E i n s t e i n Obse rva to ry because of t h e rapid f a l l of i ts e f f e c t i v e area above 3 keV. As kT of many c l u s t e r s are t y p i c a l l y 5-8 keV, good re sponse i n t h a t ene rgy range is needed t o measure t e m p e r a t u r e g r a d i e n t s and consequen t ly t o d e r i v e t h e i r masses. 239 Large Scale S t r u c t u r e o f t h e Universe One of the i m p o r t a n t o b j e c t i v e s i n astronomy is to s t u d y t h e s t r u c t u r e of t h e u n i v e r s e on v a r i o u s s i z e scales and to s e a r c h for other c l u s t e r i n g h i e r a r c h i e s . There is r e a s o n to bel ieve t h a t X-ray measurements w i l l p rove to be v e r y u s e f u l fo r t h i s s tudy . A. Fabiar. h a s p o i n t e d o u t a t t h i s workshop t h a t d i f f u s e X-rays may be t h e best p robe of t h e u n i v e r s e i n t h e r a n g e 10-100 Mpc. The f a c t o r of 50 larger th roughpu t of LAMAR compared t o t h e E i n s t e i n Obse rva to ry and even more above 3 keV w i l l e n a b l e it t o examine r e g i o n s of much lower s u r f a c e b r i g h t n e s s which e x t e n d f u r t h e r from t h e c l u s t e r c e n t e r . I n p r i n c i p l e , t h i s w i l l permit t h e search for s t r u c t u r e on a larger scale t h a n c l u s t e r s . As X-ray measurements (> 2 keV) a r e so s p e c i f i c t o e x t r a g a l a c t i c o b j e c t s , LAMAR is expec ted to f i n d a l a r g e number of new e x t r a g a l a c t i c objects. The d i s t r i b u t i o n of some c a t e g o r y or sub-ca tegory may e x h i b i t a s t r u c t u r e o r c l u s t e r i n g h i e r a r c h y t h a t is n o t a p p a r e n t i n v i s i b l e l i g h t a g a i n s t t h e opt ical background of s ta rs and normal g a l a x i e s . ?or example, t h e tendency t o form s u p e r c l u s t e r s may be more apparent i n t h e c o r r e l a t i o n or lack t h e r e o f between r e g i o n s of d i f f u s e X-ray e m i t t i n g gas t h a n i n g a l a x i e s . S t r u c t u r a l f e a t u r e s of l a r g e scale may be e v i d e n t i n s t u d i e s of t h e X-ray background. Recent p a p e r s s u g g e s t two possible mechanisms t h a t may indeed r e su l t i n l a r g e r scale s t ruc ture . A 100 Mpc vo id has been r e p o r t e d i n Bootes (Ki r schne r e t a l . 19811.6 I f t h e X-ray background is l i n e a r l y p r o p o r t i o n a l t o t o t a l mass a long t h e l i n e of s i g h t out t o t h e Hubble d i s t a n c e of , I 240 say 4000 Mpc (i.e, t h e sum of a l l ac t ive g a l a c t i c n u c l e i and possible hot gas) , t h e n a 100 Mpc void would be detectable as a diminution i n t h e X-ray background over t h e e n t i r e lo f i e l d , 100 Mpc 4000 Mpc = 0.025 Source confusion noise may make it impossible t o detect an effect of t h i s amount i n any one f i e l d b u t t h e observation of many f i e l d s should be revealing of s t r u c t u r e of t h i s magnitude. Another possible source of ' Jariations i n t h e X-ray background has been suggested by Ostriker and CowieO7 I n t h e i r picture , t h e agent of galaxy formation is mult iple supernova explosions i n t h e ambient gas occurring a t t h e epoch z = 5. The process of galaxy formation i n t h e i n t e rga lac t i c medium is analogous t o t h a t of s t a r formation i n our own galaxy. A consequence of t h i s process is t h e c rea t ion of hot cav i t i e s ; T = 1 O * K , w i t h a diameter of 15 Mpc. Their surface b r i g h t w s s is about 2 x lo-* ergs/cm2-sec-ster ( i . e . , equal t o t h e i so t rop ic background) and angular s ize about 15' t o 30'. T h i s should lead t o s ign i f i can t var ia t ions i n t h e X-ray background w i t h i n t h e LAMAR lo x lo f i e l d of view t h a t a r e e a s i l y detectable. The Einstein Observatory, a s well a s EXOSAT and ROSAT, do not have s u f f i c i e n t throughput above 2 k e V t o de tec t va r i a t ion i n t h e ex t r aga lac t i c X-ray background. t 2.3 Quasars and Other Galaxies w i t h Active N u c l e i ',ne of t h e s ign i f i can t fea tures of X-ray measurements is t h e i r s p e c i f i c i t y for ex t raga lac t ic ob jec ts fo r data above 2 k e V I 24 1 i n energy and above 20° i n g a l a c t i c l a t i t udes . X-ray images a r e dominated by point-l ike a c t i v e g a l a c t i c nuclei and t h e d i f f u s e emission of c l u s t e r s of galaxies. I n pa r t i cu la r , measurements by LAMAR would r e s u l t i n f i n d i n g a la rge number of new quasars. Optical observations would s t i l l be needed t o determine r edsa i f t s , but t h e s e l ec t ion process based upon X-ray cha rac t e r i s t i c s would make t h a t a r e l a t ive ly straightforward and routine procedure a s t h e sample should be r e l a t i v e l y f r e e of ga lac t i c objects. The key p o i n t about LAMAR is that it would allow s t u d i e s and c l a s s i f i c a t i o n of quasars according t o t h e i r X-ray cha rac t e r i s t i c s , s u c h a s luminosity, spec t r a l index, and possible X-ray emission or absorption l ines , and temporal benavior. Large throughput is needed t o obtain a level of s ignif icance much beyond t h a t required t o merely e s t ab l i sh existence. I n t h a t regime, Temporal Behavior The s t u d y of temporal va r i a t ions promises t o be a very important diagnostic t oo l for ac t ive g a l a c t i c n u c l e i a s described by A. Lightman a t t h i s symposium. There a r e theore t ica l reasons t o believe and considerable observational evidence t o ind ica te t h a t ac t ive g a l a c t i c n u c l e i w i l l e x h i b i t f a s t time va r i a t ions i n t h e i r X-ray f l u x . LAMAR is e s s e n t i a l for observing temporal var ia t ions i n a l l b u t the few very br ightes t and nearest objects. Imaging is necessary for removing background and avoiding source confusion. Figure 2 is a simulation of a type of temporal var ia t ion expected t o occur of ten i n a QSC? (c.f. t h e energy source is accret ion onto a black hole. A O S 0 of Lightman) if 1 i 242 0 * I 1 + r cu k ~i I t .& Q, 0 II) v) m rrl Y. s C 0 l l R) 0 3 h) cu cu I u 0 c) cu .b ORIGINAL PAGE IS OF POOR QUALITY -1 243 300C 200( I OO( SIMULATED OBSERVATION OF OX I69 (after Tononboum et ol., 1979) i i € 0 BACKGROUND RATE 10000 30000 50000 UT SECONDS OF DAY X Figure 3 244 moderate X-ray i n t e n s i t y has been se lec ted for simulation. Sinusoidal periods of decreasing length correspond t o a s e r i e s of th ree f i n a l o r b i t s around t h e nuc leus . A more spectacular temporal var ia t ion is simulated i n Figure 3. T h i s is based on an actual E i n s t e i n Observatory measurement of a f a i n t quasar i n which only 47 counts were obtained. There is an indicat ion of la rge v a r i a b i l i t y i n a shor t time.8 The coupling of spec t r a l measurements t o temporal w i l l provide even more diagnost ic information about t h e environment surrounding a n ac t ive g a l a c t i c n u c l e u s . There is evidence from a combination of Ariel 5 and E i n s t e i n Observatory data for systematic co r re l a t ions between t h e t o t a l X-ray emission from Seyfer t galaxies and t h e r a t i o i n f l u x between a soft (0.5-1 keV) and a harder component (2-10 k e V ) ( E l v i s and Lawrence 1981Io9 Variable i n t r i n s i c absorption of s o f t X-rays plays a ro l e i n t h e correlat ion. LAMAR w i l l be ab le t o carry out spectral/temporal s t u d i e s i n d e t a i l for many objects. 2.4 Compact Objects i n t h e Galaxy One of t h e g rea t contr ibut ions of X-ray astronomy t o astrophysics is t h e discovery of c lose binary sys tems containing compact objects t h a t e x h i b i t a remarkable range of time var ia t ions of intensi ty . These sys tems present an opportunity t o s t u d y t h e behavior of neutron s t a r s and other compact ob jec ts i n an environment where t h e y a r e accret ing subs t an t i a l streams of matter. The X-ray Timing Explorer (XTE) w i l l be undertaken a s a dedicated low cost mission t o s t u d y these sys tems us ing la rge area non-imaging detectors . While XTE is expected t o produce 245 s ign i f i can t r e s u l t s based upon observations of t h e hundred or so most intense g a l a c t i c ob jec ts , we can already a n t i c i p a t e the need for follow-on inves t iga t ions w i t h a more sophis t icated instrument t h a t can deal w i t h f a i n t e r objects. W i t h i ts imaging capab i l i t y and large area, LAMAR w i l l be ab le t o observe much f a i n t e r objects because background e f f e c t s and source confusion a r e eliminated. Thus , it can address a var ie ty of new systems s u c h as neutron s t a r s i n other environments, a s well a s cataclysmic var iables containing degenerate dwarfs. Neutron $3- The greater s e n s i t i v i t y of t h e LAMAR extends t h e scope of neutron s t a r invest igat ions considerably by increasing t h e number of channels of ooservation. They a r e described below. (1) Compact Objects i n Supernova Remnants A number of supernova remnants have been shown by t h e Einstein Observatory t o contain compact X-ray e m i t t i n g objects. These inc lude WSO (SS433), 6109-1, RCW103, t h e Vela SNR, and others. A l t h c u g h t h i s co l l ec t ion of ob jec t s represents a mixture of X-rays from accretinq binary s y s t e m s , synchrotron accelerat ion of p a r t i c l e s and hot neutron s t a r surfaces , t h e y a r e a s a group rather young neutron s t a r s . They may present aspects of neutron s t a r behavior t h a t a r e differe.1, ,ran compact b inar ies which a r e generally much older objects . ( 2 ) W i o P a a r s The Columbia University group has detected X-ray ?mission f tom several radio pulsars w i t h the Imaging Proportional Counter of the Einstein Observatory (Helfand 1981.) . l o The 246 o b j e c t s seem t o be s p a t i a l l y ex tended i n d i c a t i n g t h a t X-rays are prodwed as a r e s u l t of a c t i v e p a r t i c l e a c c e l e r a t i o n as in t h e Crab Nebula and Vela. P u l s a r s a r e i s o l a t e d n e u t r o n s t a r s and r e p r e s e n t objects i n t e r m e d i a t e i n age between those found i n supernova remnants and t h o s e i n compact b i n a r y systems. With 50 times t h e throughput of t h e E i n s t e i n Obse rva t ry , LAMAR shoould be a b l e t o s t u d y many p u l s a r s . ( 3 ) Possible Pers i s ten t X-ray Sources a t Gamma Ray B u r s t Pos i t i o n s S e v e r a l models f o r gamma r a y b u r s t s (e.g. Woosley and Wallacelll p r e d i c t t h a t t h e r e w i l l be p e r s i s t e n t X-ray emission. There is marg ina l ev idence for such a cor respondence i n an E i n s t e i n Obse rva to ry o b s e r v a t i o n . I f t h e f l u x e s are w i t h i n an o r d e r or two of magnitude of p r e d i c t e d , t h e n t h e catalog of p r e c i s e gamma r a y b u r s t p o s i t i o n s can be examined w i t h LAMAR for a s tudy of h i g h l y magnet ized n e u t r o n s ta rs . ( 4 ) F a i n t Compact Binary Systems (Neighbor G a l a x i e s ) The d i s c o v e r y of i n d i v i d u a l sys tems e x h i b i t i n g a new type of behav io r , s u c h as Cyq X-1 , Her X - 1 , and AM Hei-, has had a profound effect upon our p e r c e p t i o n and unde r s t and ing of t h e p h y s i c s of compact objects. Thus, t h e d e t e c t i o n of a d d i t i o n a l o b j e c t s w i t h u n i q u e tempora l behavior cou ld have a g r e a t s i g n i f i c a n c e t h a t is d i f f i c u l t t o prede termine . The imaging c a p a b i l i t y of LAMAR w i l l allow it t o s t u d y much f a i n t e r compact binary sys tems i h e t h e r t h e y a r e w i t h i n our own g a l a x y and of r e l a t i v e l y low i n t r i n s i c l u m i n o s i t y o r i n neighbor g a l a x i e s where t h e y a r e f a i n t because of distance. Thus , LAMAR would make a c o n s i d e r a b l y l a r g e r number of n e u t r o n s t a r compact b i n a r i e s 247 accessible t o study. While t h e r e is no reason t o believe a p r i o r i t h a t t h i s l a rge r group w i l l contain phenomena or f ea tu res t h a t a r e not seen among t h e more i n t e n s e gteap, t h e previous his tory of compact ob jec ts indicates t h a t even a s i n g l e object can have an impact t h a t is revolutionary. Cataclysmic Variables I n addi t ion t o compact binary systems containing neutron s t a r s , there exists f a n t e r ob jec ts containing degenerate dwarfs which includes cataclysmic var iab les ( C V ) . Their time sca l e s of temporal change a r e not l i k e l y t o be a s rapid b u t not necessar i ly less in te res t ing . T h i s means t h a t w i t h an imaging detector f a i n t objects can be s t u d i e d because we can in t eg ra t e for longer periods of time without background or confusion becoming a fac tor . The recently discovered X-ray C V ' s (AM Her, 2A0311-227, V1223 Sag, ,12237-035) show bath op t i ca l and X-ray pe r iod ic i t i e s ana quas i -per iodic i t ies which allow diagnosis of the s t ruc tu re and mass t ransfer i n these systems. A s an example of how LAMAR w i l l perform i n t h e s t u d y of the temporal behhvior of cataclysmic var iab les , we consider t h e f a i n t e s t object t h a t is l i k e l y t o be of in t e re s t . As described by D. Lamb a t t h i s workshop, it is one t h a t emits a t lo1 erqs/sec a t a dis tance of 1000 pc. There a r e hundreds of ob jec ts more i n t e n s e than t h i s . (More d i s t a n t ob jec ts a r e d i f f i c u l t t o s tudy opt ica l ly . ) I n LAMAR t h i s source w i l l count a t a 0.5 c t s / sec w i t h e s sen t i a l ly no background. We can e a s i l y s t u d y temporal behavior of individual f l a r e s on a time sca l e of 50 seconds or more. Search for pe r iod ic i t i e s and spec t r a l changes 248 w i l l be accomplished q u i t e eas i ly . The spec t ra l reso!.ution of LAMAR w i l l permit a s t u d y of correlated changes of s o f t an3 hard components. 2.5 S t e l l a r Coronae One of the major contr ibut ions of the HEAO program and t h e E i n s t e i n Observatory (HEM-2) , i n pa r t i cu la r . has been t o e s t ab l i sh X-ray measurements a s a major diagnost ic t oo l for s t e l l a r coronae. X-rays have been detected from s t a r s of v i r t u a l l y every spec t r a l type and f l a r e a c t i v i t y has been found. Some moderate resolut ion l i n e spectroscopy was obtained for t h e few most intense objects. As impressive a s t h e Einstein Observatory r e s u l t s may be, t h e y represent only t h e beg inn ing phases of what could be a major new means of fundamental invest igat ions of s t e l l a r coronae. The e f f ec t ive area of the E i n s t e i n Observatory telescope was only 200 cm2. Because i t s e f f ec t ive area w i l l be 50 times t h a t of the Einstein Obervatory and 1 0 times t h a t of AXAF, t h e LAMAR i s a much more powerful too l for s t e l l a r photometry. W i t h the use of more e f f i c i e n t dispers ive techniques, LAMAR'S increase of throughput over E i n s t e i n for moderate resolut ion spectroscopy ( E / h E : 100) is poten t ia l ly even much la rger than t h a t for imaging and photome t r y . I t should be possible t o make a grat ing t h a t is a t l e a s t 1 0 times a s e f f i c i e n t a s t h e object ive transmission grat ing of Einstein. Reflection grat ings, a s discussed by W. Cash a t t h i s symposium, w i l ; allow much higher l i n e d e n s i t i e s and, consequently, much higher dispersions. The key point is t h a t t h e 249 i d i s p e r s i o n would be l a r g e enough t.0 make an Imaging P r o p o r t i o n a l Counter f e a s i b l e as t h e detector b~ t h e d i s p e r s e d spectrum. The I P C ' s e f t i c i e n c y is about f o u r times h i g h e r t h a n t h a t of t h e h i g h r e s o l u t i o n imager of t h e E i n s t e i n Observa tory . Hence, even i f o n l y a f o u r t h of t h e f u l l area of t he L A . W were devoted t o s p e c t r o s c o p y , t h e i n c r e a s e i n throughput r e l a t i v e t o E i n s t e i n would be la rger by a factor of: 5014 x 10 x 4 : 500. T h i s would be a l a r g e enough i n c r e a s e t o make modera te r e s o l u t i o n spec t roscopy appl icable t o many s t e l l a r o b j e c t s . Assuming t h a t 114 of t h e LAMAR'S t o t a l a r e a is dcvoted t o r e f l ec t ion g r a t i n g s and assuming t h a t t h e g r a t i n g e f f i c i e n c y is - - * , t h e estimated c o u n t r a t e i n t h e d i s p e r s e d spectrum of a s t a r e m i t t i n g lo2 ' e r g s / s a t a d i s t a n c e of 25 pc is 0.5 c o u n t d s i n t h e 0.5 t o 1.5 k e V band. T h u s , an o b s e r v a t i o n of 2 x 1 0 seconds w i l l p rov ide some l o 4 c o u n t s , and a comparable number of background coun t s . For a r e s o l u t i o n of 1 0 0 , t h e average energy b i n w i l l c o n t a i n 1 0 0 c o u n t s , and many of these w i l l be i n t h e form of l i n e s so that many b i n s w i l l c o n t a i n s e v e r a l hundred coclnts. T h i s is s u f f i c i e n t t o p rov ide meaningful measurements of t empera tu re and i o n i z a t i o n e q u i l i b r i a . 4 Photometry i n c o n j u n c t i o n w i t h low r e s o l u t i o n spec4:roscopy ( E / A E : 2) a t 1 k e V can be a p p l i e d t o many s ta rs . T h i s is u s e f u l f o r moni tor ing f l a r e a c t i v i t y and d e t e c t i n g changes i n t empera tu re t h a t a r e c o r r e l a t e d w i t h i n c r e a s e s i n i n t e n s i t y . F i g u r e 4 is an i l l u s t r a t i o n of t h e spec t rum of EL s t a r a t a 250 distance of 40 pc e m i t t i n g e rgs / s as observed i n LAMAR for a time of l o * sec. Spectra a r e shown for a pure thermal spectrum (Raymond & Smith) of kT = 0.17 keV, a s well as for various amounts of an addi t ional component f l u x w i t h kT = 1.0 keV. is no d i f f i c u l t y i n detect ing a small percentage Of t h e higher temperature component. ‘In t h e example taken, t h e s t a r could be i n t h e Hyades c lus te r . When the LAMAR is pointed t o that. region, we would expect t h a t several s t a r s would be observed simultaneously w i t h i n t h e lo f i e l d of view of LAMAR. There The fea tures of s t e l l a r coronae t h a t could be s t u d i e d w i t h X-ray measurements have been discussed by Linsky a t t h i s symposium. They are: Temperatures, range and dependence upon luminosity Densit ies Flow ve loc i t i e s Total energy input and heating mechanism Energy balance Geometry, f r ac t ion of t h e volume f i l l e d by loops Wind accelerat ion mechanisms In te rac t ion of winds w i t h i n t e r s t e l l a r medium Flares on s t a r s of various types The types of measurements needed t o carry out these s t u d i e s are: ( a ) Imaging fo r measurement of f l u x w i t h some spec t r a l resolut ion (low resolut ion spectroscopy) t o d i s t i n g u i s h between s o f t and harder bands. (b) Iden t i f i ca t ion of new s t e l l a r sources w i t h high t t .. . .. 1 251 IOS: I I I i - -- - c - STAR EMITTING IO2* ergs /sec (0.2 - 4.0 keV - - - AT 40 pc ( HYADES) - lo4 5 - - - I .o 2 .o 3 .O ENERGY (keV) Figure 4 252 s e n s i t i v i t y p r o v i d i n g t h e c a p a b i l i t y t o o b t a i n X-ray l u m i n o s i t y f u n c t i o n s of v a r i o u s c a t e g o r i e s of s ta rs . (c ) Temporal s t u d i e s of f l a r e s a n d o t h e r a c t i v i c y , s i m u l t a n e o u s t e m p e r a t u r e - f l u x measurements . (d) Moderate r e s o l u t i o n s p e c t r o s c o p y ( E / A E 2 100) . (e) High r e s o l u t i o n s p e c t r o s c o p y ( E / b E 2 1 0 0 0 ) . LAMAR w i l l be able t o p r o v i d e measurement capabi l i t i es ( a ) , ( b ) , and (c) for many s t a r s when u s e d i n i t s imaging mode. W i t h t h e u s e of r e f l e c t i o n g r a t i n g s f o r w a r d of t h e mirror assemblies o v e r a t least some of t h e m i r r o r s (e.g. a t 25% of t h e t o t a l area) it c a n p r o v i d e ( d ) , a moderate r e s o l u t i o n s p e c t r o s c o p y c a p a b i l i t y . High r e s o l u t i o n s p e c t r o s c o p y ineasure t e n t s w i l l be o u t by AXAF. Thus, LAMAR w i l l be able t o address many of t h e areas of p r i n c i p a l c o n c e r n i n s t e l l a r c o r o n a e . 2.6 I n t e r s t e l l a r Medium I t has been estabiished t h a t a l a r g e f r a c t i o n of t h e volume of t h e i n t e r s t e l l a r medium (IS91 c o n t a i n s a h o t g a s e o u s component of low d e n s i t y . The s i g n a t u r e of t h i s component is t h e s o f t X-ray background which is t h e p r e d m i n a n t source of d i f f u s e X-rays i n t h e 0.1-0.3 k e V band. S e v e r a l y e a r s of s o u n d i n g rocket measurements by t h e U n i v e r s i t y of Wiscons in g r o u p have p roduced a s o f t X-ray map of t h e e n t i r e s k y i n 6 O x 6O b i n s . T h i s map shows a h i g h l y s t r u c t u r a l gap t h a t is p e r h a p s w i t h i n a hundred p a r s e c s or so of t h e Sun. W i t n t h e h i g h t h r o u g h p u t imaging ( - 1 0 4 cm2 e f f e c t i v e a rea , 1 1 a n g u l a r r e s o l u t i o n , 10 f i e l d ) and its spectral a b i l i t y which c o n s i s t s of low r e s o l u t i o n t 253 (E/A E 2) spectroscopy fo r d i i f u s e regions and moderate resolution spectroscopy (E/A E t 100) f o r po in t sources, it is poss ib le t o ca r ry o u t add i t iona l s t u d i e s of t h e ISH t h a t a re re levant t o t h e t o p i c s discussed by Cowie and S h u l l a t t h i s symposium. (1) Imaging of t h e Ga lac t i c Component of t h e X-ray Backaround kn Several Spectral Bands, 0.1-0.3 keV, 0.5-1 keV, and 1-2 k e L Because of absorpt ion w i t h i n t h e plane of our galaxy, w e detect higher energy X-rays out t o larger distances. The 0.1-0.3 keV band is pr imar i ly l o c a l , up t o 200 pc, t h e 0.5-1 k e V band can reach out t o 1000 pc, whi le t h e 1-2 k e V band extends t h e reach almost t o t h e galactic center . With t h e LAMAR, w e have s u f f i c i e n t throughput t o image t h e background on t h e scale of a f r a c t i o n of a degree. The count r a t e i n a lo f i e l d would be 20 c o u n t d s i n t h e 0.1-0.3 keV band, on t h e average perhaps a f a c t o r of 3 lower i n t h e 0.5-1 k e V band, and another f a c t o r of 3 lower a t 1-2 keV. (Above 2 k e V t h e extragalactic component is dominant.) With i n t e g r a t i o n times of l o 3 seconds or more for eacn lo f i e l d , w e would be able t o cons t ruc t sur face br ightness maps w i t h considerable s ign i f i cance i n each of t h e three energy bands on a s c a l e of a f r a c t i o n of a degree. We would detect t h e sur face br ightneas v a r i a t i o n s t h a t r e s u l t from old supernova shells t h a t a r e no longer i d e n t i f i a b l e as d i s c r e t e rad io sources. We would a l s o detect d i f f u s e regions t h a t are heated a8 a resu l t of s t e l l a r p a r t i c l e emission. 254 (2) Resolution of Discrete Sour-~m Wkic;@ i o n t r i b u t e t o Beatinq of t h e ISU. Resolving d i s c r e t e sources is e s s e n t i a l for i d e n t i f y i n g t h e residual d i f f u s e component t h a t represents t h e i n t e r s t e l l a r gas. Furthermore, it w i l l lead t o t h e i den t i f i ca t ion of d i s c r e t e objects t h a t a r e s i g n i f i c a n t sources of energy input t o t h e ISM. Probable examples of t h e l a t t e r a r e t h e X-ray e r h t t i n g OB s t a r s and associat ions detected by t h e E i n s t e i n Observatory. I n t h e rl Carinae region and i n Orion, there is circumstant ia l evidence for process by t h e proximity of d i f f u s e emission t o a h igh d e n s i t y of d i s c r e t e X-ray sources. t h i s (3 ) X-ray Absorption Spectra a s a Probe of t h e ISH. Absorption f ea tu res i n t h e continuum spectra of d i s c r e t e sources can be s tudied w i t h t h e moderate resolut ion spectroscopy capabi l i ty of t h e LAMAR. The LAMAR'S h igh throughput w i l l make these s tud ie s f eas ib l e for many objects by providing a probe of t h e galaxy along t h e many d i r ec t ions t o f a i r l y i n t e n s e sources. We would detect t h e 0 and N e edges of cold mater ia l and perhaps t h a t of matter i n a higher ionizat ion s t a t e . ( 4 ) Imaqinq of-t B- Although these objec ts a r e of la rge diameter, good angular resolution is s t i l l required for imaging because of t h e need t o resolve and remove numerous d i s c r e t e sources. The h o t component of t h e ISM is undoubtedly related t o shock heating by old S N R ' s a s described by Cox and Smith, McRee, Cowie, and Ostr iker , among others. We should see t h e d i f f u s e shells of old S N R ' s p lus 255 " g i a n t bubbles" l i k e t h e one found i n t h e Cygnus r e g i o n t h a t may have o r i g i n a t e d from t h e i n t e r a c t i o n of m u l t i p l e SNR's. 12 ( 5 ) Studies of t h e ISM Neiwor G- Our unde r s t and ing of t h e ISM of our own g a l a x y w i l l be g r e a t l y aided by images of ne ighbor g a l a x i e s , s u c h as t h e LMC, SMC and M 3 1 . For t h e LMC and SMC, w e w i l l o b t a i n a complete p i c t u r e of t h e hot component of t h e i r ISM and t h e r e l a t i o n of t h e major spatial f e a t u r e s t o S N R ' s and other sources of h e a t i n g . ( 6 ! Co ronae of Normal G a l a x i e s With in 3 HPC. The p r o d u c t i o n of h o t g a s by supernova e x p l o s i o n s and a c t i v e s t e l l a r objects w i l l lead t o t h e P s t a b l i s h m e n t of a s t e a d y hot corona around normal g a l a x i e s . The &ona is an i n t e g r a l effect of t h e h e a t i n g processes. The features of t h e corona , such as its d e n s i t y , scale h e i g h t , and r e l a t i o n t o t h e p l a n e , c a n be measured. There are theoret ical c a l c u l a t i o n s by Bregman, which s u g g e s t t h a t LAMAR s h o u l d be able t o measure t h e co ronae of nearby edge on g a l a x i e s w i t h i n a few Mpc. F igure 5 i n d i c a t e s t h e s i g n a l expec ted a t v a r i o u s h e i g h t s above t h e p l a n e i n an o b s e r v a t i o n of 1 0 seconds. 13 4 3.0 HIGH THROUGHPUT SPECTROSCOPY WITH LAMAR Requirements for h i g h throughput are even greater for s p e c t r o s c o p y t h a n for imaging. The e x i s t e n c e of an X-ray source and even its p o s i t i o n can be de termined from r e l a t i v e l y few photons. When background is n e g l i g i b l e , 1 0 pho tons i n t h e image a re s u f f i c i e n t . However, t o c a r r y out s p e c t r o s c o p y s tud ie s , many ORiGlfJAL PAGE IS OF POOR QUALrn cu 0 9sz 257 more photons a r e needed. For example, i f kT = 4 k e V , s i m u l a t i o n s show t h a t 4000 pho tons are r e q u i r e d i n order t o d e t e r m i n e a v a l u e for t h e t e m p e r a t u r e w i t h *lo% p r e c i s i o n by f i t t i n g model spectra t o continuum data o b t a i n e d w i t h a telescope sys tem having good response a t h i g h e r energy , such as LAMAR or AXAF. (Witkout good response a t 4 keV, t h e measurement takes much l o n g e r or c a n n o t be done.) I f t h e spec t rum is a power law w i t h -1.5 as t h e number-energy spectral index , i n order t o o b t a i n + lo% accuracy i n t h e spectral index , 500 c o u n t s a re r e q u i r e d i f t h e spec t rum is n o t s e l f - abso rbed (NH = 2 x i o 2 ) , b u t t h e number needed climbs r a p i d l y as i n t r i n s i c a b s o r p t i o n i n c r e a s e s . A t NH which is n o t u n l i k e s e v e r a l S e y f e r t g a l a x i e s s t u d i e d , l o 4 c o u n t s a r e needed f o r a 10% p r e c i s i o n i n t h e spectral index measurement. Thus, for broad band spectral measurements, w e r e q u i r e from a factor of 50 t o 1000 more pho tons t h a n fo r merely e s t a b l i s h i n g e x i s t e n c e . For d i s p e r s i v e l i n e spe'-sroscopy, t h e de t a i l s of t h e d i s c u s s i o n are d i f f e r e n t , b u t t h e g e n e r a l c o n c l u s i o n is t h e same; a large number of pho tons is needed t o a r r i v e a t a q u a n t i t a t i v e r e s u l t . For example, t o measure t empera tu re , w e may need t o d e t e r m i n e t h e r a t i o of two l i n e s t o b e t t e r t h a n 10% p r e c i s i o n . A t l eas t 200 pho tons a r e needed i n each l i n e . The c o u n t s i n l i n e s a re g e n e r a l l y o n l y a f r a c t i o n of t h e t o t a l number of i n c i d e n t photons. Taking t h e f i n i t e e f f i c i e n c y of t h e d i s p e r s i v e e l emen t i n t o accoun t , w e conclude t h a t t h e c o l l e c t i i i g area of t h e system must take i n s e v e r a l hundred t o s e v e r a l thousand t o t a l pho tons for l i n e s p e c t r o s c o p y t o p r o v i d e a q u a n t i t a t i v e l y meaningfu l r e s u l t . of pho tons We c o n s i d e r two forms of s p e c t r o s c o p y that , c o u l d be carr ied out by LAMAR. One is non-dispersive spectroscopy for gross spectral characteristics of sources dominated by continuum, temperature and temperature gradients in diffuse sources, such as clusters of galaxies, and spectrophotometry for faint and variable sources. These are carried out in conjunction xith imaging. The other form of spectroscopy is a dispersive option that requires additional hardware. 3.1 Non-Dispersive Spectroscopy For many faint and diffuse sources, there is no alternative to non-dispersive spectroscopy. In cases where lines are weak, such as quasars, non-dispersive spectroscopy may be the only form of spectroscopy that is feasible. As an example of non-dispersive spectroscopy in a diffuse source, we consider a measurement of the temperature distribution of the cluster A2069 in cells of 1' x 1' (Fig. 1). Simulations show that >IO4 sec are required for a measurement resulting in AT/T < -1. The temperature gradient in combination with the surface brightness map could then be used to create a map of the mass distribution. The detector for non-dispersive spectroscopy could be the imaging proportional counter. Recent devices provide as good an energy resolution as conventional proportional counters, E/AE : 5 at 6 keV. With this level of energy resolution, it was possible for proportional counters aboard Ariel 5, OSO-8 and HEM-1 to detect Fe lines in clusters of galaxies and spectral features in supernova remnants. Use of a scintillating imaging proportional counter as described by W. Ku14 at this meeting ond eisewhere and AndersonlS could provide about twice as good energy 259 resolut ion. T h i s type of de tec tor is s t i l l i n t h e development s tage , so it is not y e t known what loss of spatial r e so lu t ion is involved compared t o an ordinary IYC or how r e l i a b l e these de tec to r s would be i n long term operation. 3.2 Dispersive Spectroscopy A paper presented by W. Cash a t t h i s meeting descr ibes a method f o r d i s p e r s i v e spectroscopy by use of r e f l e c t i o n g ra t ings i n t h e extreme off-axis configurat ion, forward of t h e mirror. The ru l ings a r e near ly p a r a l l e l t o t h e i nc iden t d i r e c t i o n of t h e rad ia t ion . The r e f l e c t i o n e f f i c i e n c y of t h e g ra t ings are h igh , b e l G w 1.5 keV. Because t h e ru l ing dens i ty is high , 104 l i n e d m m , d i spe r s ions are l a r g e enough for an imaging proport ional counter t o be used as t h e de tec tors . A system w i t h 1' of angular r e so lu t ion would provide an energy reso lu t ion , E/AE of about 100 a t 1 keV. As a LAMAR imaging sys tem is compatible w i t h t h e requirements of t h e r e f l e c t i o n g ra t ings , it is worthwhile t o consider a combined imaging-spectroscopy LAMAR system. There a r e two p o s s i b i l i t i e s . I n one, every module has a dual function. Gratings would be placed before each mirror fo r spectroscopy measurements. When t h e g r a t i n g s a r e removed, each module operates i n an imaging mode. The second p o s s i b i l i t y is t o have two t y p e s of modules, one for imaging, t h e o ther optimized f o r spectroscopy. The advantage of removable g ra t ings is higher throughput as t h e f u l l power of t h e LAMAR is available i n any given observat ion whether t h e ob jec t ive is imaging or spectroscopy. The advantage of t h e second conf igura t ion is s impl i c i ty , compactness, and no moving pa r t s . Study is needed t o 260 determine which configuration is preferred. Ass igning reasonable values t o t h e various e f f i c i e n c i e s and obscurations, one can estimate t h a t i n t h e d i spers ive spectroscJpy mode t h e LAMAR would have an e f f ec t ive area of about lo3 cm2. T h i s is subs tan t ia l and would permit d i spers ive spectroscopy t o be car r ied out on many objects. 4.0 MODERATE COST APPROACH FOR LAMAR Although t h e LAMAR is a la rge area f a c i l i t y , it can be developed w i t h rhe cost cons t r a in t s of a moderate mission. There a r e two considerations which mi t iga te t h e cost . One is t h e f a c t t h a t t h e resolut ion goal is only about a minu te of a r c w h i c h allows a number of telescope manufacturing techniques t o be f e a s i b l e t h a t would f a i l a t t h e a r c second leve l . For example, h igh cost f iguring and polishing techniques a r e not required. The other fac tor is the modular construction ot LAMAR. The problem of developing an array of l a rge area reduces t o one of finding amanufac+,?!ring t e c h n i q u e t h a t is e f f i c i e n t a t making many copies of a module. Numerically controlled machines and computer ass is tance a r e two techniques t h a t a r e useful i n t h i s regard. The module i t s e l f can be of optimum s i z e w i t h respect t G problems of construction, t e s t i n g , and integrat ion. Furthermore, the modular approach has another cost advantage over a monolithic device i n t h a t it allows t h e e s sen t i a l elements of t h e system t o be f u l l y t es ted and understood from s t u d i e s of one or a few sample modules. No "scale-up" problems, changes i n technique, OK other uncer ta in t ies a r i s e when embarking upon the major phase of construction. One key t o cont ro l l ing c o s t s is t o use a s i n g l e 26 1 1 technology for each of the three items, mirror assemblies, detectors, and gratings. At present, there are several technologies for the development of mirror assemblies that seem promising for LAMAR. To provide some credibility to the idaa that LAMAR can be developed at moderate cost, we consider the particular techniques under study at the Center for Astrophlrsics for mirrors and detectors as at example. Our approach is based upon the construction of mirror assemblies consisting of nested plates of float glass with imaging proportional counters as detectors. The elements of this method are listed in Table 4-1. It is based upon the use of commercial materials and the avoidance of polishing for the reflecting surface. Numerically controlled machining and computer assistance are used for the formation of the figure. 4.1 The Mirror Assembly The mirror assembly can be constructed of commercially available float glass with a gold coating. X-ray reflectivity is good at short wavelengths. Glass plates are constrained mechanically to form a figure that is almost parabolic, A nested system equivalent in size to m e LAMAR module has been used successfully on a series of rocket flights.16 The theoretical angular resolution of such a system is limited by elastic deformations in the bent plates. Several measures can be taken to improve the resolution as summarized in Table 4-2. Mechanical tolerhnces are expected to be important long before reaching the ultimate resolution of a pre-figured or "slumped" plate. 262 Table 4-1 MODERATE COST APPROACH FOR lo2 MODULES FOR LAMAR Nested Plate Mirrors Reflecting Surfaces: Figure Formation: Commercial float glass (Au coated) No polishing required Automated interactive microprocessor control of positicn adjustments based upon sensing of visible light image by reticon diode arrays Mirror Assembly Structures: Mass production using numerically controlled machines to fabricate top, bottom, and side plates, .001" accuracy easily achieved Mass production of screw machine products relatively inexpensive in quantity Imaging Proportional Counters Electronics: No critical elements, extensive use of integrated circuits Bodies : Numerically controlled machines No critical tolerances Wire Planes: Special precision tooling facilitates wire winding Reflection Gratinas Surfaces: Replication 263 ORtGlNAL PAGE IS OF PGCR QUALITY TABLE 4-2 LAMAR MIRROR SYSTEM - PERFJRMANCE (ENCiRCLED ENERGY vs. SYSTFhl TYPE) ADJUSTED BENT PLATE (ON-AXIS OPTIMIZED) 50%=0.63 orc minutes diometet MASKED ADJUSTED BENT PLATE (OPTIMIZED MASKED) 50%=0.59 arc minutes diameter AREA LOST * 18% I I STIFFENED FLAT BENT PLATE 50 % ss 0.45 arc minutes diameter I ADJUSTED PESWMPED PLATE I I 50% zz a16 orcminutes diameter I I I OPT1 CAL AXIS ADJUSTED SHAPE L - \SLUMPED SHAPE I PERFECT NESTED SYSTEM 50% ~0.12 orc min&.s diameter R NOTES: - ALL NUMBERS APPROX. -UNIT REFLECTANCE ASSUMED - FOCAL LENGTH BI 213 cm -30.5cm SOUARE PLATES ASSUMED -No INITIAL SURFACE RIPPLES ASSUMED 264 Cumputer ass i s tance would be u s e f u l f o r bending of t h e p l a t e s t o the desired shape. Figure 6 i l l u s t r a t e s t h e method t h a t we plan t o s t u d y . A microprocessor cont ro ls t he posi t ion of a s l i t t h a t can be driven alcng a precise l i nea r encoder. A port ion of a p l a t e is illuminated by v i s i b l e l i g h t through t h e s l i t . The l i n e image is read a t t h ree points by diode arrays. The centroidal posi t ion of t h e th ree images a r e determined by t h e microprocessor. T h i s information is used t o dr ive stepper motors which apply bending moments t o p l a t e u n t i l t h e images a r e acceptable. The s l i t is moved t o another pa r t of t h e p l a t e and the process is repeated. As experience is gained i n t h i s procedure the microprocessor program w i l l be updated t o incorporate refinements i n t he p l a t e adjustment procedure. 4.2 Reflection Gratings Although more de f in i t i on of t h e r e f l ec t ion gra t ings is needed a t the present time, it is expected t h a t mass production methods a r e applicable. A nested array of ident ica i grat ing p l a t e s would seem t o be a good match t o t h e op t ics of t h e nested p l a t e telescope. Replication seems extremely promising a s a method for producing a large number of i den t i ca l gratings. 4 . 3 The Detectors Imaging proportional counters (IPC's) i n t h e i r various fc;ms can be manufactured by well es tabl ished and straightforward methods. The techniques ii:.:lude c i r c u i t board manufacturing, extensive use of integrated c i r c u i t s a s components, and numerically controlled machining. L i t t l e 6 i f f i cu l ty is I ! 265 I * i I . .." 256 anticipated in assigning these tasks to conventional manufacturing shops. 5.0 SUNNARY AND CONCLUSIONS The high throughput capabilities of LAclAR will satisfy many of the requirements cf future programs in X-ray astronomy. It will excel1 in measurements requiring: moderate resolution imaging for flux measurements an3 positions detection of low surface brightness features timing for temporal variability spectral measurements The high throughput of LAMPR is unique and will greatly surpass that of all other telescope instruments, including the Einstein Observatory and all other facilities being planned for the :.uture both in the U.S. and abroad. LAMAR is applicable to 8 wide range of scientific objectives and will serve many investigators. The general utility of the LAMAR may be appreciated fron an analysis of the guest observer usage by instrument of the Einstein Observatory. This is summa--ized in Table 5-1. Guest observer usage is a good indicator of how the astronomical community in general perceives the various instruments in terms of being relevant to their interests. The table shows that the Imaging Propxtional Counter was the overwi.elrcina choice Lath in terms of the nrinber of observations and the total time in the focal plens. The essential caaracteristic of the IFC that TABLE 5-1 E INSTEI. U OBSERVATORY GUEST OBSERVER S'LhTISTICS BY I N ~ c T R U h d N T Total Time Done IPC - 6518.68 ksec 78.2% HRI - 1550.99 18.6% sss - 229.10 2.7% FPCS - 39.60 . 5% TOTAL - 8338.37 k s e c - Total Number of O b s e r v a t i o n s IPC - 1571 B R I - 13 8 sss - 20 FPCS - 2 90.7% 8.0% 1.2% . 1% TOTAL - i731 r e s u l t e d i n i ts be ing selected so f r e q u e n t l y was i ts h igh throughput . LAMAR w i l l exceed t h e IPC of t h e E i n s t e i n Observa tory by a factor of 50 i n t h a t c a p a b i l i t y which was found i n practice t o be most u s e f u l t o t h e a s t r o n o m i c a l commupity as a whole. Despite i t s large c o l l e c t i n g area, t h e modular approach i n t r i n s i c t o LAMAR w i l l g r e a t l y s i m p l i f v its development and w i l l allow it t o be compatible w i t h a moderate cost Exp lo re r miss ion . ACKNOWLEDGEMENTS We would l i k e t o thank Wallace Tucker and Dan ie l F a b r i c a n t f o r i n f o r m a t i v e d i s c u s s i o n s and a i d i n t h e e s t i m a t i o n of performance of t h e LAMAR. We w u l d a l so l i k e t o thank Lester Cohen and Paul BKP 8 for t h e i r c o n t r i b u t i o n t o t h e d i s c u s s i o n of t h e n e s t e d plate m i r r o r system. 267 i f 268 REFERENCES 1. 2. 3. 4. 5 . 6. 7. 9. 9 . 10. 11, 12. 13. Schwartz, D.A. (19761, Ap.;. (Le t te rs ) , 206, L95. Mushotzky, R.F., H a r s h a l l , F.E., Boldt, E.A., Holt, S.S., and S e r l e m i t s o s , P.J. (19801, Ap.J., 235, 361. Forman, W. and Jones, C. (19821, t o be pub l i shed i n Annual ReJiews of Astronomy iind As t rophys ic s , V o l . 20. Gioia, I.M., Geller, M . J . , Hvchra, J., Maccacaro, T., and Stocke, J. (19811, submi t t ed t o Ap,J. F a b r i c a n t , D., Lecar, M., and Gorenstein, P. (1980) , Ap.J., 241, 552. R i r schne r , R., Oemler, A., J r , , Sc i iec te r , P.L., and Shectman, S. (19811, Ap.J. (Let ters) , 248, L57. Os t r ike r , J. and Cowie, L. (19811, Ap.3. (Le t t e r s ) , 243, L127 . Tananbaum, H., Avni, Yo, Branduardi , G., E l v i s , M., Fabbiano, G., Feigelson, E., Giacconi, R., Henry, J.P., Pye, J.P., Soltan, A - , and Zamorani, G. (1979) , Ap.J. (Le t t e r s ) , 234, L9. E l v i s , M. and Lawrence, A. (19811, submitted t o Ap.J . Helfand, D., Becker, R.H., and Novick, R. (19811, Bul l . A.P.S. , 26, 570. Woosley, S.E. and Wallace, R.K. (1981) , L i c k Observa tory p r e p r i n t . Cash, W., Char l e s , F., Bowyer, S., Walter, F., Garmire, G., and Riegler, G. (19801, Ap.J. (Let te rs ) , 238, L71. Breqman, J . N . (19801, Ap.J., 237, 681. 14. Ku, W. and Ha i l ey , C.J. ( 198 i1 , I.E.E.E. Trans . on Nuc. I 269 Sci., Vol . NS-28, 830. 15. Anderson, D . F . (19811, I . E . E . E . Trans. on Nuc. S c i . , Vol . NS-28, 842. 16. Gorenstein, P I f Gursky, H., Harnden, F . R . , Jr., De Caprio, A., and Bjorkholrn, P. (19751, J . E . E . E . Trans. olr Nuc. Sci., Vol. NS-22, 616. A Wolter Type I LAMAR L..C. Catura, W.A. Brown and E.G. Joki Lockheed Palo Alto Research Laboratory Dept. 52-12, Bldg. 255 3251Eanower Street Palo A l t o , California 94304 1 Noveaber 1981 "hi- paper presented at the CeJOard Workshop on X-Ray As'tonomy and Spectroscopy 5-7 October 1981 272 A Wolter Type I LAMAR R.C. Cdtuxa, W.A, Brown and E.G. Joki Lockheed Palo Alto Research Laboratory Abstract Observational objectites for the WLNAR and their influence on the tnstru- Pent design are discussed, parameter is the angular reeolution of the WWAR modules since it so strongly influences sensitivity, optical identifications, source confusion, spectral resolution for objective gratiuqs and the ability to resolve small extended sources. ware status discussed and the perfonance of a LAMAR observatory presented. promising technique for enhancing the reflectivity of Wolter Type I X-ray optics in a selected bandpass at high energy b s been investigated and the performance of the LAMAR module, utilizing thts method, has been calculated. It is concluded that the most important design A high resolution Woltei Type I LAMAR module is described, its hard- A I . Introduction There are a number of important objectives in X-ray astronumy which require large collecting area to achieve high sensitivity in timing measurements, spec- troscopy end the study of faint sources. and source confusion problems, the only practical way of achieving such large area instruments is to utilize X-ray Imaging with good angular resolution. h e to the properties of X-ray reflection, however, 1arS.c effective areas cannot be achieved at X-ray wavelengths with a single telescope and thus the concept of an array of co-aligned telescopes, in modular units, has arisen. This concept has becme camn@nly known as the Large Area Modular Array of Reflectors and is Identified by the acronym, W R . general terms, the objectives of LAMAR observations and to show how achieving these objectives very strongly influences design of the instrumentation. the current status of a hardware program to develop WolterI X-ray optics for the LAMAR will be described and the design of a W 4 R module utiliziilg these optics will be discussed. Capabilities and performance of a 24 module XAMAR have been calculated and compared with those of the Einstein Observatory and AXAF. Wolter I optics by deposition of multilayered diffraction coatings on their reflecting surfaces will be discussed. Because of detector background noise The purpose of this paper is to discuss, in Also, Finally, a promising technique for improving the high energy response of 11. Iaportant LAMAR Characteristics Many specific objectives in X-ray astronomy which can be addressed by LAMAR observations were presented during the first day of this workshop. general observational Dbjectives for the LAMAR and how their achievement very strongly influences the LAMAR design are discussed below. Several 1. Surface BriRhtness Measurements of Paint Diffuse Emission. This objective includes study of emission from intracluster gas in galaxy clusters, galactic halo emission, shadowing of the dxffuse component of the extragalactic X-ray backgrcund by absorption in nearby galaxies and 273 investigation of soft diffuse emission within ;he Galaxy. Since higher angular resolution will not concentrate the diffuse X-rays onto a smaller detector area, sensitivity to extended emission depends primarily on maximizing the LAMAR effective area and minimizing detector background rates. lowever, good angular resolution is also very important in providing increased sensitivity for detecting and subtracting the con- tribution from discrete sources. Since the temperature of cluster sources Is 5 keV it is important that the LAMAR sensitivity extend to as high an energy as possible. Also, since the detector in each module adds noise, it is important to achieve the large effective area with the feiest number uf modules . 2. High sensitivity T i m i n g Measurements Studying the size of the emitting volume and the efficiency of energy conversion in active galactic nuclei are Important objectives of these ewaeurwnts. Also, such measurements allow the investigation of evolution in binary, pulsating and burst sources during periods of low mass transfer, the study of stellar flares, cyclic variability in stars and the identi- fication of time variable sources in nearby galaxies. For brighter sources, where background is negligible, statistical uncertainties in timing measurements depend only on LAMAR effective area. However, for faint point sources, where sensitivity to time variability is statistically limited by background, it is critical to minimize the count- ing rates from detector background. Also, the importance oc assigning observed variability to a particular source requires minimizing source confusion by achieving good angular resolution. Since sensitivity to the largest possible region of the sky maximizes observational efficiency by allowing concurrent study of many sources, it is impcrtant for the LAMAR modules to have as large a telescope field as possible. 3) Survey Observations Such observations may take the form of limited surveys of particularly interesting regions of the sky or an all-sky survey. provide the resource for extending luminosity functions of various classes of objects to much lower luminosities and for various statistical studies in correlatiw X-ray characteristics with properties measured at other wavelengths. An all sky survey would provide a source of reference data for many studies, identifications, archival data for tests of models and theories without requiring further observations. Perhaps most important, a deep survey would discover interesting new objects of low population which answer or raise Important questions in astronomy or provide an imediate test of theory. These surveys will The sensitivity of a survey, of course, depends on maximizing signal from the sources, minimizing detector noise and being able to survey as large a field as possible in a single observ8tion. Thus, survey observa- tions require noL only large effective area but also good angular resolu- tion to minimize noise, reduce source confusion and improve ctmnces for optical identifications. In addition, the W R telescapes must provide uniform response over their fields to allow a survey of miform sensi- 274 tivity . 4) Spectroscopic Measurements The importance of spectroscopy at optical wavelengths is well known and it will provide the same advantages to X-ray astronomy. Study of tempera- tures, electron densities, chemical abundances, emlssion mechanisms and red shifts all became possible with the capability for highly sensitive medium resolution - 150) spectroscopy. maximizing effective area and aininizing background noise. resolving power of objective grating spectroscopy depends linearly on the telescope resolution and its uniformity over the telescope field. addition, the spectral range covered by the instrument is dependent on the size of telescope field. Broad band spectroscopy with non dispersive detectors again requires Eowever, the In Obviously all of the above discussion can be summarized by saying we need a LAMAR with largest effective area over the otidest energy range, lowest back- ground noise, best angular resolution with uniform response over the broadest possible field. of the telescopes has the most important impact on LAMAR performance. However, assdng comparable effective area, angular resolution The image sensors in a LAMAR will experience an appreciable counting rate from both the diffuse X-ray flux, Bd, and charged particle background, B , that will degrade the instrument'? sensitivity to point sources. counting rates, summed for all modules of the LAMAR, are given by: 2 -1 counts sec These backdound (1) (2) Bd = FR A B = K ( R f ) N counts sec-l 2 P where F is the intensity of diffuse X-rays, A, R and f are the LAMAR effective area, angular -esolution and focal length respectively, K is the charged parti- cle counting rate per unit detector area, and N is the total number of detectors (modules) in the LAMAR- The total background counting rate, Bt, is then: The product Rf is the pixel size on the detector. qt = Bd + Bp - R2(FA+iCf%) (3) The extragalactic cogqogent of-she-fiffugf X-gfy background, with DO interstel- lar itbsorption ('1 E ph ca 8 keW s t 1, is a reasonable lower limit to the flux from most places in the sky at energies a - p 0.5 keV.,Zln tbf range 0.5 z26 keV chis prowides an intensity, P = 2 x 10 min) prgxiqtely 5 x 1G counts sec per mm' of detector area. For a L.MAR with 10 rate, B, is: photons an sec (arc For ioagigg proportiow# counters iu the range .5 - 6 keV, K IF ~ p - cm effective area, and a focal length of 2m the total backgromd counting . (4 1 -1 Bt R2 (.02 + 1.5 x lo4 N) counts sGc where lt is measured in arc minutes. It Is evident :hat the diffuse X-ray back- ground is the nriac-pal source of noise in a LAMAR. e v e r , if tto nu:tfwr of rodu?es gtms to of order 100 the particle background is no lorqet negligible. ORIGINAL PAGE 1s OF POOR QUALln 275 Also, if an appreciable fraction of the diffuse X-ray background may be resolved into discrete sources, as it now appears from Einstein observations, particle background rates will become dominant for a LAMAR with many modules. When backgfound is negligible, the faintest detectable source strength varies as (AT) When the X-ray source must be detected above random fluctuations in the background counting rate the faintest detectable source intensity, I, varies as: , where T is the observing time. 112 112 112 (Bt u (R~PAT) R(=) P AT AT I = root of other parareters. faintest detectable source (5 counts detected with zero background or suf- ficient detected counts to be 3 stan- dard deviations above the background) is plotted as a function of observing time, for three values ot angtlar2 resolution in a LAHAR with 10 cm effective area. These calculations utilize the background rate in equa- tion (41, neglecting B . The limit- ing sensitivity of a thescope-yith perfect resolution varies as T and is indicated by the solid line. Telescopes with finite angular reso- lution depart t r w this line as they begin to acquire background counts and thereaf~ff~their sensitivity varies as T , indicated by the dashed lines. A LAMAR having an angular resolution of 1 arc min or larger becomes background limited in less than 100 sec of observing the. Figure 1 indicates that If X-ray optics of 20" resolution instead of 3' resolution are utilized it is This effect where the particle background in eq. 3 has been neglected. tivity, therefore, varies linearly with its resolution, but only as the square A telescope's sensi- is shown in Figure 1 where the w V rr 2 C v1 y h F.. G I E7 " E $ E E- w c - 4 rs 1"-8 1 o2 10' 1 o4 OBSFHVINC TIME (SEC) 4 2 Figure 1. Sensitivity of a 10 cm LAMAR as a function of observing time for various values of its angular resolution. I \ I \ I .01 HER X-1 - p0ssi))e to imprgye the LAMAR sensitivity by a factor of 10. 4ThUS, sources dovn to 10 ergs sec (-01 Her X-1) can be detected in M31 in 10 time and with a spatial resolution of -70 pc, if 20 arc sec optics are employed far LAMAR. sec observing Figure 2 show8 the effect of LAMAR angular resolution on the ability to detect time variability of faint sources. 614 is assumed to occur In the intensity, I, of a source at the. center of the 10 s observation (eg. a partial X-ray eclipse). significance of 3 standard deviations above background noise is assumed for these calculations. A step function change of intensity, Detection of this change with a Figure 2 indicates nearly a factor of 10 improvement in 276 ORWUAL PAGE 13 OF PO02 QUALIF ability tion is to detect faint eclipsing binaries (dI/I=l) if the LAMAR angular resolu- improved from 3' to 20". Other advantages of the higher angular resolution, not related to sensitivity are: 1. 2. 3. 4. Improved ability to optically identify newly discovered sources. Higher spectral resolution in using objective gratings. For a telescope with 20" resolution and a 6000 l / m grating one can obtain L'AA - 200 while 3' resolution ellows A / AX of only 20 . Ability to distinguish extended sources from point sources on a much finer scale. This is im- portant in distinguishing emis- sion of distant extended clus- ters from that of compact galac- tic nuclei and in separating dis- crete low luminosity galactic sources from dii'i-se emission. Studies of the angular diameter- I I I Wl Ability to detect time 1 o-2 10-I IO0 Figure 2. variability in faint sources as a func- tion of LAMAR telescope an&ular resolu- tion. redshift relation in distant cluster X-ray sources provide an even more important application for a telescope of improved angular resolution, since the distant clusters are expected to be smaller than a few arc minutes in angular diameter. Less source confusion. The limit that there should be no more than one source per 40 pixels becames an important factor in studies of faint sources. This will be dis- cussed in more detail later. 111. Eardware ctatus A large Wolter-I X-ray tele- scope has recently been fabricated in a joint program involving the Mull-rd Space Science Laboratory and the National Physical Labora- tory In the U.K. and our own labo- ratory here In the U.S. Wolter-I telescope is the same op- tical design that was used on the Binstein Obqervatory and Involves The PARABOLOID HYPERBOLOID 5 A _ . . _ ,. . i---. . 'fl u...:,', ,,. ' I?.. .%L..-A.- ::. i.. . L Figure 3. The Aries rocket telescope shown in cross section. successive X-ray reflections from paraboloidal and hyperboloidal mirror ele- ments. The telescope, which was flown on a N*SA-Aries sounding rocket, has a focal length of 2.3 m, a grazing angle of 1.9 , an eftrance aperture 66 cm in diameter and a geometrical collecting area of 380 cm . shown schematically in Figure 3, were made from rolled ring forgings of 5083 aluminum alloy. turning, which utilizes a precision air-bearing lathe an4 a diamond cutting tool to machine the required curves to within 1 micron. then plated with a thin coating of electroless nickel and polished to obtain the final X-ray reflecting surfaces. The telescope mirrors, These mirror blanks were figured by the process of diamond These figured surfaces were Prior to flight, the telescope performance was measured in the 1000 ft. X-ray calibration facility at the Marshall Space Flight Center. were made at X-ray energies of .277, .>72, -705, -933, 1.5 and 2.05 keV. Pre- liminary results of the measured effective area which the mirrors present to a distant point source are shown in Figuti 4. tive area calculated from the X-ray optical constants (Brshov, Brytov and Lukirskii, 1964) of the nickel surface. while the crosses show the measured Measurements The solid disks indicate the effec- values. The statistical uncertainty in the measurements is negliglble, however, there are systematic uncer- tainties in there data due to impre- cise knowledge of the imaging propor- tional counter efficiency, which is now being determined. The data points at .277 ke9 disagree appreciably because some X-ray events at this energy fall below 'he lower level pulse amplitude discriminator on t..e imaging proportional counter (IPC) output and therefore fail to be coun- ted. Corrections for this effect are also being determined. The measured effective area at .933 keV falls well above the calculated value because this is in the vicinity of the L-shell X-ray absorption edges in the nickel reflecting surfaces where the optical constants are very poorly known. At .572 and -705 keV, where the sy:.tema- tic uncertainties are smallest, the measured values are approximately 60% of those calculated. Since there are two reflections in the telescope, these data indicate the polishing process has achieved - 75% of the theoretical re- flection efficiency. 0 CALCULATED X MEASURED x X x x 0 0 x 0 . 5 1 . 0 1.5 1.0 ENERGV I h N l Figure 4. Comparison of measured and calculated effective area for mirrors in the Aries rocket tele- scope . The Point Spread Function (PSF) of the telescope (mirrors plus IPC) mea- sured at 1.5 keV is shown in Figure 5. centroid of the X-ray source image and swning the counts in IPC pixels which lie within successive annuli of increasing radius. annulus is then divided by the number of pixels contributing to the sum to obtain the normalized PSF. The PSF ls obtained by determining the The total counts within each The PSP is shown on a semi-log plot where the hori- eontal coordinate, which is given in IPC pixels, corresponds to off- axis angle in the telescope field. Since each pixel is 15 arc sec in size, the gntire plot covers the central .4 radius of the field. This point spread function shows the typical response of an X-ray telescope in having a very intense central core followed by a low in- tepitp tail (down by a factor of 10 ). due to the IPC is removed from the telescope PSF we obtain a value of 30 arc sec FWEM for the PSF from the mirrors alone. This is very encouraging since the design goal for figuring the mirrors was a blur circle diameter of 40 arc sec. This result, along sith having achieved 75% of theoretical X-ray reflectivity from the mirror sur- faces, on m r first attempt at fabricating a telescope, is very encouraging and giycs us confidence in using this technique of mirror fabrication for LAMAR applications. Figure 6 shows an Image of Cyg X-1 obtained during the Aries rocket flight. This object was observed as a point source calibration late in the flight and for spectral obser- vation by a companion experiment. When the image broadening Our laboratory, in collabora- tion with our U.K. colleagues, is currently in the process of fabri- cating two additional mirror pairs utilizing refinements in production technique gained f ram experience with the present telescope. Dia- mond turning of the mirrors will be carried out in the U.K. on their newly commissioned diamond turning facility. These marrors will be nested within the existing set to form a three-element telescope which will be used with objective reflection gratings fot s p e c t w scopic observations. This spec-' troecopic application is undertaken la collaboration with Webster Cash at the University of Colorado, who will discuss objective reflection FIRfES X-RFIY TELESCOPE MSFC C Z ~ L I ~ R G T I O N RNALVSIS POINT SPRERD FUNCTION: TEST 29 E=l.500 1. 10.' Figure 5. Aries telescope, including IPC, at an energy of 1.5 keV. Point spread function for the . . . . . . .. L. I 1 1 2 0 8 3 8 9 - rigute 6. during the Aries rocket flight. An Image of Cyg X-1 obtained ORIGINAL FASE IS OF POOR QUALITY grating spectroscopy later in this workshop. Another telescope fabrication effort, also utilizing diamond turning to This effort differs from that described here in the figure Wolter I mirrors, is being carried out in parallel by Gordon Carmire at Penn. State University. preparation of the X-ray reflecting surfaces. coated with an acrylic lacquer which provides a very hLgh quality surface finish. deposit to provide the final X-ray reflecting surface. at GSFC, has pioneered thi; technique of fabricating X-ray reflectors and will describe it in more detail later in this workshop. The diamond-turned mirrors are This thin (- 10 um) lacquer coating is then overcoated with a metal Peter 9erlemitsos, here IV. A Wolter Type I LAMAR The definition phase of a NASA funded Spacelab investigation has recently been completed, in which a single Wolter-I LAMAR aodule was designed. definition study was carried out by a consurtium of seven institutions including Penn. State University, University of Washington, Mullard Space Science Laboratory, University of Leicester, The U.K. National Physical Laboratory, University of Cambridge and Lockheed Palo Alto Research Laboratory. goals for the telescope were to maximize the effective area of the module and provide the best possible angular resolutic- over the broadest telescope field within the cost constraints. The resulting Nigh Resolution LAMAR (HRL) module is shown in Figure 7 and consists of a ten element nested array of Wolter-I urrors. The HKL focal length is 3.6 m and the mirror diameters range from 90 This The design , . . . , - - . I I . - 5 0 1 R A W I ! i I f . \ . . 4 Figure 7. The EiBh Resolution LAMAR (HRL) mirror assembly utilizing ilolLer-I X-ray optics. 280 cm to 30 cm with grazing angles between 1.7' and .56O. long in their axial diaensions, the telescope has a gemtrical area of 1350 em and the design goal was to achieve an on-axis image blur of 20 arc, sec radius or less. manner as described earlier for the Aries rocket telescope. However, if the technique of coating the diamond turned surfaces with acrylic lacquer prmes effective in preserving the 20 arc sec figure, it would be utilized to reduce costs. plate, which also serves as an aligaPent fixture for the nested array. This alignment is obtained by utilizing the precision machintng capability of the diamond turning facility to machine the marim surfaces of the paraboloids, hyperboloids and the support plate. The rating flange surfaces of each mirror are machined at the same time as its interior surface is figured and thus the flange provides a true reference for the mirror axis. Surfaces of the support plate are diamond turned to be flat and parallel, for axial aligment, and the inner surfaces of raised lugs, present on the six radial webs are also diamond turned to provide mirror aligsment in a direction normal to the telescope axis. This alignment tecbnnique utilizes the precision of the diamond turning machine to greatly reduce, what otberotise would be a time coasming and expensive effort in the telescope production. three mirror nested array cor the Aries rocket program. The mirrors are 36 cm It is anticipated that these mirrors would be fabricated in the same As shown in Figure 7, the mirrors are attached to a cental Support This method is being utilized in fabricating the Figure 8 is a schematic of the BIU. telescope shaving the mirror assembly on the left and the focal plane on the right. includes a rotary interchange mechanism to position either of two IPC detectors at the HBL focus, is attached to the mirror support plate by a tripod structure. A shutter door, which seals the telescope entrance aperture, also serves as a sun shade wben observations are being conducted. in diameter and slightly over 4 m long, is designed for flight on the Space Shuttle utilizing a pointing control system. have been discussed by Catura et al. (1981). The focal plane assembly, which This payload, which is 1.1 m Further details of the BRL Payload The calculated response of the HBL mirror assembly is shown in Figures 9 and 10, assuming the outer 3 mirrors are N i coated and the inner 7 mirrors have Au reflecting surfaces. The effective area as a function of energy, which the LAUNCH AND RE-ENTRY CLAMPS TRIPOD SUPPORT- \". SHUTTER DOOR AND MECHANISM /- c - - - . .-..A I - , - . THERMAL PRE - COLLIWTOR- r - CONTAMINANT - SEAL INTERCHANCE MECHANISM CRUCIFORM X-RAY BAFFLE AFT MOUNT NESTED MIRROR ASSEMBLY MIRROR SUPPORT P U T € V-BAND FORWARD MOUNTS (2) Figure 8. A drawing of the BB1 payload deslgned for Spacelab observations. I e' I .* I e* R a i I.' . .. .. b. . I *' I @ i * ,/ Figure 9. Effective area of a single Figure 10. Image blur as a function HRL module as a function of energy. of field angle in the HRL focal plane. mirrors preaent to a focal plane lnstrument, is shown in the top curve of Figure 9 along with the response of the HRL telescope when a Xenon jllled IPC is uti- lited as an image sensor. The mirror assembly has - 1000 cm effective ar a below the Nl L-shell absorption edges at -85 keV and provides nearly 20 cm at the Fe line energy of 6.7 keV. Figure 10 shows the radius of the l q e blur as a function of field angle in the focal plane. It is somewhat energy dependent but, by defocusing the detector slightly (trading on-anle resolution for a larger telescope field), It is poesibls to mnintain an image blur of < 20 arc sec out to a field angle of neariy b.4 I in radius. Figure 11 ehows how HRL telescopes could be conceptually arranged in a 4 Figure 12 module eubasstmbly of the LAMAR. Spacelab facility instrument or areembled into a full male LAHAR. acheamtically Illustrates a 24 module LAMAR, ohown here as a free flying srtel- lite. The Wolter-I LAMAR illustrated in Figure 12 could be placed in orbit by a single shuttle launch. with a mnaller number, for example, on the Space Platform and add further modules as budgets and shuttle availability penaltted. array perforunce could be ptogresoively enhanced and the specif ic capability of array Prrsbers could be altered to reflect chaalng scientific priorities and developments in tho instrumentation f leld. Such a subassembly could be flown as CI However, it would be possible to establish the array In this way the overall -. _. ___ Figure 11. 4 nodule LAMAR Subassembly. Figure 12. Schematic of a 24 module free flying Walter-I LAMAR. ORIGINAL FAGE IS OF POOR QUALITY 283 The effective area of a 24 module UWAR as a function of X-ray energy is compared in Figure 13 with the effective areas of the Einstein Observatory and the future Advanced X-Ray Astrophysics Facili- ty (W). Tbe areas shown are for the mirrors alone, as presented to a focal plane instrrrent. The data of Figure 13 has been used to com- pare sensitivities of these three observatories as a function of ob- serving time. For purposes of this comparison, unit detector efficien- cy is asstmed. Charged particle coqtiag rategluereZtaken to be 5 x 10 counts s mr and the extra- galactic diffuse X-ray Intensity (the largest background for t p 4 lAl+)-ps rgyresgpted as 11E ph cm s keV sr . A Crab-like energy spectrum was a s s d for the source and was folded through the telescope responses given in Figure 13. The relative sensitivities for detecting faint point sources with the three observatories, under the above assumptions, are indicated in Figure 14. The dashed curve indi- cated for the LAMAR would result if the entire extra-galactic diffuse X-ray background could be resolved into discrete sources and only the detector noise from charged parti- cles remained. The three horieon- tal lines indicate the source con- fusion limits for three different values of pixel sizes determined by the telescope axular resolution. These limits are independent of the observatory and use only the numberflux relationship: -16 s-1.53 NOS) = 2.7 x 10 (6 1 -1 sources sr determined by Maccacaro et. al. (1981) and the confusion limit defined as 1 source per 40 pixels. It is amarent that confusion 1 0' 1 0' I d Q un " a w I lo* > r 0 Y u Ir w I ! 0' 24 -t 1- ------- ----- 0.1 c.2 0.5 I .O 2.0 5.0 10.0 mm. c1 Figure 13. three observatories as a function of energy . Effective collecting area of Figure 14. Sensitivity for detecting point X-ray sources as a function of observing time for three obwroatories. becaras-6 problem for an observatory with 31 angular resolution at the sensitivity which Binstatn achieved in a 10 Remlts of the Einstein deep survey (Giacconi, et al. 1979) show no apparent turnover In the sec observation. i . P i 284 -1 num%t-flux relation for exLtagalactic sources down to- 2 x sec fied. fusion lirit in a 600-?4c observatiqp, unless ipre i? a turn over in the nuber counts between 2 x 10 and 4 x 10 ergs CI sec . the must conclude that angular resolution is a critical factor for extragalactic studies in a highly sensitive observatory such as the LAMAR. ergs cm , SO that the confusion limit for 1' resolution is observatiomlly veri- Even with 20" pixels, the 24 module Wolter I WLNAR will be at its con- V. X-Ray Reflectivity Enhancerent with Hultilayers The technology for depositing extremely thin alternating layers of high and low density materials, which act as Brakg diffractors, is now becoming well established (Underwood, Barbee and Keith, 1979; Spiller and Segmuller, 1980 and Barbee, 1981). These coatings are applied by vacuum deposition with highly uniform layer thicknesses down to lOA, essentially replicating the surface finish of the underlying substrate. coatings on the reflecting surfaces of a WolterI telescope offers the potential Of increaasing its effective area in a selected bandpass at high energy. If the underlying mirror finish is preserved by the multilayers, the low energy specu- lar reflection of the telescope will not be dlminished. However, it is neces- sary to arrange the high energy band pass well above the energy at which the nonnal mirror reflectivity cuts off, since interference effects will othervise degrade the specular reflection. reflect efficiently only below about 2.5 keV, they will in principal allow reflectivity enhancement for Pe- line emission at 6.7 keV by mul- tilayer deposition without dis- turbing their low energy res- ponse. Deposition of these multilayer diffraction Since the outer mirrors of the BBL module The Bragg reflectivity of gold-carbon multilayers of various thickaesses and number of laperpairs has been calculated with the aid of a computer code which predicts their response as a function of wavelength. A re- sult of these calculations is shown in Figure 15 for 15 layer pairs of gold and carbon opti- aieed for 1.85A X-ray at a grazing angle of 1.7 (appro- priate for the outer mirror of the HRL module). The peak re- flectivity of this rocking curve is 42% and its PWtfM is .14A, which 8"s an angular equivalent of -13 Since the grazing angle varies slightly along the length of a Wolter I mirror, the width of the rocking curve should be broad from t'he Bragg equation. h c l v ( L F W 3 ~ ~ P Y G C ' L " L Figure 15. multilayer with 15 layer-pairs as a functkon Reflectivity of a Cold-Carbon enough to encompass this variation of wavelength. The layer thickness is 16#- Also, because two reflections are involved, the telescope efficiency depends on the square of the multilayer reflectivity. rocking curve width are cmpeting parameters in the multilayer design and so they must be optimized. structure having 15 layer pairs where each layer is 16A thick. peak efficiency for double reflection of - 16% and has a rocking curve wider than all but the largest mirror. HOOFever, peak reflectivity and Figure 15 is the optimum in this case and represents a This provides a For purposes of calculation, the outer 4 mirror pairs -re assued to be coated with multilayers of appropriate layer spacing such that the rocking curve peaked for the grating angle at the center of each mirror. tracing program used for the calculations of Figure 9 was modified to include the aagular dependence of the Bragg reflectivity of the multilayers, assuning no degradation of the low energy specular reflection. response when the multilayers on all four outer mirrors are appropriate to reflect 1.8SA X-rays. The multilayers Increase the response at 6.7 keV over that from specular reflectio3on the inner mirrors by a factor of 7, providing a peak effective area of 125 cm in a baod2pass which is .5 keV FUHM. module LAMAR, one would obtain - 3000 cm at this energy. 9 off-axis response of the telescope is also shown in Figure 16 at angles of 0.1 The computer ray Figure 16 indicates the HRL For a 26 and 0.3O and indicates that- the multilayer res- ponse degrades approximately the same as that for specular reflec- tion at this energy- It is possible to broaden the band pass of the telescope at the expense of peak response by tuning each mirror pair to reflect a slightly different energy. Figures 17 and 18 show results of these calculations where the peak wave- lengths for each of the four mirror pairs are separated by -04A and .08A respectively. While the results of these calculations are encouraging, a number of practical matters need to be addressed. These include the practicality of depositing uniform multilayers on such large surfaces and whether tho, low energy specular response can be maintained after multilayer deposition. VI . Coaclusions Angular resolution of the X-ray optics in a LAMAR most strongly in- fluences its performance. A LAMAR with 20" resolution has a factor of Figure 16. Effective area of an HRL module with the outer 4 mirror pairs coated by adtilayers and optimized for 1.8% X-rays. Three off-axis 285 I i t ! 1 10 better sensitivity than one of the same effective area but with 3' angu- shown angles in the telescope field are 286 ORiGlNAL PAGE IS OF POOR QUALITY 4 2 lar resolution. Also, a LAMAR with 10 cm effective area and 3' angular resolution will be source confusion limited in about two minutes of observing time at the sensitivity level reached by Einstein in about 800 see. The com- parable confusion limit for a LAMAR of 20" resolution is 20 tires better, at a sensitivity 3 times below the level of the Einstein deep surveys. The spectral resolution achievable with objective gratings is linearly related to angular resolution of the X-ray optics. Consequently, it is critically important to the LAMAR Observatory's performance to utilize the highest angular resolution possible within the mission cost constraints. The technology for producing Wolter Type I X-ray optics with 20" angular resolution is rapidly reaching urturity and a detailed design and a development plan now exists for a high zesolution LAMAR module which utilizes these optics. Deposition of multilayered diffraction coatings on the outer mirrors of this telescope module appears to be H promising way of enhancing the high energy performance in a selected bandpass. I e' I e' 0 0 m 2 I 18' > t 0 W Y - I e' I e" -1 . . \ ', \ Figure 17. Response of the HRL module Figure 18. Response of the HRL module when the peak wavelengths for multi- layers on the outer 4 mirrors differ by .O& by .08R. when the peak Wavelengths for multi- layers on the outer 4 mirrors differ Acknowledgements 28 7 Portions of this research have been supported by NASA Contracts NAS5-23563 We are grate- and W5-26090 and by the Lockheed Independent Research Program. ful to Dr. J.L. Culhane for very useful discussions and for providing the drawing of the 24 module LAMAR. members of the HRL consortium on portions of the design of that instrument which have been reported here. We also acknowledge indispensable help by the References Barbee, T.W. 1981, Proceedings of AIP Topical Conference on Low Energy X-Ray Diagnostics, Monterey, CA., to be published. Catura, R.C., Lt al. 1981, S.P.I.B. Proceedings, 284, to be published Ershov, O.A., Brytov, 1.A. and Lukirskii, A.P. 1964, Optics and Spectroscopy, 22, 66. - Giacconi, R. et al. 1979, & J . (Letters), - 234, L1. Maccacaro, T., et al., 1981, CFA preprint No. 1519, submitted t o Ap. J . March 1981. Spiller, E. and Segmuller, A. 1980, Appl. Phys. Lett., 37, 1048. Underwood, J.H., Barbee, T.W. and Keith, D.C. 1979, S.P.I.B. Proceedings, 184, 123. PERFORMANCE OF SINGLE PHOTON-COUNTING X-RAY CHARGE COUPLED DEVICES G . R . Riegler R . A. S t e m K . L5ewer F . Vesct?lus J e t Propulsion Laboratory California Ins t i tu te of Technology Pasadena, CA 91109 J . A. Nousek and G . P . Garrnire Department of Astronom;? Pennsylvania State University State College, Pennsylvania 290 PERFORMANCE OF SINGLE PHOTON-COUNTIN3 X-RAY CHARGE COUPLED DEVICES G. R. Riegler , R. A. Stern, K. Liewer, and F. Vescelus Pasadena, CA 91109 Jet Propulsion Laboratory, Ca l i fo rn ia I n s t i t u t e of Technology and J. A. Nousek and G. P. Garmire S t a t e College, Pennsylvania Department of Astronomy, Pennsylvania S t a t e Universi ty ABSTRACT Resul ts of i n i t i a l performance tests on x-ray sensing p rope r t i e s of charge-coupled devices (CCDs) are presented. CCDs have demonstrated ex- c e l l e n t spa t ia l r e so lu t ion and good s p e c t r a l reso lu t ion , super ior t o t h a t of non-imaging proport ional counters . I. INTRODUCTION A charged coupled device (CCD) can be expected t o combine t h e bes t p rope r t i e s of high-resolut ion imaging de tec to r s and so l id - s t a t e spectro- meters, namely high s p a t i a l reso lu t ion and moderate s p e c t r a l reso lu t ion , a t high e f f ic iency . Because of t h e importance of such a device f o r x-ray astronomy, a number of researchers have begun laboratory s t u d i e s of x-ray CCDs ( G r i f f i t h s et . al. 1981 and re ference the re in ) . This paper de- s c r i b e s t h e s t a t u s of x-ray CCD s t u d i e s a t t h e Jet Propulsion Laboratory. Elsewhere wi th in t h i s conference, reviews of t h e known prope r t i e s and open quest ions f o r a wide range of g a l a c t i c and ex t ra -ga lac t ic sources have been presented. EIAE - 40 a t E " 6 keV, which CCDs are expected t o provide. The a b i l i t y t o produce a spectrum f o r each image element (p ixe l ) is important f o r t h e s t u d i e s of supernova remnants, c l u s t e r s of ga lax ies , and wherever source confusion would be a problem f o r non-imaging s o l i d state spectrometers. These reviews have c a l l e d for moderate r e so lu t ion , e . @ . , 11. DESCRIPTION OF THE TEST CAMERA Several CCD cameras have been b u i l t a t JPL f o r t h e study of CCDs and f o r t he t e s t i n g of CCDs t o be used by the Gal i leo J u p i t e r o r b i t e r and t h e Wide Fie ld Planetary Camera of t h e Space Telescope. i n these tests, one f o r t h e v i r t u a l phase devices (Janesick e t a l . , 1981) and the o ther f o r t he th ree phase devices (Blauke e t a l . , 1981). and amplitude of t he clock pulses required t o s h i f t t he da ta out of the two types of CCDs d i f f e r , but a l l the ampl i f f e r s and s i g n a l processing c i r c u i t s are ideiit ical . Two cameras were used The timing The s igna l processing c i r c u i t r y uses a double co r re l a t ed sampler where output is d i g i t i z e d t o 16 b i t s . 5 x lo4 p i x e l s per second which minimizes t h e noise assoc ia ted with sampler. Thcse itells p lus very c a r e f u l camera layout allow us t o achieve a no i se fip(ure of 4.5 e lec t rons RMS f o r the camera sys tem and an ampl i i i e r no i se of 7 e lec t rons f o r t h e v i r t u a l phase CCD o r 13 e l ec t rons f o r t he th ree phase CCD. The d i f fe rence i n read noise is due t o d i f f e rences i n t h e on chip ampl i f ie rs . The p ixe l s are clocked out a t a r a t e of The CCDs are contained i n a vacuum chamber which is evacuated t o a Cooling is accomplished by a regulated flow of pressure of a few Torr. l i q u i d ni t rogen. with a s t a b i l i t y of l? . l °C. exposure f o r all. the sources used. The temperature can be var ied from room temp t o -190°C A s h u t t e r i n t h e vacuum chamber con t ro l s t h e The removable f ron t f ace of t h e vacuum chamber can hold an o p t i c a l window, a beryll ium f o i l window, o r t h e window can be replaced by a f lour - escent X-ray source with interchangeable t a r g e t s which is ac t iva t ed by a lmCu source of a lpha particles. The CCDs used i n these tests were a l l manufactured by T e x a s Ins t ru- The t h r e e phase devices were thinned ments Co. square p ixe l s i n 800 x 800 ar rays . t o approximately l o p and operated with t h e r ad ia t ion inc ident from t h e non- c i r c u i t s i d e (backside). The v i r t u a l phase CCD is a t h i c k device and t h e r ad ia t ion is inc ident on t he c i r c u i t s ide . Because t h e v i r t u a l phase CCDs have only a s i n g l e ga t e s t r u c t u r e , t h e c i r c u i t l aye r i s t h i n (0.6 micron). There is an add i t iona l 0.5 micron p ro tec t ive overcoat. While t h e absorb- t i o n i n these layers would requi re t h a t t h e CCD be thinned and run i n t h e back i l luminated mode f o r energies below one k i l o v o l t , th inning was not re- quired f o r these tests (80% transmission a t 1.5 keV). Both t h e th ree phase and v i r t u a l phase devices have 15.2 micron Although t h e v ix tua l phase device i s 250 microns t h i c k it is a layered s t r u c t u r e with only a 10 micron th ick layer adjacent t o the ga t e s t r u c t u r e chat a c t s as an a c t i v e de t ec to r region €or x-rays. t h i s temperature t h e dark current is near 10’2e-/pixel/sec f o r t he 3-phase device and 10-3e’/pixel/sec f o r t h e v i r t u a l phase. vary from 1 t o 30 seconds depending on which source is used and so dark cur ren t is completely negl ig ib le . The v i r t u a l phase devices have an ad- d i t i o n a l no ise source which comes froru cu r ren t s generated within t h e de- v i ce when it is clocked. This noise source amounted t o 40 e lec t rons RMS f o r t he device used i n these tests, For the tests described here the CCDs were cooled t o about - lOO°C. A t Our exposure times A more complete descr ip t ion of t he 3-phase device was given by Blauke e t a l . (1981). (1981). The v i r t u a l phase device w a s described by Janesick e t a l . 292 111. X-RAY PERFORMANCE Both three-phase and virtual-yhAse CCDs were il luminated with 5.9 keV x-rays from a Fe 55 aource. pseudo-image which d isp lays r e l a t i v e amount of charge f o r each p ixe l . With the a r r ay o r i en ta t ion shown i n Figure 1, events are t ransfer red out (charge-coupled) i n a downward d i r ec t ion i n each column. Some "smearing" along the v e r t i c a l d i r ec t ion is apparently due t o incomplete charge t r a n s f e r and is frequent ly re fer red t o as "deferred charge". The gray scale of the image is highly "stretched" t o reveal t h i s low-level incomplete charge t r ans fe r . Figure 1 shows a port ion of a The d i s t r i b u t i o n of individual p ixe l event s i z e s is shown i n The "background" curve is off scale below r e l a t i v e event The response t o Fe 55 x-rays is shown by the peak near Figure 2. s i z e 7200. event s i z e 7720. To account f o r charge smeo:ing along the read-out d i r ec t ion and Figure 3 shows the the f i n i t e probabi l i ty of charge deposi t ion i n t w o adjacent p ixe ls , var ious da ta summation techniques have been used. histogram of the number of events as a funct ion of r e l a t i v e event s i z e f o r p i x e l s when the deferred charge is included with the main event charge. S t a r t i n g with the main event, the computer algorithm f o r t h i s approach adds the charge f o r successive p i x e l s i n a v e r t i c a l coluum below the i n i t i a l event u n t i l the p ixe l charge is less than two standard deviat ions above an average background l eve l . Figure 4 shows the same set of da t a as i n Figure 3, except that those events have been excluded f o r which the main charge deposit ion was observed i n two adjacent p ixe l s . It is apparent t h a t , compar2d t o the histogram i n Figure 3, the background events below the energy peak have been s i g n i f i c a n t l y reduced. Figure 4 represents the bes t performance obtained so f a r i n an x-ray CCD. The f u l l width a t ha l f maximum response (FWHM) is - 12%, corresponding t o an energy reso lu t ion of 700 e V FWHM. In order t o obta in the equivalent RMS noise f igure , which is frequent ly used t o charac te r ize CCDs, w e use a conversicn f ac to r of 8.5 (3.6 evlelectron-hole p a i r x 2.35, the number of standard deviat ions per FI.rHM). The noise performance in Figure 4 corresponds t o 74 e lec t rons equivalent RMS noise. Figure 5 was derived from the same set of da t a as i n Figure 2 through 4 except t h a t a 3 x 3 mask was used -33 the a r ray of p ixe l s t o be included f o r each event. A somewhat b e t t e r performance is obtained i n Figure 6 where a rectairgular mask of 2 x 6 p ixe l s w a s used; the mask was aligned along the charge readout d i rec t ion . In the l a t t e r two cases, no add i t iona l cor rec t ion f o r incomplete charge t r ans fe r w a s made. The performance f o r virtual-phase CCDs Is comparable t o t h a t of 3-phase CCDs except that a poorel- noise performance was obtained. From tests a t o p t i c a l wave lengths it was observed that addi t iona l charge is created in the serial r e g i s t e r under the clocked w e l l . charge, which appears t o be a funct ion of t he clock p u l m shape, contr ibuted t o poor noise performance e t o p t i c a l and x-ray wavelengths. Figure 7 shows the response of a vir tual-phase CCD t o Fe 55 x-rays. Comparable graphs f o r s u l f u r x-rays (2.3 keV) and alumj.num x-rays (1.49 keV) are shown i n Figures 8 and 9. This amount of IV. PRESENT AND EXPECTED PERFoRr4ANCE The noise performance achieved t o date with v i s i b l e l i g h t was 8.8 electrons IPS f o r a three-phase device and 22 electrons r m s fo r a virtual-phase CCD. tests are 22 electrons and 44 electrons, respectively. Compared t o these values, the r e su l t s f o r mono-energetic x-rays of 5.9 Lev are st ill s igni f icant ly poorer. The average widths observed in vis ible- l ight me most recent analysis of x-ray ca l ibra t ions shows that the depth of the ac t ive regions deduced from op t i ca l tests is i n f a c t consis tent with the detection efficiency as determined from x-ray absorption measure- ments and the manufacturer's specif icat ions, i.e., 10 microns. Spat ia l resolution obtained is comparable t o the s i z e of a p ixe l , i-e., I5 microns x 15 microns. resolution of ! /3 arc second f o r a telescope of 10 meter focal length, for example. fo r an AXAF. This pixel s i z e corresponds t o an angrlar The preceding data show conclusively that CCDs have excel lent s p a t i a l resolution and good spec t ra l resolution, superior t o non-imaging proportional caunter resolution. However, fur ther research is required t o understand the difference between the noise performance a t op t ica l and x-ray wavelengths. Various technfques t o optimize CCD performance f o r x-ray appl icat ions need t o be explored, 5 u t these preliminary r e s u l t s a r e c l ea r evidence tha t CCDs w i l l becora an iaportant too l fo r x-ray astronomy. i 1 294 REFERENCES Blauke, M . , Janesick, J . , Hall, J . , and Cowens, M. 1981, Prc:. S . P . I . E . , 290. - G r i f f i t h s , R. E . , Polucci, C . , Mak, A., Pturrdy, S . S., and Schwartz, D. A. 1981, Proc. S.P.I.E., 290 ( i n press) . Janesick, J . , Hynecek, J . , Blouke, M. 1981, Proc. S.P.I .E. , 290. 296 ORIGINAL PAGE If OF POOR QUAL^^ 3-PHASE CCD: Fe55, ADJACENT EVENTS INCLUDED 3-PKQSE CCD: Fe55, ADJACENT EVENTS EXCLUDED I t 1 90 30 20 10 0 0 200 400 6al 800 loo0 1200 MNl SIZE (Dn) O9!CINAL GAGE 1E OF POOR QUALITY 3-PHASE CCD: Fe55, 3 x 3 MASK 0 400 600 800 MNl SIZE (On) FIGURE 5 3-PHASE CCD: Fe55, 2 x 6 MASK 100 I I I 1 1 1 I 1 1 I I 1 90 80 70 ul FrM t5 50 2 = z?! 30 20 10 0 297 NEM SiZE (On) FIGURE 6 298 VIRTUAL PHASE CCD: FeSS 1 100 80 v, W E m 8 Q 4" P 20 0 FIGURE 7 VIRTUAL PHASE CCD: S K a FIGURE 8 VIRTUAL PHASE CCD: AI K a la- 299 FIGURE 9 AN IMAGING GAS SCINTILLATION PROPORTIONAL COUNTER FOR THE DETECTION OF SUBKILOELECTRON-VOLT X-RAYS Charles J. Hailey, William H.-M. Ku, and Michael H. Vartanian Columbia Astrophysics Laboratory, Columbia University ABSTRACT A large area imaging gas scintillation proportional counter (IGSPC) has been constructed for use in X-ray astronomy. The IGSPC consists of a gas scintillation proportional counter (GSPC) with a 1 wm coupled to a multiwire proportional counter (MWPC) via a calcium fluoride window. The MWPC, filled with a mixture of argon, methane, and tetrakis (dimethylamino) ethylene, detects the W photons emitted by the xenon gas in the GSPC. energy resolution of 17.5% (FWHM) and 1.9 mm (FWHM) spatial resolution at 1.5 keV. polypropylene window Over a sensitive area of 21 c d the instrument has a measured , 302 I. INTRODUCTION Recent research into gas scintillation proportional counters by ourselves (Anderson et al. 1978; Ku et al. 1979; Hamilton et al. 1980) and others (Charpak, Policarpo, and Sauli 1980; Andresen et a1 1978; Hoan et al. 1980; Peacock et al. 1980; Maneo et al. 19801, has demonstrated the enormous versa- tility of this device for X-ray detection. These counters offer a factor of 2 improvement in energy resolution over conventional proportional counters in the energy range 0.1 to 60 keV (Ku et al. 1979; Hamilton et al. 19801. The use of a photomultiplier tube array to collect the W light from the GSPC has demonstrated the feasibility of achieving mlllimeter resolution imaging capabilities (Hamilton et al. 1980; Charpak, Policarpo, and Sauli 1980; Andresen et al. 1978; Hoan et al. 19801. Finally, the well defined pulse shape corresponding to X-ray capture in the GSPC allows rise-time discrimina- tion, enhancing the usefulness of thi.3 instrument in high background environ- ments such as outer space (Ku et al. 1979, Mane0 et al. 1980). Three years ago, Policarpo (1978) suggested using a photoionization chamber in place of a photomultiplier tube to detect the W radiation. TEA was found to be a suitable gas for krypton scintillation (Policarpo 1978; Charpak, Policarpo, arld Sauli 1980) but a more versatile gas ("MAE) was sug- gested by Anderson (1981). We extended the usefulness of the GSPC by combin- ing the good endrgy resolution capabilities of the GSPC with the good spatial resolution capabilities of the MWPC I:Ku and Hailey 1981). with its 100 pm aluminum foil window, was aseful above several kilovolts. We have extended the spectral range of our instrument to the subkiloelectron-volt That instrument, X-ray region. We discuss the design and operation of the IGSPC a3d present da ta acquired with our counter. 11. INSTRUMENT The instrument we have designed is shown i n Figure 1. The GSPC is a pillbox-shaped chamber of 15O-mm diameter. mic material (Macor). dow. ensure t h a t the window is an e l e c t r i c a l ground. O-ring seal so t ha t the X-ray window may be replaced. absorbed by xenon atoms i n the d r i f t region. xenon atoms d r i f t t o a l i g h t producing region defined by two s t a i n l e s s steel meshes held a t high voltage. photons by excimer deexci ta t ion. f l uo r ide window i n t o the 30 c m X 30 cm x 5 e m MWPC. The MWPC is f i l l e d with TMAE and P-20. Photoelectrons e j ec t ed from the TMAE molecdes d r i f t down through a low f i e l d region defined by a s t a i n l e s s s t e e l mesh located on the lower sur face of the calcium f l u o r i d e window and a cathode w i r e plane located below the window. The photoelectrons are acce lera ted through the cathode plane toward a high voltage anode plane. wires on the upper cathode plane def ines the t o t a l ac t ive region. ching e l ec t rons a r e co l lec ted on t h e anode while ions c o l l e c t on the cathode wires. The charge signals induced on :he two cathode planes set perpendicu- l a r ly t o one another are processed d i g i t a l l y t o yiefd the cent ro id of t h e It is made of a machineable cera- X-rays e n t e r the counter through a polypropylene win- The polypropylene is coated with approximately 100 A of aluminum t o The GSPC is designed with an The X-ray photons are Photoelectrons e j ec t ed from the The e l ec t rons are acce lera ted , producing W The W photons escape through the calcium "MAE moleculres absorb a la rge f r ac t ion of the W photons. A lower cathode plane with wires set orthogonal t o t h e The avalan- ! 304 charge d i s t r i b u t i o n i n orthogonal d i r ec t ions (Reid et al. 1979). Energy information is obtained by co l l ec t ing the e l ec t rons i n a charge-sensit ive preamplif ier a t tached t o the anode plane. Important parameters of the IGSPC are summarized i n Table 1. 111. DESIGN The goal was t o bui ld a s o f t X-ray IGSPC with 18% (FWHM) energy resolu- t i o n and 1.0 mm (FWHM) s p a t i a l r e so lu t ion a t 1 keV. Good energy r e so lu t ion requi res maximizing W l i g h t output from the GSPC and de tec t ion by the MWPC. Good W l i g h t production is obtained by using as high a d i f f e r e n t i a l vol tage as possible on the GSPC without allowing charge mul t ip l ica t ion . The W photons produced are proport ional t o the thickness of the l i g h t producing regicn so t h i s must be l a rge (Conde, Fe r re i r a , and Ferreira 1977). co l l ec t ion requi res t ransparent W window and g r i d s and a la rge s o l i d angle of in te rcept ion . The calcium f luo r ide d isk is made as t h i n as mechanical con- s t r a i n t s allow and provides approximately 80% transmission. reso lu t ion s e t s add i t iona l cons t r a in t s on design (Ku and Hailey 1981). A f l a s h l e s s g e t t e r i n a separa te chamber connected by two tubes t o the main counter chamber maintains the pu r i ty of the xenon gas. The upper O-ring s e a l i n the GSPC allows easy replacement of t h i n wlndows which a re mounted on aluminum carrier p l a t e s . an O-ring and may be removed t o provide access t o the MWPC chamber. Good W Optimal pos i t i on The e n t i r e GSPC assembly is at tached t o the MWPC v i a 305 I V . MEASURED PERFORMANCE OF THE IGSPC a. Gain and Energy Resvlution The GSPC was f i l l e d with 760 t o r r of high pu r i ty (99.995%) xenon gas. The MWPC was f i l l e d with 6?0 t o r r of P-20 and 0.3 torr of TMAE. Resul t s similar t o those described below were a l s o obtained with an MWPC f i l l of 608 t o r r of argon, 152 t o r r of isobutane, and 0.3 t o r r of TMAE. source (5.9 keV Mn-K) was epoxied t o a window support r i b t o allow cont inual A small '5Pe monitoring of gain while measurements were taken at lower energies with a var i e ty of fluorescence X-ray sources. GSPC s e t t i n g s of 0.5 kV and 3.8 kV on the upper and lower g r ids , respec t ive ly , and 0.5 kV on the MWPC cathode were chosen as nominal. Figure 2 shows a typ ica l energy reso lu t ion versus anode high voltage curve a t 0.94 keV (Cu-L). 2.2 kV, which was chosen as the nominal anode opera t ing voltage. The curve e x h i b i t s a minimum at about Figure 2 a l s o shows the t o t a l cathode plane output as a funct ion of MWPC anode HV. The system gain tends t o run away above about 2.3 kV. Figure 3 summarizes the best energy reso lu t ion performance of the IGSPC a t the energies measured: 0.28 keV (C-K), 0.68 keV (F-K), 0.94 keV (Cu-L), 1.5 keV (Al-K), 2.3 keV (S-K), 5.9 keV (Mn-K). The 8.5% (FWHM) reso lu t ion obtained a t 5.9 keV is cons is ten t with energy r e so lu t ion The r e s u l t s agree w e l l with the expected E'1'2 sca l ing . measurements taken on a photomult ipl ier tube using the same polypropylene window. The gain of the counter was measured as a funct ion of posi t ion. The gain is uniform t o b e t t e r than 10% across the e n t i r e 51-mm diameter of t he X- r a y entrance window. The good gain uniformity is a result of the small X-ray window diameter with respect t o the t o t a l GSPC diameter (ensuring minimal f i e l d f r ing ing a t the window edge) as well as the l a rge area of t he MWPC which 306 ensures e f f i c i e n t W co l l ec t ion even f o r events near the edge of the window. Figure 4 shows the energy reso lu t ion as a funct ion of pos i t i on i n the d i rec- t i o n p a r a l l e l t o the anode wire d i rec t ion . The energy r e so lu t ion is uniform across the e n t i r e window as expected from the good gain uniformity. S imi la r r e s u l t s are obtained i n the orthogonal d i rec t ion . b. Pos i t ion Resolution and S e n s i t i v i t y We have mapped the counter response over the e n t i r e 51- polypropylene window. Figure 5 shows a p lo t of the weighted average pos i t ion versus the source pos i t ion p a r a l l e l t o the anode wire d i rec t ion . were obtained f o r t he d i r ec t ion peroendicular t o the anode wire d i rec t ion . The measurements were taken a t 0.94 keV (Cu-L) with a copper anode Henke tube mounted approximately 300 cm from the window plane of the counter and con- nected v i a a vacuum vesse l t o the test chamber. Movable slits defined a 150 pm x 150 pm beam s ize . The counter response is l i n e a r over almost the e n t i r e range, slowly f l a t t e n i n g towards the window edges. This f l a t t e n i n g is due t o the lo s s of l i g h t in te rcepted by the 130 mm x 130 am MWPC as the X-ray source is displaced from the center . The e f f e c t is q u i t e small, i n marked con t r a s t t o r e s u l t s we had previously obtained with a 75 rn x 75 am MWPC (Ku and Hailey 1981). i n going t o a l a rge r s ized MWPC. obtained at d i f f e r e n t X-ray energies. Various col l imat ion schemes allowed us t o obta in measurements a t 5.9 keV (Mn-K), 1.5 keV (Al-K), and 0.29 keV (C-K), i n add i t ion t o our measurement at 0.94 keV (Cu-L). The da ta have been cor- rec ted f o r the e f f e c t s of f i n i t e source size. E11'2. Again, similar r e s u l t s The des i r e t o improve our pos i t i on s e n s i t i v i t y was a prime motivation Figure 6 shows posi t ion-resolut ion da ta The da ta scales roughiy as Figure 4 shows the pos i t ion r e so lu t ion versus source pos i t i on p a r a l l e l 307 t o the anode wlres. obtained ( i n both direct ions) . As v i t h the energy resolut ion, good uniformity is C. Bi8e-Tf- and Backgroud-Rejection Efficiency: Pulso-shape d i sc r i a ina t ion has been appl ied with good success i n GSPCS to reject non-X-ray background events (Andresen et al. 1977). T k t i m e between i a i t i a t i o n of secondary l i g h t m l t i p l i c a t i o n and the complete co l l ec t ion of a l l the l i g h t is a function of the e l ec t ron cloud size, t h e d r i f t ve loc i ty i n the absorption region, the s ize of the s c i n t i l l a t i o n region, and the d r i f t ve loc i ty i n the s c i n t i l l a t i o n region. For aur counter geometry and xenon, t h i s rise time is 2.1 mfcroseconds f o r X-rays captured near the vindow. The spread i n the rise time is a funct ion of the variance on the s i z e of t h e e lec t ron cloud as it en te r s the s c i n t i l l a t i o n region, va r i a t ions i n d r i f t ve loc i t i e s a r i s i n g from d i s t o r t i o n s i n the electric f i e l d s , and va r i a t ions i n the Ce:h a t which the X-rays are absorbed. by maximizing the d r i f t ve loc i ty and minimizing the f i e l d d i s to r t ions . p a r a l l e l g r id geometry of our present counter is i d e a l f o r t he formation of X- ray pulses. The rise tjme is e s s e n t i a l l y the same f o r a l l X-rays i n the energy range 0.1 to 6 keV within the cen t r a l 51- a c t i v e area of the counter, although the spread i n rise times increases f o r lower energy X-rays due t o the s t ronger e f f e c t of poor f i e l d s near the polypropylene window. times f o r minimum ionizing particles and y-raps is m c h less well-def ined and ravges from very shor t times €or those events hJsorbed i n the s c i n t i l l a t i o n region t o very long times f o r those events which t rack through the whole counter. The narrowness of the r ise-t ie d is t r i t ru t ic f o r X-rays then, allows us t o set a very narrow tlse-time window f o r ' , c e p t t ~ c e and reject a la rge These va r i a t ions can be minimized The The spread i n rise times is less than 0.06 microsecoids (1 u). The pulse formation 308 percentage of t h e non-X-ray background events . Background rejection r a t e s of 95% have been achieved while maintaining 90% X-ray acceptance rates. More- over , more than ha l f of t h e cosmic ray induced background events occur near t h e edge of t h e GSPC an4 can be r e j e c t e d by b i r t u e of t h e i r l o c a t i o n o u t s i d e t h e c e n t r a l 5!-mm f i e l d of view. V. CONCLUSION The prel iminary r e s u l t s from our s o f t X-ray IGSPC demonstrate t h e poten- t i a l usefu lness of t h i s instrument. However, t h e r e are s e v e r a l s t e p s we w i l l t a k e t o improve t h e energy and p o s i t i o n reso lu t ion . Improvement of our g e t t e r i n g system should lead t o purer xenon gas and thus l a r g e r W l i g h t output . Reducing t h e calcium f l u o r i d e th ickness through t h e use of double pumping and reducing t h e d i s t a n c e from t h e lower GSPC g r i d t o t h e calcium f l u o r i d e window should allow better W c o l l e c t i o n e f f i c i e n c y and also lessen t h e i n t r i n s i c l i g h t cloud spread. F i n a l l y , a sys temat ic program t o examine var ious f i l l gases f o r our IGSPC w i l l be undertaken. These s t e p s should h e l p us a t t a in our goal of 18% (FWHM) energy r e s o l u t i o n and 1.0 mm (FWHM) s p a t i a l r e s o l u t i o n a t 1 keV. The au thors wish t o acknowledge h e l p f u l d i s c u s s i o n s with R. Novick. This work was supported by t h e Nst iona l Aeronautics and Space Adminis t ra t ion under g r a n t NCR 33-008-102. This is Columbie Astrophysics Laboratory Cont r ibu t ion No. 214. 1ABLE 1 Parameters of ICSPC's Design Parameter IGSPC GSPC - Gas mixture" - Gas p r e s s u r e ( to r r )* - Absorpt ion d e p t h (m) - Ligh t m u l t i p l i c a t i o n d e p t h (mm) - Bottom g r i d t o W window (mm) - W window material - W window dinenst ions (mm) - X-ray window material (vm) - X-ray window dimension (mm) - Grid t r a n s p a r e n c y - Nominal HV (kV)*V3, V1, V2 - Gas mixture" c.is p r e s s u r e ( torr )* Absorpt ion d e p t h (mm)* Anode-cathode d i s t a n c e (mm) Anode wire dimension (mm) Cathode wire dimension (mm) Cathode group s i z e Anode wire s p a c i n g (mm) Cathode w i r e s p a c i n g Grid t r a n s p a r e n c y (ground) Nominal *dV (kV)* va, vc, vG x t ion 760 10 1 1 4 CaF2 7.0 x 150 i polypropylene 51 die. 9LZ 0, 0.5, 4.0 "MAE + P-20 0 3 + 620 12 4.5 0.02 x 130 0.063 x 130 6.0 2.0 0 .55 90% 2.2, 0.5 , 0 *Ad j u s t able 31 0 REFERENCES Anderson, D. F. 1981, Nucl. Ins t r . Methods, submitted. Anderson, D. F., Ku, W. H.-M., Novick, R., and Scheckman, M. 1978, - IEEE Trans. Nucl. Sci., -15, 813. Andresen, R. D., Leimann, E. A., Peacock, A., and Taylor, B. G. 1977, Nucl. Ins t r . Methods, 146. Andresen, 8. D., Leimann, E. A., Peacock, A., and Taylor, B. G. 1978, - IEEE Trans. Nucl. Sci., RS-25, 800. Charpak, G., Policarpo, A,, and Sau l i , F. 1980, IEEE Trans. Nucl. Sci., 19s-28. Conde, C. A. N., Fer re i r a , L. R., and Fe r re i r a , F. A. 1977, IEEE Trans. IJucl. Sci., As-24, 221. Hamilton, T. T., Hailey, C. J., Ku, W. H.-M., and Novick, R. 1980, - IEEE Trans. Nucl. S c i . , 19s-27, 190. Hoan, N. N., Jeanjean, J., I toh, H., and Charpak, G. 1980, Nucl.Instr. Methods, 172, 603. Ku, W. H.-M., and Hailey, C. J., Ku, W. €?.-H., Anderson, D. F., Hamilton, T. T., and Novick, R. 1981, IEEE Trans. Nucl. Sci., US-28. 1979, - IEEE Trans. Nucl. Sc i . , -26, 490. Manzo, G . , Peacock, A., Andresen, R. D., and Taylor, 8. G. 1980, Nucl. Ins t r . Methods, 173.. 301. Peacock, A., Andresen, R. D., kimenn, E. A. , tong, A,, Mdneo, G . , and Taylor, B. G. 1980, Nucl. Instr. Methods, 169, 613. Policarpo, A. J. P. L. 1978, Nucl. Instr . Methods, 153, 389. Reid, P. B., Ku, W. R.-M., Long, K. S., Novick, R., and P l sa r sk i , R. L. 1979, IEEE Trans. Nucl. Sci., IS-26, 46. FIGURE CAPTIONS 311 FIG. 1 - Schematic diagram of the imaging gas scintillation proportional counter (IGSPC). FIG. 2 - Total pulse height from the cathode planes plotted as a function of anode high voltage (XI; energy resolution plotted as a function anode high voltage (*I. the GSPC at (3.8 - 0 .5 ) kV. The data are for Cu-L with Vc = 0.5 kV and FIG. 3 - The energy resolution vs.X-ray photon energy of the IGSPC. The solid line is proportinal to E’“*. The energy ( 0 ) and position ( * ) resolution for Cu-L as a function of source position parallel to the anode w i r e direction. FIG. 4 - FIG. 5- Position sensitivity and linearity in the direction parallel to the anode wires in the MUPC. The divider output is plotted VS. the source positon. FIG. 6 - The IGSPC position resolution VS. X-ray photon energy. The solid line in proportional to ~ - 1 1 2 . -.e.-.”.- ORlGlNAP PAPUT ?$ OF POOR QUALITY MWPC ANODE HV ( K V ) 3 13 3 14 I 1 * b l . . l I 0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 IO X-RAY ENERGY ( k e V ) FIGURE 3 315 CI v) != z a6 - 140- a s120,i 3 80- cn E 100 c 0 W PT * c3 W z W 1E I 3 LL a CI v) r- z 3 cri 0 a 5 0 0 o o o o O 0 z 0 c- - - =? s - I40 -120 z -eo Z a b 0 . ‘IooE -1 - . . . . . . 0 . 0 . 0. H I * LL - 1 1 I 1 I 1 1 I 1 POSITION (cm) FIGURE 4 3 16 2950 6- ----I t - 2750 !z f 2550 m a S A _ _ v) 2350 I- J POSITiGY (cm) FIGURE 5 317 FIGURE 6 SOFT X-RAY CAMERA Robert Pet r e Massachusetts Institute of Technology Cambridge, MA ABSTRACT We describe a Wide-Field S o f t X-ray Camera (WFSXC) s ens i t i ve i n t h e 50 - 250 eV band. w h i c h is p r e s e n t l y u n d e r deve lopment by MIT and Le ices te r U n i v e r s i t y . The camera f e a t u r e s Wol t e r -Schwarzsch i ld o p t i c s w i t h an 8 degree f i e l d of view a n d 300 cm2 c o l l e c t i n g a r e a . The f o c a l p l a n e i n s t r u m e n t is a m i c r o c h a n n e l p l a t e d e t e c t o r . Broad-band et lergy d i s c r i m i n a t i o n is p r o v i d e d by t h i n - f i l m f i l t e r s mounted immedia t e ly i n f r o n t o f t h e f o c a l p l a n e . The WFSXC is c a p a b l e of d e t e c t i n g s o u r c e s w i t h i n t e n s i t i e s > 5 per cent o f HZ 43 d u r i n g t y p i c a l s o u n d i n g r o c k e t exposures, a n d i t would approach t h e same s e n s i t i v i t y r a n g e a s EUVE d u r i n g a t y p i c a l e x p o s u r e from t h e S h u t t l e . The X-ray Astronomy s o u n d i n g r o c k e t g r o u p a t MIT, a l o n g w i t h c o l l a b o r a t o r s from L e i c e s t e r U n i v e r s i t y , have deve loped a Wide F i e l d S o f t X-rsy Camera (WFSXC). T h e i n s t r u m e n t f e a t u r e s Wolter-Schwarz- s c h i l d o p t i c s ( W o l t e r 1 9 5 2 a , b ) w i t h an 8 d e g r e e d i a m e t e r f i e l d o f view and a m i c r o c h a n n e l p l a t e d e t e c t o r a t t h e f o c a l p l a n e . The WFSXC was d e s i g n e d t o c a r r y o u t an a l l - s k y s u r v e y i n t h e u l t r a s o f t X-ray band (50 - 250 eV; 50 - 250 A ) , w i t h m o d e r a t e s e n s i t i v i t y and a rc -minu te a n g u l a r r e s o l u t i o n . m i s s i o n o r S h u t t l e pay load a r e i t s w i d e f i e l d and i t s o p e r a t i o n i n an e n e r g v band t h a t is complementary t o t h e t r a d i t i o n a l s o f t X-ray band ( e , g . , t h e 0.1-4 keV band of E i n s t e i n ) . Al though t h i s camera by i t s e l f would n o t w a r r a n t an e x o l Q r e r micaior t , e s p e c i a i l y i n l i g h t o f t h e pending EUV E x p l o r e r , WFSXC would be a n i d e a l complement on a n e r i p l o r e r m i s s i o n t o a more c o n v e n t i o n a l s o f t X-ray te lescope o r o t h e r s o f t X-ray s p e c t r o m e t e r , Pe rhaps t h e most a p p r o p r i a t e a p p l i c a t i o n o f t h e W F S X C would be a s a S h u t t l e pay load . S i n c e i t i s a s u r v e y i n s t r u m e n t , i t c o u l d s i m p l y b e mounted i n t h e S h a t t l e bay w i t h o u t any i n d e p e n d e n t p o i n t i n g mechanism, o b s e r v i n g in w h a t e v e r d i r e c t i o n t h e S h u t t l e p o i n t s . A l t e r n a t i v e l y , its s h o r t f o c a l l e n g t h makes i t e a s i l y a d a p t a b l e i n t o an Exper iment of O p p o r t u n i t y ( E O P ) package . The p r imary a t t r a c t i o n s o f u s i n g WFSXC a s p a r t of an explorer A s c h e m a t i c o f t h e WFSXC a p p e a r s i n Fig . 1. A p h o t o o f t h e pay load ( p r i o r t o f i n a l w i r i n g f o r a s o u n d i n g r n c k e t e x p e r i m e n t ) is shown i n F i g . 2. Br ie f ly , t h e experiment c o n s i s t s o f three nested Wol t e r -Schwarzsch i ld t y p e I m i r r o r s . w i t h a m i c r o c h a n n e l p l a t e d e t e c t o r i n t h e f o c a l p l ane . Broad-band e n e r g y d i s c r i m i n a t i o n is p r o v i d e d by t h i n - f i l m f i l t e r s mcunted i m m e d i a t e l y above t h e f c z a l p l a n e . a r e a o f “300 cm . I n o r d e r t o o b t a i n a r e a s o n a b l e a n g u l a r r e s o l u - t i o n o v e r s u c h a b i d e f i e l d , ?t rJas n e c e s s a r y t o machine t h e mirrors i n a Wolter-Schwarzsch!ld t y p e I c o n f i g u r a t i o n , r a t h e r t h a n t h e more The m f r r o r s have an 8 d e g r e e f i e l d of view and a g e o m e t r i c a l 320 commonly used Wolter t y p e I p a r a b o l o i d - h y p e r b o l o i d . The Wol te r - S c h w a r z s c h i l d o p t i c s a r e s i m i l a r t o t h o s e o f t h e W o l t e r t y p e 1 ( W o l t e r 1952a1 , except t h a t t h e s u r f a c e s e x a c t l y f u l f i l l t h e Abbe c i n e cond i - t i o n ( W o l t e r 1952b) . The optimum f o c a i s u r f a c e is t h e r e f o r e f r ee o f t h o s e a b e r r a t i o n s which grow l i n e a r l y w i t h o f f - a x i s a n g l e (i.e.s coma). Hence t h e Wol t e r -Schwarzsch i ld is t h e s u p e r i o r d e s i g n f o r w i d e - f i e l d g r a z i n g - i n c i d e n c e o p t i c s (Chase and VanSpeybroeck 1373) . T h e d e s i g n cf s u c h a w i d e - f i e l d m i r r o r a l s o n e c e s s i t a t e d a s m a l l f o c a l r a t i o and t h e a s s o c i a t e d l a r g e g r a z i n g a n g l e s - The m i r r o r s t h e r e f o r e hav2 l i t t l e r e s p o n s e above -250 eV. T h i s is i n d i c a t e d i n F ig . 3 a , which shows t h e e f f e c t i v e a r e a o f t h e o p t i c s a s a f u n c t i o n o f o f f - a x i s a n g l e f o r v a r i o u s w a v e l e n g t h s . Approximate ly t w o - t h i r d s o f t h e e f f e c t i v e a r e a i s preserved o u t t o 4 d e g r e e s o f f a x i s . The m i r r o r s e s s e n t i a l l y compr i se an f : l o p t i c a l system, w i t h a 55 cm f o c a l l e n g t h and a 38 cm e n t r a n c e a p e r t u r e . T h e r e s o l u t i o n of t h e m i r r o r s (rms b l u r c i r c l e r a d i u s ) a s a f u n c t i o n o f o f f - a x i s a n g l e is i l l u s t r a t e d i n F i g . 3b. The d o t t e d curve shows t h e f o c a l p l i , n e c h a r a c t e r i s t i c s f o r a f i a t d e t e c t o r l o c a t e d a t t h e o n - a x i s f o c u s . S i n c e a ma jo r a b e r r a t i o n is c u r v a t u r e o f f i e l d , ' t i s p o s s i b l e t o s i g n i f i c a n t l y improve t h e o f f - a x i s p o i n t r e s p o n s e by c u r v i n g t h e f r o n t m i c r o c h a n n e l p l a t e t o match t h e optimum f o c a l s u r f a c e . The optimum f o c ? l n h a r a c t e r i s t i c s o f t h e m i r r o r s a r e r e p r e s e n t e d by t h e s o l i d l i n e i n F i g . 3b. The Leicester U n i v e r s i t y g roup i s d e v e l o p i n g s u c h a c u r v e d - p l a t e m i c r o c h a n n e l p l a t e d e t e c t o r f o r f u t u r e u s e w i t h t h e WFSXC. A t t h e ;)resent time, a f l a t m i c r o c h a n n e l p l a t e c j e t e c t o r h a s been r e p o s i t i o n e d 1.25 mm i n f r o n t o f t h e o n - a x i s f o c a l p o s i t i o n t o p r o v i d e t h e best a v e r a g e imaging p r o p e r t i e s o v e r t h e inner 5 degree d i a m e t e r of t h e f o c a l p l a n e , a s represented by t h e dashed l i n e i n F i g . 3b. The m i r r o r s were machined o u t o f 18 cm aluminum p l a t e s t o 2 k . S i n c e e a c h o f t h e two r e f l e c t i n g s u r f a c e s is so s h o r t , i t was p o s s i b l e t o machine bo th s u r f a c e s from a s i n g l e piece of aluminum. T h e m i , - r a r s were machined on an o r d i n a r y n u m e r i c a l l y - c o n t r o l l e d l a t h e and p l a t e d w i t h 250 m i c r o n s o f . = l e c t r o l e s s n i c k e l . The f i n s 1 s u r f a c e s were t h e n cu t on t h e s i n g l e - p o i n t d i amond- tu rn ing l a t h e a t t h e Y-12 p l a n t a t Oak Ridge, Tennessee. The r e f l e c t i n g s u r f a c e s were p o l i s h e d t o a low-sca t - t e r f i n i s h and e l e c t r o p l a t e d w i t h 1000 A o f g o l d t o enhance t h e i r r e f l e c t i v i t y below 250 eV (Mal ina and Cash 1 9 7 8 ) . I n o r d e r t o a t t a i n even a modest ("1 a r c m i n u t e ) a n g u l a r r e s o l u t i o n ( t h e s c a l e of an image on t h e f o c a l p l a n e i s 10 a r c m i n u t e s / m m ) , a s wei l a s d e t e c t X-rays w i t h e n e r g i e s l ess t h a n "!OG eV, i t was n e c e s s a r y t o use a m i c r o c h a n n e l p l a t e d e t e c t o r a s t h e f o c a l p l a n e i n s t r u m e n t . The d e t e c t o r c o n s i s t s o f two m i c r o c h a n n e l p l a t e s i n t h e chevron c o n f i g u r a t i o n w i t h a r e s i s t i v e anode r e a d o u t (Lampton and i a r e s c e 1971'). The d e t e c t o r was d e s i g n e d and c o n s t r u c t e d by t h e Leicester , ~ n i v e r s i t y g roup u s i n g p l a t e s s u p p l i e d by Mul l a rd . L t d . anr! a r e s i s t i v e anode f a b r i c a t e d b y E M I , L t d . . S i n c e t h e r e f l ec t ed X-rays a r e i n c i d e n t on t h e m i c r o c h a n n e l p l a t e a t l a r g e a n g l e s ("30 d e g r e e s ) and mic rochanne l p l a t e s a r e r e l a t i v e l y i n e f f i c i e n t a t these a n g l e s , a t o p p l a t e w i t h a 0 d e g r e e b i a s was chosen . t h u s p r e v e n t i n g a s y m m e t r i e s i n t h e camera e f f i c i e n c y a c r o s s t h e f o c a l p l a n e . The m i c r o c h a n n e l p l a t e e f f i c i e n c y was enhanced i n t h e s o f t X-rzy r e g i o n by c o a t i n g t h e t o p p l a t e w i t h magnesium f l u o r i d e (Lapson and Timothy 1976) . The r e s i s t i v e shee t r e a d o u t p r o v i d e s p o s i t i o n a l i n f o r m a t i o n a c c u r a t e t o "0.1 mm ( 1 a r c m i n u t e ) . 321 1 I Broad-band f i l t e r s a r e p l a c e d i n f r o n t of t h e f o c a l p l a n e t o p r o v i d e c r u d e s p e c t r a l i n f o r m a t i o n a s well a s t o f i l t e r ou t unwanted geocoronal background r a d i a t i o n ( 3 0 4 A and 584 A ) . The t r a n s m i s s i o n s of t h r e e s u c h f i l t e r s a s a f u n c t i o n of w a v e l e n g t h a r e shown i n F i g . 4a . We h a v e chosen t h e p a r y l e n e N and t h e B e r y l l i u m / p a r y l e n e N f i l t e r s f o r our f i r s t s o u n d i n g r o c k e t f l i g h t . F o r t h e s o u n d i n g r o c k e t e x p e r i m e n t we have s i m p l y mounted t h e two f i l t e r s s i d e by s i d e , e a c h c o v e r i n g h a l f t h e f o c a l p l a n e . ( A f i l t e r whee l would be used on a S h u t t l e o r e x p l o r e r mission.) The camera w i l l p e r f o r m i t t o b s e r v a t i o n s s c a n n i n g a t a c o n s t a n t r a t e i n t h e d i r ec t ion t h a t a l l o w s a s o u r c e p a s s i n g a c r o s s t h e f o c a l p l a n e t o be o b s e r v e d t h r o u g h b o t h f i l t e r s . I n a d d i t i o n t o t h e two t r a n s m i t t i n g f i l t e r s , an opaque f i l t e r c o v e r s a s m a l l p o r t i o n of t h e d e t e c t o r s u r f a c e so t h a t t h e c o u n t i n g r a t e from non-imaged events may b e m o n i t o r e d d u r i n g f l i g h t . F i g . 4b shows t h e o n - a x i s e f fec t ive a r e a of t h e e n t i r e camera ( f i l t e r , t e l e s c o p e and d e t e c t o r ) f o r t h e v a r i o u s f i l t e r s a s a f u n c t i o n of w a v e l e n g t h . The s e n s i t i v i t y of VFSXC a s a f u n c t i o n of time i s shown i n F ig . 5. I n s u n d i n g r o c k e t e x p o s u r e s ('10 sec) w e c a n d e t e c t sources down t o '1/20 t h e i n t e n s i t y of HZ 4 3 , t h e S r i g n t e s t known u l t r a s o f t X-ray / EUV s o u r c e (Lampton e t a l . 19?5) . I n a t y p i c a l e x p o s u r e a v a i l a b l e d u r i n g a S h u t t l e f l i g h t (1C3-1000 s e c ) , t h e experiment w i l l be 10-1CC times more sens i t ive , p l a c i n g i t i n t h e same s e n s i t i v i t y r a n g e a s EUVE. Of course, t h e pr imary a d v a n t a g e of t h i c i n s t r u m e n t i s i t s w i d e f i e l d of v i e d , wh ich makes it p a r t i c u l a r l y s t i t a b l e f o r s u r v e y a p p l i c a t i o n . The e n t i r e c e l e s t i a l s p h e r e car! be imaged i n '1000 exposures. An example of s u c h an a p p l i c a t i o n is on t h e German s a t e l l i t e ROSAT. The KFSXC complements t h e e n e r g y band o f t h e main RCSAT t e l e s c o p e , a n d i s i d e a l l y s u i t e d f o r t h e s u r v e y mission ROSAT i s t o u n d e r t a k e . I n f a c t , a s c a l e d - u p v e r s i o n o f UFSXC h a s been p roposed by a c o n s o r t i u m of B r i t i s h g r o u p s a s an a n c i l l a r y e x p e r i m e n t on ROSAT, and h a s been t e n t a t i v e l y i n c o r p o r a t e d i n t o t h e s a t e l l i t e d e s i g n . The rocket-borne v e r s i o n o f t h e WFSXC was l a u n c h e d from W h i t e Sands Missile Range on O c t o b e r 1 6 , 1981. I t s f l i g h t p l a n c o n s i s t e d o f a se r ies o f s c a n s a c r o s s '100 d e g r e e s of s k y i n wh ich s e v e r a l p o t e n t i a l u l t r a s o f t X-ray / E U V sources a r e l o c a t e d . T h e d a t a f rom t h e f l i g h t a r e p r e s e n t l y b e i n g p r o c e s s e d , and t h e i n s t r u m e n t i s b e i n g r e f u r b i s h e d f o r a s p r i n g , 1982 l aunch . REFERENCES Cha Lam Lam La P Ma 1 Wol Wol se 9 p t o n p t o n scn, i n a , t e r , t e r , R.C., and VanSpeybroeck, L.P. 1973, AP. O p t . , . M. . MarRon, 9. . P a r e s c e . F. . S t e r n . R . . and . Ap. J.-(Lette;s), 203 , -L711 . M . , and P a r e s c e , F . 7 7 4 , Rev. S c i I n s L.B., and T i m o t h y , J .C. 1976, A D . 1 R.F., and Cash, W. 1978, Ap. o p t . , G D t . . 17 . - - . . H. 1952a , Ann. P h y s . , 10 . 94. F. 1952b, Ann. P h y s . , 1 3 , 286. t r u m . 151 3309. 2 12 Bowy 45 , 2TB. 1042. e r , S. 1098. 1976 u 32 3 w I - o % l m l s o m z SDOm WAVELENGTH 31 Figurn 4 : (a ) Transmission of th.3 thin-film filters vs wavelength. ( b ) Effective a:-ea of the Wide-Field Soft X-ray Camera vs. wavelenct-h for each of the three filters. These areas include the reflcctivities of the X-ray mirrors, txansmisslon of the filters, and efficiency of thc microchanriel plate detector. I IO 100 1000 10000 OBSERVATION TIME (SI Figure 5: Sensitivity cf the WFSXC vs. observation time. Sensitivity represents the minimum source flux observable with 5 sigma significance over imageu background for the Parylene N filter. Imaged back- ground rate is approximatel- 300 photons s-1 sr-1. %’ 325 r R82* 2 6 0 7 0 X-RAY IMAGING ABOVE 3 keV M e l v i l l e P. Ulrner Department of Physics and Astronomy Northwestern Univer i s t y Evanston , I 1 1 i noi s 60201 I . INTROD2CTION My o r i g i n a l mot iva t ion for the designs presented here was t o produce energy dependent x - r a j maps of c lus te rs of galaxies. can a l so be used t o study supernova remnants, ga lac t i c nuc le i , etc. Of course, these designs 11. PENCIL REAM DESIGN The f i r s t design i s t ha t f o r a stacked, etched g r i d co l l imator , shown i n Figure 1. This could, i n p r i nc ip le , be pu t on the XTE. This would a l low the XTE s a t e l l i t e t o be used f o r d i f f u s e object study as wel l a s for po in t sources i n confused regions o f the galaxy. ezi;sion. The AXAF’s la rge focal plane scale i s not optimal f o r 10‘ t o lo extents, however. out t o 15 o r 20 keV q u i t e eas i l y , and AXAF o ~ l y has a token e f f e c t i v e area above -7 keV. Admittedly, AXAF w i l l study d i f f u s e Furthermore, the stacked, etched g r i d design can be used The p a r t i c u l a r design and experiment parameters tha t we present below a r e hased on a Space Lab proposal done i n co l labora t ion w i t h G. F r i t z and R. Cruddace. The co l l imator design has more than one so ut ion; the one presented here was done by the Rendix Corporation. design on the XTE i s t h a t the po in t i ng requirements are higher than those o f the XTE s a l t e l l i t e . Also, note t h a t the 8 x good, and i s achievable because o f the small f i e l d o f view. It demonstrates the power of t h s r e l a t i v e l y simple experiment. Note, the only drawback t o usage o f t h i s UFU s e n s i t i v i t y i s q u i t e 326 Experiment Summary Table 1 Cbject ive To stbdy c lus te rs of galaxies, ga lac t i c nuc le i and other extended and po in t x-ray sources with h igh s e n s i t i v i t y and l a r g e dynamic energy range. Technique Passive co l l imators and gas f i l l e d propor t ional counters. F i e l d o f View 4' x 4 ' and 12' x 12'. Col l imators 9 u n i t s o f stacked electroformed invar gold coated mesh. Detectors Low background wi re wal l propor t ional counters. - Energy Range -25 - 15 keV. E f fec t i ve Area S e n s i t i v i t y 3000 cm' a t 2-10 keV, 1000 cm2 a t 0.1-0.25 keV and 10-15 keV. ( 5 0 ) Point source 8 x UFU i n lo5 seconds, 2-6 keV. Required Point ing S t a b i l i t y and accuracy 1' o f arc, measurement t o b e t t e r 20" of arc. Accomodation Mass 215 kg power 100 watts. 1 meter square by 0.6 meter i n height. - Table 2: Col l imator Propert ies F i e l d o f View 4 x 4 arc min and 12 x 12 arc min Transmission 50 per cent E f fec t i ve Area (each) 320 cm' Size (each c o l l imator) (Over ac t i ve area of propor t ional counter) 230 mn x 280 mn x 360 mn high Weight 13.1 Kg Thennal Heat Leak (each) 7Y @ O C (10 watts rF 30° C) Total Ueight (9 co l l imators , op t i ca l 135 Kg bench, thermal cover) Total Heat Leak ( 9 c o l l imators 81W and thermal cover) 32 7 111. M I R R O R ARRAY DESIGN The second design we present overlaps wi th the c a p a b i l i t i e s of AXAF, but the design i s optimized t o work a t 7 keV. the present design i s qu i te good fo r sources tha t are t o be s tud ied on scales >1' Relow, we show below t h a t a t 7 keV, The basic design was used f o r a Space Lab proposal which I submitted i n co l labora t ion w i t h the Un ivers i ty o f Birmingham group (A. P. Willmore, P.I.). A s ing le m i r ro r module consis ts of 4 nested Wolter Type I mi r ro rs (Figure 2). Then 80 o f these modcles would be combined as shown i n Figure 3. separate images would be formed, and combined l a t e r by software. and sens i t i v i t i es a re sumna r i zed bel ow. The 80 The design Experiment Sumnary: Table 3 80 m i r r o r un i ts , each comprising 4 nested sections, focal leng th 200 cm, maximum mi r ro r -12 cm diameter, -40 cm long (Figure 2) Grazing angle 25' a l lowing operat ion t o a t l e a s t 8 keV Resolution 2 ' ; F ie ld of view -20' diameter Large 2 f fec t i ve area from one p o s i t i o n sens i t i ve propor t ional counter f o r de tec t ing the images from 20 m i r r o r un i ts . minimized by la rge guard region surrounding each image region and manufacturing problems reduced. Cost minimized by: * * * * Background consequently * (1) ( 2 ) 'Mass' producing m i r ro rs by e l e c t r o forming rep l i cas from a Aiming f o r moderate resolut ion, as above superpol i shed master 328 Table 4: S e n s i t i v i t y i n 4 x l o 4 seconds (50) Energy Point Source 20' Diameter Source 2-7 keV 2 x UFU 2 x UFU 7-9 keV 6 x UFU 6 x 1W2 UFU a t 6.7 keV, i r o n l i n e 5 x 1W6 photons/cm sec 5 x photons/cm sec 2 2 Table 5 Source Detected Time Required ergs/sec a t the Galact ic Nucleus..................200 seconds 5 lo3' ergs/sec ir t M31 Nucleus+. ........................ .4 x 10 seconds NGC4151-like ob ject a t 2 . -13.. ...................... .4 x 10 seconds I ron l i n e from Coma-like Cluster a t Z . .3.......4 x 10 seconds 4 4 r, Cluster 1/3 as b r i g h t as Coma w i t h 10' isothermal sphere 80 reso lu t i on elements > ~cJ........ 4 x 10 secdnds 3 Cluster 1/30 as b r i g h t as Coma--simple detect ion if 10' radius..............................4 x 10 seconds '2' reso lu t ion al lows a region of .2kpc rad ius t c be iso la ted. _.... We calcu lated the e f fec t i ve area of the e n t i r e system, and t h i s i s shown i n Figure 4. estimated losses due t o scat ter ing, shadowing, and propor t ional counter response. This area o f -300 cm2 compares q u i t e wel l w i t h the AXAF area a t 7 keV o f -200 cm'. The AXAF quoted area does not take i n t o account losses due t o sca t te r i ng o r the propor t ional counter. e f f e c t i v e area could be as low as 100 cm2, and hence the design we present here has 1.5 t o 3 times the area of AXAF a t 7 keV. The focal plane scale o f AXAF i s -5 times tha t of our design, and therefore, fo r d i f f u s e sources, w i t h scales >I1, the s ignal t o noise r a t i o w i l l be s i m i l a r f o r AXAF and the design presented he r e . Note, t h i s area i s on ly -303 o f the geometrical area, because of We est imate t h a t the t r u e 1V. MIRROR REPLICATION Is i t poss ib le t o make many m i r ro rs cheaply? We have been re-evaluat ing and o l d technique--electro-forming. "low sca t te r " o r "super pol ished" f i n i s h can be q u i t e e a s i l y reproduced. Total in tegra ted sca t te r i ng (Bennett, 1978) t e s t s were performed on both the mandrel ( p r i o r t o use) and the 14th electro-formed n i cke l pieces. For both pieces, the r.m.s. surface roughness was 15 2 2 A (Bennett, p r i v a t e comnunication). analyzed. beams from the f l a t s had c 1.4' h a l f power r a d i i , and the t o t a l r e f l e c t i v i t y o f the f l a t s was > 60 Z. Electro-forming op t i ca l f l a t s i s ac tua l l y more d i f f i c u l t than e lect ro- forming c y l i n d r i c a l pieces. Thus, we are o p t i m i s t i c about t h i s technique, and we are proceeding ahead w i t h plans t o reproduce super-polished c y l i n d r i c a l pieces. We found t h a t op t i ca l f l a t s t h a t have a 0 The 7 keV x-ray sca t te r ing r e s u l t s we performed are being The present pre l iminary analys is showed t h a t the re f l ec ted x-ray 3 30 V. CONCLUDING REMARKS I n conclusion, designs of modest p ro jec ts tha t are not covered by AXAF are d i f f i c u l t t o f ind. The designs presented here would more observing t ime fo r p ro jec ts tha t would not requ i re the h reso lu t ion c a p a b i l i t i e s o f AXAF. Given the funding s i t u a t i o n i n t e r e s t i n -7 keV astronomy, modest instruments w i t h capabil overlap XTE and AXAF should be given consideration. REFERENCES Bennett, H. E. 1978, Opt ical Engineering, - 17, 480. Rake ava and e gh spa t ia l and the h igh t i e s t h a t XTE 1 a3 332 ORIGINAL PAdk Is OF POOR QUAL^^^ Ezz e??zzz? I k I i I I I I I -+ Figure 2 Four ne,:ed Wolter Type I mirrors. One o f 80 modules 333 ORIGINAL PAGE tS OF POOR QUALITY Figure 3 Four u n i t s , each comprised of 20 nests o f 4 m i r r o r s and approximately .64x.64x2.5m a r e mounted 2 on each s i d e o f the Birmingham SL2 p o i n t i n g system. e tc . , a r e s i t u a t e d i n the c e n t r a l space. The s t a r t r a c k e r , gyros, 3 34 I ooc I O 0 I C I 0 ORIGINAL PAdE IS OF POOR QUALITY 2 \ 5 Energy ( k e V ) IO figure 4 The effective area versus energy for the f u l l 8Cl mirror/ proportional counter system Raymond J. Proctor Center for Astrophysus and Space Sciences UCSO. San Oiego, 92093 ABSTRACT lhe two types ot apertdire modulation techniques useful to X-ray imaging are reviewed. The use of optimum coded temporal aperture modulation is shown, in certain cases, to offer an eowantage over a spatial aperture modulator. Example applications of a diffuse anisotropic X-ray background experiment and a mde field of wew hard X-ray imager are discussed In aH hews of astronomy imaging satishes the prime need for oo11rce locations and structure. In X-ray astronomy imaging also prondes a freedom from source confusron and an answer to the question of ''which part ot the detected flux is source and whleh part is detector background?". The latter requiiement is partw*llarly relewd at photon energtes greater than 10 Kev. the regime of *%ad X-ray" astronomy. Unfortunately at energtes above 10 KeV R is not efftuent to use reflecting opttcs for imaging and some other method is necessary. The means to produce images at energtes greater than 10 KeV appear to be limited to aperture moduletior. or scanning methods Aperture modulabn is the encoding of the photons inadent on a detector such that thetr angular dependence can be recovered. This method has a mumplex ahntage over more dasstcal scanning methods (Fellget 1958) and is thus to be preferred There are two bascc types ot aperture modulation. spattal and temporal aperture modulatmn. Spatial aperlure modulation is a statcc .nethod pronding time independent imaging and is epnomized by Dlcke's random pnb le camera (Dldce 1968). whilst temporal aperture modulation is a dynamic method and is epdomned by Mertz's mack interferometer (Mertz 1966) which is more generally known as the rotatton moduletlon collimator of X-ray astronomy (Oda.1965; Schnopper.1968). The bauc pnnclples of the two methods are illustrated in F I ~ 1. The spatial aperture modulator casts an X-ray shadowgram onto the posltion sensitwe detector and hence requires N detector elements to detect N mash elements whilst each element of the temporal aperture modulator detects the fuil N elements of the modulator. atbelt in a hnite time. This basic h e to produce an image is the image cycle time dehnad as the time It takes the complete N aperture elements to traverse the detector element. For both methods the reconstruction ot the source direction can be achteved by back-projection ot the count rate modulation through the mask onto the sky. Both methods have been improved by the use ot optimum coded apertures based on Cyclic Difference Sets (Gunson and hlychronopulos.1976) or one of their less generalized subsets such as PN codes. Hadamard codes or Uniform redundsnt arrays (Hsrwrt 6 Sloane.1979, Fentmore 8 Cennon.1978). The optimum encoding allows the unrquo decoding ot cornplcatfd point and extended source held& Optimum coded spatrl aperture modulation has been used in X-ray astronomy to image the galactic center between 2 and 10 KeV (Proctor.Sbnner and Willmore.1978) whilst optimum tempore! aperture nwtulatmn has been usmi in numerous lntra-red spectroscopy and imagtng appleattons (HsFmt and Sloane,l~;s) The tuture of spatial aperture modulation WIVI Its tune independent imaging haa been recognized by many peaple. An optimum coded telescope to image dusters of galaxies to 3' in the energy range 2 to 25 KeV has been buill by the Unwerslty of Birmingham and is scheduled to fly on Spacelab 9 (Skinner.1980). Wide held 01 new spatlal aperture modulatmn cameras have been proposed tor the European XBO and Japanese ASTRO B missmns to mondor large fractions of the sky tor transient sources. At extreme separations between the coded aperture and the detector, the spatial aperture modulation techntque allows the possibility of very good angular 336 ORIGINAL PAGE I3 OF POOR QUALIN resolution (Hudson and Lin.1978) The PmholelOcculter Factllty Science Worlung grcup has incorporated this feature into a strawman model tor a spatially coded aperture modulation telescope for providing 20 arc second images of the sun combined mth a Fourier Transtorm telescope to provide marc second images over a smaller held of view (Hudson et a1.1980). The concept is shown IF F I ~ 2 When constant or slowly varylng X-ray sources are of interest then the technque of temporal aperture modulatm can be used. It otters the unique feature that one detector element can produce an image of many elements of sky Thus a high resolution. wide held of view camera can be built. A spatial aperture modulation system cannot provlde this option because it is constrained to have N detector elements to image N sky elements, thus n only prowdes options of high resolution.narrow held of wew or poor resolution. mde field of mew A high resolution camera normally involves the difhcult problem of attitude solution but a high resolution wtde held of view camera can see the brlghter X-ray sources and generate Its own internally conslstent attitude solutions One potential disadvantage appears to be that vanable background and source components create image baclqround fluctuations Provlded the time to complete one image cycle of the dyname mask is less than one minute a temporal modulator mll experience no difficulty from normal cosme ray background variations in near earth orbd. A variable source in the held of new adds a component to the image only of amplitude bI/d2/N where bl is the amp!itude change and N is the number of mask elements. Additionally varisble sources will have a period uncorrelated wdh the image cycle period hence over M image cyde perlods the rewiual image ~uctuatws WIN be unoMrebted and introduce a further I / @ reductton in thetr rebtive amplitude. Two example instruments. DAXBE and FOXI. mll be discussed to illustrate that the features of temporal aperture modulation urn be applied to X-ray astronomy. 4. THE DIFFUSE ANISOTROPK: X-RAY BACKGRWND EXPERMEKT ( OAXBE 1. 0 Most COSCIIIC rsdiatlons are thought to have "dlttuse" background components. The mmwave 27 K background and neutnno background are believed to be truly dlftuse and date from the ' B g Bang" of current cosmology whilst the X-ray background is believed to orginate from discrete sources since the epoch of galaxy formation !.e. at redshtfts of z t 3 . Very !ittle is as yet known about a grawtatmnal radiation background. There may be a difference in the anisotropes of the microwave and neutrino backgrounds which are eftectlvely at redshifts of zm1OOO as compared to the locally produced X-ray background at r<3 Wolfe.19~ Fabian & Wamdrl97% Rees,1380). Unfortunately the neutnno background has not as yet been cbserved but mtcroww observations from balloons have determined a 24 hour anisotropy in the approximate direction of our local supsrcluster (Conklin.1969; Smoot et a1..19TI, Cheng et a1,1979) Assuming the dipole effect is totally due to the moton of the sun and not intrinstc to the 27 K background then the effect is equivalent to an infall of our gelaxy towards the local supercluster at a veloclty of 401fl9 kmslsec (Cheng et SI-1980). Recent studies on me redshlfts of 300 sptral galaxies gwe a net solar motion mth 'aspect to nearby galaxiea ((30 Mpc) of comparable velocity but of mdely different direction from the Ip*tlon wtth respect to the 27 K background (Rubtn et aL.1976; de Vaucouleurs et al ,1981). The 24 hour Afuse anisotropy has been measured in the range 2-10 KeV by the HEAO-1 A2 experiment to be 0.005f0.0009 of the dlftuse flux (Boldt.1981). The pole of the distributlon lies at the same position IS the mwowave, at approximately the posttion of the local supercluster. However Boldt also shows that this anisotropy may not be inconsistent m h a reasonable luminosity of X-ray sources in the local supercluster, It may be possible to test this hypothesis if the anisotropy can be detected Mhout using the data from the local supercluster. The sduation is made more interesting by the discovery of an inhomogenedy in the universe whicl: IS larger than the local supercluster. An apparent dehctt in the number of optical galaxles exists in the constellation of Bootes. It is interpreted as a void of size 10 &stance of maOO Mpc mth 6MIM-1 (Kirshner.Oemler,Schecter and Shectman.1981L It is calcvlated by Kirshner et al. that such an inhomogeneity will gwe an approximately 150 Kmlsec perturbation to our galaxy's velocity relahve to distant galaxies (t-ll In the near Mure the Cosmic Background Explorer mll make very detailed microwave 6 far infrared observations of the cosmological background radmtion to prowde the large wale picture of our galaxy's peculiar 0 6 3 Mpc at a ORIGINAL PAGE Is OF POOR QiJALm velocity whilst surveys or ine redshifts of nearby galaxie- will provide the local picture of our galmy's pcrcultor wlouty. Better X-ray data are needed to complement a& complete these studies by providing the intermediate plcture and allow the local and cosmological e h t s to be separated. HOW WELL CAN WE MEASURE ruE X-RAY ANISOTROPY? 0 The baw 27 K 24 hour eftect has an amplitude of 0.0013 of the diWuae beckground. In X-rays the Compton-Getting effect (Compton 6 Getting.193SI. magnihes the amplitude to Qoo6 of the diffuse background. Hence our detection sensitivity must be at least 0.001 in order to obtain a 5 stgma detection. The mensurement of the anisotropy to an accu*scy of at least 0.001 requires an approach tree of systematic problems. T h e requirement is eft8ctively '.I map the shy with suthcient accuracy to exclude known X-ray sources. The conventional method involves the scanning of an asymmetric detector in the local anisotropic raemtion held which cspeccalty at energies > 20 KeV produces a variable background problem. The use of wde held. optimum coded temporal aperture modulation allows the use of a symmetrical detector and gives a much greater s8gnal to noise ratm such that the local background becomes negltgible The problem of experimental systematic eHects should be greatly reducea and because only a single detector element is needed tor imaging the experiment is very simple to build and operate. An enample of how such an instrument could Be operated is shown in F I ~ 3 The hgure shows no superstructure but only the splnning detectors and coded mash The detectors are simple detectors ot either gas counter or scintillating type optimised for a symmetric response. No position sensitivity is required. The detectors are constrained by the superstructure to see the ditluse bachground only through the coded aperture. low level leskage through the superstructure is not critical as only photons coded by the mask are "imaged". The spinning subsatellite receives an optimally modulated count rate from which an image of the fluctuations in a strip of the shy is made Taking the dimensions of Table 1, the statistical sensitivity of OAXBE in one month is given in Fig 4 in terms of a one sigma sensitivity as a fraction of the diffuse background flux. The sensitivity is per image elemetit If the sensitivity to a 24 hour component of the background is required then this may hr? considered as the comparison between two hemispheres of shy. Thus the sensitiwty is obtained by dividing by &I14 where 0 1s the number of source tree image elements used in the analysis. The tundamental Iimrtstion on the precision that can be obtained tor the anisotropy is from the fluctuation in the indiwdual !rnage element intensities due to source9 lust below the detection threshold. For the example instrument outlined here h g with 43 elements) the limit is dlll~O.304 per image element (Wamck,Pye 6 Fabian.1980). This translates into a 24 hour anisotropy detection of 0005f0.0006 ot the diftuse bachgrobnd. This is much worse than the 5-60 KeV statistical limits from one month of observation, e.g. for 5-15 KeV its f0.00015 ( f 1 2 k m s l s e c ) The statistical limit can be reached if external information on the number of bright extragalactic sources per image element is known because it is mainly the sources rust below the detection threshold (-1 UFlJ for DAXBE) which cause the fluctuations. This information should be forthcoming trom optical surveys and ROSAT (the German X-ray shy survey satellite) The presence ot high latitude galacttc components 0 1 0 times the 2-10 KeV anisotropy) complicates the analysis still turther But these components tall more rapidly wrth energy than the diftuse X-ray flux (Iwan et ak.1981) Hence more reliable measurements wll be obtained it energies greater than 20 KeV are used where the galactic components are smaller. It is noted that any incomplete galactc component subtraction leaves a 12 hour component and hence the minimisation of the galactic component gives more confidence in any detected 12 hour component Such components are expected horn the shear on the X-ray background produced By large mass inhomogeneities (Fabian i Warmch.1979). 5. FtRR/S-WHE€l ORWING X-RAY /MACER FOX! ) The study ot the energy generation mechanisms in actwe galactic nuclei (AGN's) is complicated by the nee4 to monitor the variability of their total energy output with time (Lightman, these proceedings). Prewous observations have shown that the majority of the power emitted by many Seyfert galaxies. some QSO's and Centaurus A lies in the high energy X-ray range The typical variability observed tor AGN's is of order 1 day w i h the record for the fastest time of variation of 7'100 seconds held By NGC 6814 (Tennant et a1.1981). The problem then is to monitor as many AGN's as possible for variability on time scales of minutes to years ovw I 337 338 as wide a photon energy range as possible. A possible method uses a Ferris-wheel Orbiting X-ray Imager (FOXI) imaging a strip of the sky every 3-4 secs (an eighth of the rotation period). The concept is shown in Fig S. F@XI is similar to OAXBE in principle but the dimensions are enormously inflated to 50 meters diameter. The use ot a flexible jointed mask which unturls in orbit after a Space Shuttle launch, allows such a rotating canhguration to be attained. The rotating mask modulates a large sold angle of sky in its rotation thus provlding the large held of view neccessary for a survey instrument To obtain good angular resolution the central detector must be one dimensionally position sensitwe in its azimuthal direction a 0 be fixed wth respect to the stars i.e. non co-rotating. The parameters of a system composed of 100 cm long, lcm wde stnp detectors imaging 4% of the sky to 3' x 140' over a photon energy range of 3-500 Kef' are given in Table 2 The instrument would be constrained to image perpendicular to the ecliptic in order to limit differential hmtmg of the mask This constraint means a complete sky survey would occur every 6 months The time spent on any one source in this time would be 4 days The minimum detectable source fluxes are given in Table 3. These minimum fluxes combined wth the linear relationship between X-ray and M optical luminosity (Zamorani et al.1981) suggest that the sensitmty is such that 0's of the mean or greagr L I L ratio will be detected down to M =+15 at 80-120 KeV and down to M =+17 between 3 and 20 KeV. It 13" n a that the large scatter in the ratio of L /l curve of Bracedi ?al.~(r9Kl~ shows that we would expect > 50 Q W s to be detected in the 80-120 KeV energy band ad, $1000 to 9 detected 3-20 KeV. All, -in 100 Mpc (-500) would be detected if bnghter than 10 ergasec and those above a10 ergacm would be detected at 80-120 KeV. FOXl was c~ncmved as a free fwng satehte but mth the advent of long duration manned space stabocls the workrng ennronment should include spm induced "gravity". Such large spnning structures could eastly indudh a coded mask and central detector to perform FOX1 type X-ray imaging. bmilarty in a future which included O'Neill space colonies a FOX1 type imager could be added to the very large m l k m diameter structures ! 8 0 B means that some O W s of M > +15 or > +17 mH be detected. The Log N ve~fsus M B CRiGItiAL PAGE IS OF POOR QUALlN 339 Genctd System Mode Spn permi Image cycle time Sky coverage Data rate Area to a p n t source Opttons: Non-rotatmg SeomentH deteIctof TABLE 1 Paramelers lor DAXBE VALUE 2-4 week free flyer "5 seconds Same as sptn period. 40% per rotahon COMMENTS -Recoverable 7 Internal from brlght sources. 0.- diameter Graded Z Tungsten 15-43 element optimum c d ~ n g -Tradeoff za0-aox lao 5oanxzoWn dlemeter xenon andlor phasmch 5-10 KeV to 200 KeV. 1x20 Steradisnt 2x15 st edlam 3X5ooUn T Allows detectm of the whole diffuse BG and not lust the Huduatms in k -3 detectors -Allows a tactor 0.5 tor the mask -Tradeoff study. 340 General System sw P e r d sllv -ege Itnag@ cycle ttme Ma rate Attitude Solutions Chmensconr M e t e d Pettcrn Mask element dimensions Resolution Omensions Mask 4lgubr - Oetectors Meterlel Element size of a detector "dr Ip" t ot detector ekments/block Energy range So(d rngke Area to a p i n t source fAsLE 2 Peramders lor FOX/ VMU€ *30 seconds [spn period]/% 3-4 SBCS 4% per ro ta tm <30 K b d W Sun potnted and X-ray sources 03cmx100anx5000crn diemeter Graded Z Tungstw 8 times a 1023 dement code 03cm x 2cm xlOO cm 3 ' x lW lOOcm x lOOcm diameter Xenon gas counter plus Phoswich lcm wide x 5 cm x lOOcm Xenon 3-20 KeV Phoswuh 20-500 KeV 3 x 025 teradians 3x3500cm 9 COMMfNlS -1- every 6 months -Sources >5 UFU seen in one spn -3 cyltndrtcai detector Mocks. -Xenon on the outsnle Phosmch inside -Xenon and Phoswrch systems are independent. -On the c.:cum- terence of a CyllndW. -Allows a tactor 05 tor the mash TABLE 3 Foxl MINIMUM DRECTABLE SOURCE FLUXES (6 SIGMA) - 2 - 1 -1 In units ot photonscm sec KeV 1 minicte 1 hour 4 days -5 5 - 15 KeV 7x 10 I . ~ X I O - ~ 15x10 -4 (2 UFU) (a4 UFU) (urn UFU) -5 -5 -6 80 - 120 KeV 7x10 15x10 15x10 -7 200 - 300 KeV W O - ~ 8x10 -6 8x10 34 1 34 2 REFERENCES Boldt,€,Comments on Astrophysics, 1981,997. ~ a c e s s i ~ Z i t e l l t . V . ~ ~ l ~ F . a81d Formiggini,L.l-tron Astt oph.69.80 Cheng,EtSaulsan.P,Wil~n~n,D.T. and Corey&197Z&.J,232,L 139 Cheng.ES&ughn.S and W.lkinson.D.T,l~Bulletin AAS 1 ZVol 2,489 Compton.AH. and Gelttng,lA,l335,Ph~Rev.47,817. Conklin,EA, lmNature.222.971. de Vaucouleur~G,PelerqW.L..Bot(inelli,L.Gouguenheim,L and Dnke.R.H, 1 S8,Ap.J- 153.L 10 1. Fabtan.AC. and Warwi~R.S.l979,Nature.ZBQ3 Fe1lgetP.B. 1958,J.Phys.,Radiurn. 19,187 Feriirnore,€E. and Cennon.T.U.1978.Applied Opt~q17.337. GvnsonJ. and PolychronopvloS.B, 1976.Mon.NotRAstr.S~&177,485. Harwt,M. and Sbane.N JA, 1979,"Hadamard Transform Optics' ,Aced Pr Huds0n.H.S. and Lin.R.P, 1978,Space ki. Instr,l, 101. Hudson,H.S..KohI,J.L.Lin.R.P.,MacQueen.R M.Tandbwg-Hanssen,E and Dabhs.J.R,"The Rn hole/Occulter Factlity". 1 9 8 1.NASA TM-824 13. iwan.D.,BoldfE.A,Mars hall.F.E..Musholzky,R.F,Shafer,R.Aand Stott1emyer.A. 1981.ApJ.in press krshner,R P .Oemler,A,Schechter.P.L. and Schectman.SA. 1981.Ap.J, Merlz,L,1965.'7ransformations in Oytics",(New York-WtleyL Oda.M..1965,AppI.Opt,4.143 Proctor.R.J .Skinner.G.K. and WillmoreA P,1978,Mon.NoLRAslr.Sx, Rees.M.J ."'X-ray Astronomy", 1SBO.Giacorint.R and Selti,G,(Eds).y377. Rubin.V.C,Thonnard,l~,Ford.W KJr and Roberts.M.S.1976,ApJ .8 1,68> .~h~pyer.h'.W,Thompson.R.I and Watt S.. 1968,Space Sc! Rev.R.534 :.tirrne:,G K,ISBO.Jot:rnai 01 tile British loterplanelary Society 33,T' Sm0ot.G.F .Gorenstein,M.V. ana Muller,FiA. :S~,PhysRev.Letlers.~B98 T~nnant~F,Mvsholzky.R.F,Boldt,EA and SwankJH,l38l,MSA TM-82143 W~lrwuk,R S,PyeJ.P. and FabianAC.. 1 9 8 0 . M o n . N o t . R A s l r . S ~ ~ , ~ WolfeAM, 1970JpJ. 159.L61 Zemorani.G,Henry~.P,Maccecaro,T~lanan~u~,H,~ltan~vni,Y~ 24a.L57 185.745 Paturei.G, 1%: JpJ.248.408. LiebertJ,SlockeJ,Stritlmatter,P~Weymann,RJ,Smilh,M.G. and Candon JJ, 1981,Submcltc?d lo AW. FIWRE CAPTIONS 343 FIGURE 1 The basic principles of the two methods are shown. The spatiel aperture modulator casts an X-ray shadowgram onto the position sensitive detector and hence requires N detector elements to detect N mask dements whilst each element of the temporal aperture modulator detects the full N elements of the modulator. Both methods provide an N element image of t i e SIC). FIGURE 2 The layout of a "Pinhole/Occulter Facility" to be deployed from the Space Shuttle. The 50 meter boom supports a spetielly encoded aperture mash to give 61 arc second spatial resolution for hard X-rays. For solar observations an occulter will also be used to provide a shadow for coronagraphic observations. FlGURE 3 A conceptual Oilfuse Anisotropy X-ray Background Experiment (OAXBE) is shown. The concept is for a tree-flying internally powered experiment The sptnning sub-satellite causes the individual detect6rs to see an optimum inodulated count rate from separate strips of shy. The total active field of view is -40% of the sky per rotation. With precession a 100 % coverage can be obtained in 8 short time. The energy range is detector dependent but 5-200 KeV seems easily achievable. FIGURE 4 The one sigma statistical sensitivity per sky bm of OAXBE in one month ('2x10 seconds) IS shown. It i s tbouyht that the effect of source fluctuations and the galactic dtffuse component will not significantly degrade the sensrtwly (see Text). 6 FIGURE 5. 4 con# :ptuaI Ferris-wheel Orhting X-ray Imager (FCXI) ts shorn. The concept is of a free-flying satellite which iJnfurlS in orbtt after a Shuttle launch. It is envisaged that t : iP mask would rotate every 20-30 seconds whilsl the detector remained fixed relative to the stars The mask would consist of a basic 1023 elernant optmuri code repeated 8 times to give an image cycle time of one eighth of the spin perloc. The detec!or coosists of 3 w r i t s ot 1 meter diameter made up of 1 cm wide gas counter end scintiilator strip elenrenls FOX1 will image 4% of the sky every 3-4 seconds with a resolution of 3' x 140' over the photon energy range 3-500 KeV 100% shy coverage is obtained every 6 months. Over 2OOO Active Galactic nuclei should be detected (see Text). 344 A b. 0 Q E w k- a 2 0 w 0 U 3 0 1 m w I- z $ w V U 34s 346 0 0.005 z A 0.004 0.001! 0.002 0.001 34 7 S E N S I T I V I T Y OF D A X B E - FOR A O k E MONTH hlISSION 0 1 3 1 M A S K E L E M E N T S / 195 SK' / - V BINS OF 18°x120 / / . I 5 MASK E L E M E N T S *95 S K Y BINS OF l a0x2qb 1 I 1 I A. 20 60 IO0 140 I80 ENERGY ( k e V ) 348 n .--+? 349 a N82* 2 6 0 7 2 HARD X-.pAY IMAGING FROM EXPLORER J . E. Grindlay and S. S. Murray Harvard-Smithsonian Center f o r As t rophys ic s ABSTRACT Hard X-ray Astronomy (at e n e r g i e s 210 keV) h a s not y e t enjoyed t h e high s e n s i t i v i t y and high r e s o l u t i o n t h a t imaging t echn iques and t h e E i n s t e i n Observatory have allowed f o r t h e s o f t X-ray band. Many fundamental problems i n high energy a s t r o p h y s i c s r e q u i r e g r e a t l y inc reased s e n s i t i v i t y and observing t i m e a t hard X-ray e n e r g i e s . It i s p o s s i b l e t o o b t a i n such l a r g e i n c r e a s e s i n s e n s i t i v i t y as well as a n g u l a r r e s o l u t i o n w i t h coded a p e r t u r e imaging X-ray d e t e c t o r s . A new hard X-ray coded a p e r t u r e d e t e c t o r concept i s desc r ibed which would enab le very high s e n s i t i v i t y s t u d i e s of p e r s i s t e n t hard X-ray (-15-300 KeV) sources and gamma ray b u r s t s ('50-550 K e V ) . Coded a p e r t u r e imaging i s employed so t h a t '2' source l o c a t i o n s can be der ived w i t h i n a 3" f i e l d of view. Gamma b u r s t s could be l o c a t e d i n i t i a l l y t o w i t h i n '2" and X-ray/hard X-ray s p e c t r a and t iming, as w e l l as p r e c i s e l o c a t i o n s (-2'1, d e r i v e d f o r p o s s i b l e b u r s t a f t e r g l o w emission. Hard X-ray imaging should be conducted from a n Explorer mis s ion where long exposure times are p o s s i b l e . 1. INTRODUCTION With t h e E i n s t e i n Observatory, s o f t X-ray astronomy has become f u l l y competi t ive w i t h o p t i c a l and r a d i o astronomy i n t h a t v i r t u a l l y a l l major c l a s s e s of as t ronomica l o b j e c t have proven t o be d e t e c t a b l e i n t h i s energy band. Such s p e c t a c u l a r growth h a s not y e t occu r red i n hard X-ray astronomy. However, t h e r e s u l t s which are a v a i l a b l e suggest a r i c h v a r i e t y of a s t r o p h y s i c a l problems. For example, a i though E i n s t e i n o b s e r v a t i o n s have r evea led t h a t q u a s a r s and a c t i v e g a l a c t i c n u c l e i (AGN) are almost always d e t e c t a b l e i n ( s o f t : X-rays, t h e i r s p e c t r a and hence source r a d i a t i o n mechanisms are s t i l l l a r g e l y unknown. I n t h e few b r i g h t e s t q u a s a r s , and i n a somewhat l a r g e r number of st i l l b r i g h t e r S e y f e r t g a l a x i e s , hard X-ray emission ex tend ing ou t t o A t l e a s t s e v e r a l hundred keV i s d e t e c t e d . Most of t h e r a d i a t i o n and luminos i ty from q u a s a r s and AGNs may be i n t h e hard X-ray band, and t h u s r ea l i s t i c models w i l l not be p o s s i b l e u n t i l many high q u a l i t y X-ray o b s e r v a t i o n s a t e n e r g i e s up through t h e probable h i g h energy s p e c t r a l break ( a t -200-300 keV) are a v a i l a b l e . I n a d d i t i o n t o t h e q u a s a r s , a number of o t h e r fundamefital problems i n high energy a s t r o p h y s i c s may also be s t u d i e d , and i n some c a s e s u- i e r s tood , b e s t i n t h e hard X-ray band. The s c i e n t i f i c need f o r high s e n s i t i v i t y and high angu la r r e s o l u t i o n a t hard X-ray e n e r g i e s (-10-500 keV), a s d i scussed i n t h e next s e c t i o n , could be m e t w i t h a coded-aperture imaging d e t e c t o r . An Explo re r - c l a s s ha rd X-ray imaging experiment would be t h e i d e a l compliment t o f u t u r e d i r e c t imaging X-ray t e l e s c o p e s (e.g., EXDSAT, ROSAT, AXAF, and e v e n t u a l l y , LAMAR) which w i l l , of cour se , be s e n s i t i v e only below ' 5 - 8 k e V . 350 2. SCIENTIFIC NEED FOR HARD X-RAY OBSERVATIONS A wide range of fundamental problems i n a s t r o p h y s i c s can best be a t t a c k e d wi th s e n s i t i v e new o b s e r v a t i o n s a t hard X-ray e n e r g i e s . It i s p o s s i b l e t o summarize these i n t h e form of a t a b l e of p a r t i c u l a r problems and t h e approximate ranges of photon e n e r g i e s r e q u i r e d f o r t h e i r s o l u t i o n . Table 1 c o n t a i n s such a summary, i n which each e n t r y i s a n area of s tudy which r e q u i r e s o b s e r v a t i o n s in t h e photon energy range shown. Ar row t i p s mean t h a t t h e energy l i m i t ( h igh or low) is not determined, whereas v e r t i c a l t i c k s a t t h e energy range ends mean t h a t t h e range i s more or less f i x e d . w e l l I n t h e gene ra l s tudy of compact o b j e c t s hard X-ray s p e c t r a can provide a k e y test f o r t h e n a t u r e of t h e o b j e c t . White dwarfs would not be expected t o have thermal s p e c t r a w i t h t empera tu res above t h e f r e e f u l l temperatures of ‘200 keV whereas neu t ron stars could have thermal components ex tend ing t o MeV e n e r g i e s . X-ray b u r s t s have shown t h a t neutron star r a d i i can be measured from t h e blackbody s p e c t r a a t low e n e r g i e s whereas r e c e n t gamma ray b u r s t s t u d i e s by t h e S o v i e t s a t ‘30-500 keV suggest that t h e 511 keV a n n i h i l a t i o n l i n e has been d e t e c t e d a t a g r a v i t a t i o n a l r e d s h i f t of ‘ 0 . 2 - 0 . 3 , or approximately t h a t expected f o r a neu t ron stsr. Cyclotron l i n e s have been d e t e c t e d a t hard X-ray e n e r g i e s (‘60 keV) and nebcron star magnetospheres m i g h t be s t u d i e d a t e i t h e r hard (‘1OC-200 ‘ ) e n e r g i e s corresponding t o l o c a l f r e e f u l l t empera tu res a t t h e A l i v r u r a d i u s or s o f t (‘0.3 keV) e n e r g i e s i f t h e r e i s o p t i c a l l y t h i c k re-emission. Acc re t ion f lows and t h e d i s t r i b u t i o n of coo l gas and hot e l e c t r o n s i n a c c r e t i n g sys tems r e q u i r e hard X-ray obse rva t ions . In p a r t i c u l a r , t h e s tudy of Comptonization of a s o f t s p e c t r a l component by e n e r g e t i c e l e c t r o n s can lead t o hard Compton-tails o u t t o s e v e r a l hundred kel’. The r e s u l t i n g power law - t ype s p e c t r a and thermal c u t o f f s might be expected i n many m9re o b j e c t s t han j u s t Cyg X-1 where they have been observed. S t u d i e s of t h e simultaneous v a r i a t i o n s i n t h e s o f t vs . hard X-ray s p e c t r a a s a f u n c t i o n of b i n a r y phase might a l low t h e s p a t i a l temperature and d e n s i t y s t r u c t u r e of t h e a c c r e t i o n flow t o be s tud ied . Determination of t h e r a d i a t i o n mechanisms involved i n g iven X-ray sou rces obviously r e q u i r e s broad bend s p e c t r a ex tend ing t o hard e n e r g i e s . Thermal c u t o f f s a t high e n e r g i e s can d i s t i n g u i s h Comptonization of thermal s p e c t r a from non-thermal p rocesses . Sources wi th predominantly ‘10 keV thermal bremsstrahlung s p e c t r a (e.g. , most g a l a c t i c bulge s o u r c e s ) may be expected t o have unde r ly ing ha rd components from thermal p rocesses ( f ree f a l l , Comptonization, e t c . ) or non-thermal p a t t i c l e a c c e l e r a t i o n as i n SS433. F i n a l l y , and perhaps most important , t h e b a s i c unde r s t and ing of any o b j e c t r e q u i r e s knowing i t s luminosi ty and o v e r a l l e n e r g e t i c s . For q u a s a r s and a c t i v e g a l a c t i c n u c l e i (AGN) i n g e n e r a l , t h i s r e q u i r e s hard X-ray o b s e r v a t i o n s s i n c e t h e s p e c t r a a l r e a d y observed from t h e f e w b r i g h t e s t sources i n d i c a t e t h a t t h e hard band (>lo0 keV) c o n t a i n s a l a r g e f r a c t i o n of t h e t o t a l energy r a d i a t e d . It i s not p o s s i b l e t o PHOTON ENERGY: Compact Objects White Dwarf vs . -star Radius ( B u r s t e r s ) G r a v i t y (M/R) Magnetic F i e l d T a b l e 1 QUESTIONS I N X-RAY ASTRONOMY Vs. ENERGY 35 1 Magnetosphere A c c r e t i o n Flows Cold G a s (Absorp t ion ) H o t Electrons (Comptoniza t ion) Composi t ion & I o n i z a t i o n B a l . - R a d i a t i o n Mechanisms Thermal ( B B , B r e m s . , - - - I Non-thermal (Synch., Cyc l . , I C ) 1 I I 1 1 --1 1000 kiV 0 . 1 1 10 100 f------------r I I - Luminosity & E n e r g e t i c s AGN, QSOs G a l . C l u s t e r s I - I Galaxies c b Compact Sources + > S N R - S t a r s v s . DETECTORS : Graz ing Incidencc! t I Col l ima ted Prop. Counters < HARD X-RAY IVAGING * 352 c o n s t r u c t meaningful models f o r q u a s a r s without knowing where (and why) t h e i r s p e c t r a f i n a l l y cu to f f a t high ene rg ie s . Galaxy c l u s t e r s must emit some l e v e l of i nve r se Compton r a d i a t i o n from synchro t ron r a d i o e l e c t r o n s s c a t t e r i n g on t h e microwave background. T h i s should extend t o hard e o e r g i e s and i t s d e t e c t i o n would c o n s t r a i n t h e e n e r g e t i c s of r e l a t i v i s t i c gas i n galaxy c l u s t e r s . A t somewhat lower l u m i n o s i t i e s , emission from normal g a l a x i e s and compact o b j e c t s have hard X-ray components which are almost c e r t a i n l y s i g n i f i c a n t whereas SNR and stars probably have p e g l i g i b l e l u m i n o s i t y a t hard X-ray e n e r g i e s . A t t h e bottom of Table 1, w e g ive t h e approximate energy ranges of t h e p r i n c i p a l d e t e c t o r systems used t o a t t a c k t h e problems d i scussed Pbove. It i s c l e a r t h a t g r a z i n g incidence imaging, as used on E i n s t e i n , cannot d e a l w i th a s i g n i f i c a n t number of t h e s e major problem a r e a s , nor can co l l ima ted p r o p z r t i o n a l c o u n t e r s of t h e s o r t employed i n a l l X-ray astronomy missions t h u s f a r . Hard X-ray imaging, however, could a t t a c k a l l of th.. major problems. 3 . CODED APERTURE SYSTEMS FOR HARD X-RAY I U G I N G A t e n e r g i e s above t h e g raz ing inc idence l i m i t of ‘10 keV, X-ray imaging can be accomplished by a t least two techniques. Bragg d i f f r a c t i o n f l u x c o n c e n t r a t o r s can be employed t o ach ieve l a r g e e f f e c t i v e d e t e c t o r a r e a s w i t h small d e t e c t i n g elements (and t h u s d e t e c t o r backgrounds). Such a s y s t e m i s desc r ibed by Ricker (1981) but i t i s l i m i t e d t o e n e r g i e s 5100 keV. For t h e broad-band s t u d i e s desc r ibed above, where good s e n s i t i v i t y and a n g u l a r r e s o l u t i o n up t o ‘500 keV is d e s i r e d , i t appea r s t h a t coded apert’ire imaging i s t h e optimum technique. Th i s scheme employs a n a p e r t u r e , w i th a n array of t r a n s m i t t i n g h o l e s , followed by a p o s i t i o n - s e n s i t i v e d e t e c t o r . It i s t h u s j u s t t h e m u l t i p l e pinhole camera proposed by Dicke (1968). Images a r e r econs t ruc t ed (un ique ly ) by, e s s e n t i a l l y , r e - p r o j e c t i n g each d e t e c t e d photon back through each open mask ho le . A 1 1 counts from a given source are d e t e c t e d i n t h e t r u e inage p i x e l a l thcugh ( i n c o n t r a s t t o t r u e imaging) t h e t o t a l background ( i n c l d d i n g sources ) i s a l s o n e c e s s a r i l y re-projected i n t o each image p i x e l . For imaging hard X-rays, t h e requirement t h a t t h e d e t e c t o r and mask p lanes be p a r a l l e l and t h e d e t e c t o r be t h i n ( f o r minimum d e t e c t e d p o s i t i o n ambigui ty and t h u s image smearing) means t h a t c r y s t a l s c i n t i l l a t o r s are p r e f e r r e d f o r the d e t e c t o r . P o s i t i o n - s e n s i t i v e d e t e c t i o n of t h e o p t i c a l l i , e h t produced by a hard x-ray i n a c rys ta l s c i n t i l l a t o r i s then t h e t e c h n i c a l requirement f o r a coded a p e r t u r e hard X-ray imaging d e t e c t o r . We s h a l l desc r ibe i n t h e next s e c t i o n s a proposed d e t e c t o r i n some d e t a i l . F i r s t , we summarize the main advantages of a coded a p e r t u r e s y s t e m f o r hard X-rays. The angu la r r e s o l u t i o n can be h igh , and i s j u s t the angular spacing u f mask h o l e s (which become image e l emen t s ) a s seen from t h e d e t e c t o r . Thus sau rce confusion i n crowded f i e l d s ( e . g . , t h e g a l a c t i c c e n t e r r e g i o n ) c i n be e l imina ted . The f i e l d of view can be l a r g e al though the d i f l u s e background i s then a l s o l a r g e , ORIG;K!L FLGE 13 OF POOR QUALITY I - o - v I : E N - I I 2 >, - 2 - s \ s X 3 A tr I 0 -3 - -4 353 - - - but must be commensurate wi th t h e number of image elements and t h e i r angu la r s i z e as governed by t h e mask. F i n a l l y , and most important , t h e d e t e c t e d background can be measured s imultaneously wi th a source by simply comparing t h e ra tes i n t h e r e c o n s t r u c t e d image i n and ou t of t h e p i x e l ( s ) of i n t e r e s t c o n t a i n i n g t h e sou rce ( s ) . Th i s means, of coureie, t h a t cont inuous p o i n t i n g s of long exposure can be c a r r i e d o u t f o r very l a r g e i n c r e a s e s i n s e n s i t i v i t y over previous scanning d e t e c t o r s such as HEAO-A4. T h i s i s a l s o a n improvement over d e t e c t o r s which chop on and o f f source (e.g. , the NRL experiment on s i n c e chop t i m e s must always be s h o r t e r t han t h e ( u n c e r t a i n ) t i m e s c a l e s f o r background v a r i a t i o n s if t h e s e are t o be removed. GRO) I n Figure 1 w e compare t h e approximate s e n s i t i v i t y t h a t a p l a u s i b l e coded a p e r t u r e hard X-ray imaging d e t e c t o r s y s t e m might give r e l a t i v e t o the s e n s i t i v i t i e s achieved w i t h HEAO-1 and HEAO-2. The hard X-ray imaging experiments w i l l , i n g e n e r a l , not be designed t o o p e r a t e belov ‘10 keV ( t h e domain of normal imaging X-ray t e l e s c o p e s ) and w i l l not ptrform as wel l above ‘500 keV-1 MeV, where Compton s c a t t e r i n g and p a i r p roduc t ion i n t h e mask be come s i g n i f i c a n t . S e n s i t i v i t i e s could be achieved (from a n Explorer mission) i n t h e ‘10-500 keV range which a r e a t l e a s t a f a c t o r of 10-30 g r e a t e r t han agai l a b l e p rev ious ly above 10 keV even f o r t h e m o d e s t - s k d hard X-ray imaging experiment desc r ibed next. APPROXIMATE CURRENT SENSI FlVlTY LIMITS \ POSSIBLE \ \HaROX-RnY \ \IMAGING EINSTEIN -5 01 I 10 1 0 0 lo00 X-RAY ENERGY CheV) Figure 1: Approximate r e l a t i v e s e n s i t i v i t y of X-ray experiments and the Crab spectrum f o r compari:-.oii. 4. PROPOSED NARD X-RAY IMAGING EXPERIMENT ‘de now d e s c r i b e a p a r t i c u l a r experiment c o n f i g u r a t i o n f o r hard X-ray imaging from a n Explorer . T h i s i s t h e E n e r g e t i c X-ray Imaging and Timing Experiment (EXITE) which was proposed a s a second experiment t o XTE. E i t h e r t h i s experiment a s proposed, o r an en la rged v e r s i o n (EXITE i s modular) f o r an e n t i r e Explorer mis s ion , would enable t h e major a s t r o p h y s i c a l problems d i scussed above ( c f . Table 1) t o be s t u d i e d i n d e t a i l . The s c i e n t i f i c o b j e c t i v e s ( i n abbrev ia t ed form) of EXITE a r e : (1) t o measure Comptonization t a i l s and c y c l o t r o n f e a t u r e s i n source 3s4 ORIGINAL PAGE 1s OF POOR QUALlW s p e c t r a t o determine physic31 c o n d i t i o n s a t t h e source; ( 2 ) t o measure spectral- temporal v a r i a b i l i t y a t high e n e r g i e s and s tudy a c c r e t i o n f lows ; ( 3 ) t o l o c a t e and i d e n t i f y hard X-ray ! :wrces i n complex f i e l d s ; (4) t a measure high energy s p e c t r a of q u a s a r s and l o c a t e s e r e n d i p i t o u s sou rces ; and ( 5 ) t o measure t h e size spectrum ( l o g N - l o g SI, energy spectrum ( i n c l u d i n g l i n e emis s ion ) and source l o c a t i o n s f o r gamma ray b u r s t s . brcad General D e s c r i p t i o n of EXITE EXITE r e p r e s e n t 8 a f undamentally n e w concept i n hard X-ray astronomy. It a p p l i e s coded a p e r t u r e imaging t o t h e ha rd X-ray band ('15-30'3 KeV) and a t once r e a l i z e s s e v e r a l key advantages mentioned above which were p rev ious ly only p o s s i b l e a t l o v X-ray e n e r g i e s : ( 1 ) background i s measured s imultaneously wi th t h e o b j e c t f l u x ; t h e r e i s no need t o scan o r o f f - s e t p o i n t ; (2 ) a so\;rce can be r e so lved i n complex f i e l d s ; and ( 3 ) p r e c i s e ('2-5 arcmin) p o s i t i o n s can be de r ived f o r t h e f irst t i m e a t ha rd X-ray e n e r g i e s ('100 KeV) by c e n t r o i d i n g between r e s o l u t i o n elements of t h e r e c o n s t r u c t e d image. Coded a p e r t u r e imaging sys t ems are based on t h e m u l t i p l e pinhole camera (Dicke 1968) where N ove r l app ing images of a source d i s t r i b u t i o n are formed on a d e t e c t o r viewing t h e source through N p inho les i n a n a p e r t u r e mask. Although Dicke suggested us ing a random ho le p a t t e r n , images r e c o n s t r u c t e d u s i n g a "random1' p a t t e r n (never t r u l y random) c o n t a i n a n a d d i t i o n a l source of systematic no i se . To overcome t h i s w e suggested ( i n a 1976 Spacelab 2 proposa l ) u s ing a "pseudo-noise" (PN) array s i n c e t h e s e have t h e optimum prope r ty (Calabro and Wolf 1966, t h a t t h e i r a u t o c o r r e l a t i o n f u n c t i o n (ACF) i s i d e n t i c a l l y cons t an t o f f a x i s and a & f u n c t i o n on a x i s . PN arrays were independently pointed ou t by Gunsen and Polychronopulos (1976) and have been d i scussed e x t e n s i v e l y by Fenimore (1980, and r e f e r e n c e s t h e r e i n ) who c a l l them uniformly redundant arrays ( U R A ) . The ( s y s t e m a t i c ) no i se - f r ee imaging p o s s i b l e wi th a URA mask i s not compromised by a phys ica l r e a l i z a t i o n of such a p a t t e r n where t h e ho le s i z e i s smaller than t h e h o l e spacing. Furthermore, t h e URA i s c y c l i c and h a s t h e very d e s i r a b l e "window property" t h a t a complete p a t t e r n i s a v a i l a b l e t o surround eve ry element of a n extended mask. The EXITE d e t e c t o r makes optimum use of t h e s e d e s i r a b l e p r o p e r t i e s of t h e URA i n t h a t each s i n g l e d e t e c t o r (a 15 cm diameter CsT s c i n t i l l a t o r c r y s t a l ) views t h e sky through a URA mask composed of a number ( - 5 0 ) of cont iguous c y c l e s of a b a s i c URA p a t t e r n (each of dimension 13 x 11; see Figure 2 ) . A s i n g l e d e t e c t o r , which is read ou t as one d e v i c e , then e f f e c t i v e l y ( i n t h e ana lys i s ) produces '50 independent and p a r a l l e l 13 x 11 p i x e l images which a r e then co-added. T h i s a l l o w s a very much F i g u r e 2 : A 2 x 2 Cvcle Port ion o f t h e E x t e n d e d llRA Coded A p e r t u r e Col l imator . 355 faster analysis (by a f a c t o r of 50) of t h e t o t a l image than i f t h e e n t i r e d e t e c t o r area vieved t h e entire URA mask as i n a “convent ional“ coded a p e r t u r e camera. The mosaicing of t h e d e t e c t o r and mask a l s o allows t h e optimum packing of the r e c t a n g u l a r URA p a t t e r n i n t o t h e round d e t e c t o r e lements r equ i r ed (because of t h e need f o r a diode image i n t e n s i f i e r tube t o a n p l i f y the s c i n t i l l a t o r o p t i c a l o u t p u t ) . F i n a l l y , t h e URA mask i s made se l f - suppor t ing by p l a c i n g t h e (1.5 mm squa re ) h o l e s on a 1.7 mn g r i d . Th i s i s tu rned t o a d d i t i o n a l advantage by us ing t h e 1.5 mm h o l e s as t h e d e t e c t o r c o l l i m a t o r . Co l l ima t ion (2.9’ FWHM) i s achieved by making t h e mask e f f e c t i v e l y very t h i c k ( 3 cm) u s i n g spaced t u n g s t e n laminates (see Figure 3 ) . Design P r i n c i p l e s and Opera t ion of EXITE The p r i n c i p l e s of o p e r a t i o n of t h e EXITE d e t e c t o r are i l l u s t r a t e d i n Figure 4. The LRA mask p r o j e c t s a co l l ima ted image of t h e sky on to a CsI s c i n t i l l a t o r . The l i g h t produced i n t h e CsI s c i n t i l l a t o r by a n i n c i d e n t X-ray is ampl i f i ed by t h e i n t e n s i f i e r while p re se rv ing t h e s p a t i a l d i s t r i b u t i o n of b r igh tness . The i n t e n s i f i e d image is decreased i n s c a l e t o match t h e input of t h e MAMA d e t e c t o r through a reducing f i b e r - o p t i c ( 1 2 : l ) . The l i g h t l o s s e s i n t h i s r e d u c t i o n are compensated by the g a i n of t he i n t e n s i f i e r so t h a t t h e t o t a l number of photons i n c i d e n t on t h e pho towcoun t ing imaging d e t e c t o r i s t h e same a s i n i t i a l l y c o l l e c t e d from the s c i n t i l l a t o r . T h i s p r e s e r v e s t h e energy r e s o l u t i o n c a p a b i l i t y of t h e system which Figure 3: Laminated Tungsten Coded Aperture Coll imator . i s b a s i c a l l y l i m i t e d by t h e count ing s t a t i s t i c s - of t h e l i g h t f rvm t h e CsI. The imaging o p t i c a l d e t e c t o r r equ i r ed could be a r e s i s t i v e anode (Mart in e t a l . 1981) c ros sed g r i d (Kellogg, Murray, and Bardas 1979) o r multi-anode (Timothy, Mount, and Bybee 1979) t y p e s y s t e m . The Multi-Anode-Microchannel-Array (MAMA) d e t e c t o r (Timothy, Mount, and Bybee 1979) is e s p e c i a l l y w e l l s u i t e d f o r EXITE and, t h e r e f o r e , included i n t h e des ign here . The anodes of t he MAMA d e t e c t o r are 140 p a x 140 urn which corresponds t o t h e ho le spacing i n t h e URA mask (1 .7 rsn) when t h e r e d u c t i o n f a c t o r of A2:l i s considered. Light r each ing t h e MAMA from a s i n g l e X-ray r e s u l t s Ln t h e product ion of a few t e n s t o s e v e r a l hundreds of pho toe lec t rons depending on i n c i d e n t energy. These w i l l be spread ou t over s e v e r a l of t h e 140 pm p i x e l s due t o t h e t h i c k n e s s of t h e CsI c r y s t a l and t h e f r o n t window of t h e image i n t e n s i f i e r . Thus, s e v e r a l of t h e anode l i n e s a long each a x i s w i l l d e t e c t t h e event . A c e n t r o i d e s t i m a t e of t h e event p o s i t i o n i s then determined f o r each a x i s by t h e p rocess ing e l e c t r o n i c s . Since t h e pho toe lec t rons a r e spread ou t on a Large s c a l e r e l a t i v e t o t h e MCP channel s i ze 'i i i PY OR!GINAL PA&: ES OF POOR QUALITY bc C Dc m E d Y d m U d U G C c U d 3 c 0 d U U PI PI c c 0 *VI U L' PI rn I C 0 -4 U d v) 0 a w 0 357 ORIGINAL PAC2 !3 OF POOR QdALITY 358 (-15 Pm), each e l ec t ron w i l l be ind2pendcntly amplified i n the microchannel p l a t e (MCP) array (with gain - ' i t and the t o t a l charge coming out of the p l a t e w i l l be proportions; t o the incident number of photoelectrons and thus the primary X-ray energy. This w i l l be measured using a separate ampl i f ie r a t the outout of the MCP t o provide enei, j information (64 channel PHA). Single e l ec t ron events due t o thermal generat ion e i t h e r i n the MAMA or the image i n t e n s i f i e r can easily be discr iminated aga ins t using the t o t a l charge output from the MCP s ince a 20 KeV X-ray i s expected t o produce a t l e a s t 20 photoelectrons. EXITE is a modular de tec tor concept. As proposed f o r XTE, two independent MAMA de t ec to r s (each viewing th ree 15 c m diameter s c i n t i l l a t o r X-ray de t ec to r s ) a r e included f o r maximum area and the desired redundancy against s ing le point f a i l u r e s . Each de tec to r , however, i s a c t u a l l y composed of th ree independent and parallel s c i n t i l l a t o r s ( f o r maximum usage of the 25 llpll c i r c u l a r photocathode area of the MAMAS), and each s ingle 15 cm diameter s c i n t i l l a t o r operates the coded ape r tu re imaging) a s a mosaic of adjacent but independent image-f orming subde t ec to r element 8 . A1 1 detec tor subelements (each i s 2.2 cm x 1.9 cm) view the same 2.9' (FUJELM) f i e l d of view on the sky. Thus, each of the two WMA detec tor systems i e i t se l f ac tua l ly multiredundant while a t the same time easy t o t e s t , c a l i b r a t e , and read out a s a s ing le device. ( i n The EXITE dksign f o r XTE has a t o t a l a c t i v e de t ec t ion area of A,ff 438 cm2 (a geometric area times 0.4 transmission of the col l imat ing coded aper ture mask). The de tec tor employs both a c t i v e and The s y s t e m i s shown i n Figure 5 above and occupies an a rea of 81 cm x 54 cm and height It weighs 237.9 kg and requi res 3.3 Kbps (max.) of telemetry and 39 watts of power. passive sh ie ld ing t o r e j e c t and reduce background. (above the spacecraf t i n t e r f ace ) of 40 cm. Anticipated EXITE Performance A f u l l s imulation of the EXITE de tec tor and o p t i c a l imaging c h a r a c t e r i s t i c e has been ca r r i ed out . Comparison of the EXITE de tec tor system w i t h t h a t (Matteson 1978) flown on W - A 4 ind ica t e s the t o t a l background count r a t e i n the '15-300 KeV band should be '35 counts sec-! ou ts ide the SAA. %del source spec t ra were allowed t o be incident (on a x i s ) on the f u l l system f o r a given in t eg ra t ion t i m e and were "detected" with the complete s y s t e m e f f i c i e n c i e s and a gaussian b lur r ing ( u * 0.5 Fixe l ) i n the centroid determination. Images of Cyg X-1 f o r a short (lt sec) and long (500 sec) exposure and of 3C273 f o r a long ( 5 x 10 eec) exposure a r e shown i u Figure 6 as they should appear from EXITE. These images contain the f u l l 15-300 K e V bandwith of the instrument; similar images could be shown fo r any band. The f u l l 13 x 11 pixe l (2.9") image is shown; each p ixe l i s 15 arcmin. The sources a r e de tec tab le i n a '1 p ixe l rad ius about the center due t o b lur r ing i n the MAMA de t ec to r readout; I 359 Figure 6: Images (15-300 keV) from Simulated EXITE Data c e n t r o i d i n g i s t h e n p o s s i b l e and ( f o r t h e long exposures ) would y i e l d p o s i t i o n s with 2 arcmin accuracy. Add i t iona l sou rces i n t h e f i e l d which were up t o 86 times f a i n t e r t h a n Cyg X-1 and '42 times f a i n t e r t h a n 36273 would be d e t e c t a b l e (and l o c a t e d t o -5 arcmid i n t h e two "long'@ exposure images, r e s p e c t i v e l y . The t o t a l d e t e c t e d count rate of Cyg X-1 by EXITE is 110 coun t s sec-' while 3C273 is 2.8 counts sec'l. Since Cyg X-1 is t h e b r i g h t e s t known ( s t e a d y ) source i n t h e -20-300 KeV energy band, t h e 100 coun t s sec'l a l lowed f o r i n t h e t e l eme t ry has been s e l e c t c d . The s p e c t r a of coun t s d e t e c t e d i n both long expo tu res are shown i n Figure 7 as t h e p l o t t e d p o i n t s (wi th a c t u a l l y "observed" la error b a r s ) . Also p l o t t e d as l i n e s are t h e assumed inpu t (power law) spectra f o r both sources . The s p e c t r a l i n d i c e s could be d e r i v e d ?rom t h e d a t a f o r e i t h e r source wi th u n c e r t a i n t i e s Aa/a of onLy a few pe rcen t . CYC X-1 600 seconds : p * 6xlO'seconds m m Figure 7: Spec t r a Derived from Simulated Images. C 360 I I 1 1 1 1 I I I I I I I I EX ITE SENSITIVITY - n Q, 1 I 0 0) cn I > -7 2 0 t- 0 I a I 4 x lo-' e rg cm-2. This is the approximate threshold f o r coarse '20" pos i t ions and log N - log S s tudies . To achieve '2" burst pos i t ions (and high S/N spectra t o search f o r l i n e f e a t u r e s ) , a dignal-to-noise r a t i o of >30 is required i n each of t h ree detectore . 365 -c This occur f o r b u r s t s l a r g e r t han '1.0 x 10'' e r g cm -, which are expected (from presen t log N-log S r e s u l t s ) a t a ra te of per year w i t h i n t h e '277 s t e r a d i a n s f o r which a c c u r a t e l o c a t i o n s can be derived. (Note t h a t a l though b u r s t s are d e t e c t a b l e w i t h approxrlllateiy a l l - sky coverage, a c c u r a t e l o c a t i o n s are p o s s i b l e f o r about h a l f t h e sky.) Thus, even i f XTE slews could on ly be c a r r i e d o u t w i t h i n a '1000 s e c a f t e r g l o w period f o r b u r s t d i r e c t i o n s w i t h i n '45" of t h e p o i n t i n g p o s i t i o n , '10 b u r s t s per year would be a c c u r a t e l y l o c a t e d by EXITE and observed wi th high s e n s i t i v i t y by EXITE and t h e LAPC. We no te t h a t estimates by Wopsley e t a l . (1981) ior t h e a f t e r g l o w spectrum (Ly l G 3 * e r g s- , kT = 20 KeV) would p r e d i c t EXITE s e n s i t i v i t i e s of ' 2 5 G per second f o r a t y p i c a l b u r s t source a t '500 pc d i s t a n c e . Thus, high siqnal-to-noise s p e c t r a and t iming o b s e r v a t i o n s should be poss ib l e . should -40 F i n a l l y , t h e EXITE b u r s t d e t e c t i o n s y s t e m could d e t e c t ( 5 0 ) p e r s i s t e n t hard X-ray sou rces with s t r e n g t h s '113 Crab and a Crab-like spectrum i n '300 sec. Thus, hard sou rces can be monitored cont inuously and a n a l l - s k y monitor c a p a b i l i t y provided f o r XTE. 366 REFERENCES Calabro, D. and Wolf, J. 1968, Inform Control, 11, 537. Cl ine, T. and Desai, U. 1975, Ap. J. (Letters), 196, L43. Cl ine, T.L., e t a l . 1980, Ap. J . (Letters), 237, L1. Dicke, R. 1968, Ap. J. ( L e t t e r s ) , 153, L101. Fenimore, E. 1980, Applied Optics , 19, 2465. Gunsen, J. and Polychronopulos, B. 1976, MNBbs, 17?, 485. Kellogg, E., Murray, S.S., and Bardas, D. 1979, LEEE Trans. Nuc. Sci. , NS-26, 403. Ling, J.C. 1975, <' Geophys. Res., 80, 3~4;. Martin, C . , Je l insky , J., Lampton, M., Malina, R., and Anger, H. 1991, Rev. Sci. Ins t r . , i n press Matteson, J. 1978, A M 16th Arrospace Sc. Meeting. Mazets, E.P., e t a l . 1979, FTI (Leningrad), Prepr in t No. 599, 618. hr..'ker, G. * 981, these proceedings. T e r r e l l , J. 1980, T a l i a t Texas Symposium, Baltimore, MD. Timothy , J. , b u n t , G., and Bybee, B. 1979, SPE-Optics, 183, 169. Troinbka, j., e t a l . 1973, Ap. J . , 181, 737. Woosley, S., e t a l . 1981, Prepr in t . PRECEDlNG PAGE BLANK NOT FILMED The Stellar Coronal X-ray Explorer: STCOEX G. S. Vaiana Harvard-Smithsonian Center for Astrophysics A. Introduction. With the end of the observational phase of Einstein Obseri dory mission, it behooves us to consider the status of the newlyborn field of stellar x-ray astronomy. Although Einstein clearly allowed a quantum leap over previous observations of x-ray emis- sion from stars (Vaiana e2 al. lQSl), its relatively short lifespan allowed only a fraction of the variety of projects, developed in response to this instrument% capabilities, to be corn pleted. In response to the consequent perceived need for further observational capabil- ity, we have conducted a overview of the available Einstein data for the purpose of deciding which projects will boZh in all probability not be completed by future analysis of the Einstein data and, in addltion, seem to us to be of particular importance for the study of stellar surface "activitya; a natural outgrowth of this analysis was the development of design criteria for a followup mission to Einstein, one whose primary emphasis would be on stellar observations, and whose general capabilities lay within the scope of the NASA Explorer program. In the fdlowing, I will attempt to brielly review the criteria which lead us to the desired instrumental capabihYes 6 8); and pnwide an overview of the proposed instrument and some of its capabilities, as indicated by our simulatlons fi C). B. Scientific Goals and Design Criteria. One of the remarkable aspects of the stellar observations made possible by the Ein- stein Observetory was their serendipity: the instrumentation and the observing plans were not optimized for stellar observation, but rather for primarily extragalactic studies. In spite of this lack of optimization, well in excess of a third of all guest observer programs focussed on stars; and the stellar survey constituted one of the major research areas of the Consortium groups. What are some of the principal scientffic problems, and to what extent have they been resolved? In ths following, I will brielly recap some of the major issues, and attempt to define the work wtich remains to be done. 1. Dependence of C o r d Plasma Paremeters on the Characteristics 01' the Wnderly- ing' Star. The Einstein observations have enabled us to sketch in broad outline the x-ray emission pioperties of stars throughout the H R diagram; however, in many portions of the K R diagram, the data is relatively sparse (because of severe restrictions on the allowable &sewing times!. In particular, it has not been possible to assemble uniform, unbiased (vir. volumslimited) samples for every spectral types and luminosity classes. In conse quence, phenomenological studies of the variation of x-ray luminosity with stellar parame ters 8uch as rotation and mass loss rate may remain biased by selection criterla for several stellar categories because (ai least in the beginning of the mission) emphasis was largely placed on targets most likely to be x-ray sources. More severe difftculties beset the plasma dlagnostlcs (such as temperature analysis). the strong constraints on observ- ing time allowed only e very limited sample of stars to be examined by the higher resolutlon spectraneters on Einstein, such as the OGS and the SSS. Thus, questions regarding possible changes In spectral characteristics along the 'dividing line' separating low and hlgh mas8 logs latetype evolved stars (cf. Ayres el a/. 1981); soft x-ray absorp tton and possible muttiple temperature component structure in 08 stars; and flare spectros- copy, may all be only partially resolved. Furthermore, the quite limited sensitivity of rela- tively high resolutlon 8pectto;rcopy (x/u > 100) on Einstein prevented any study of line broadening 01 doppler shift effects; the latter 1s particularly of Interest In the case of 08 37 2 stars, since some models predict rapid outflow of the hot plasma (which may be obsew able wi.h A/bA - 300-1000). The observation requirements thus call for pointings with point source sensitivities of several times 1 6" erg/sec/cm*, in conjunction with spec- troscopic struments ca able of resolution In excess of 100, and with line sensitivities of 2. Temporal Variability. Again because of observing time constraints, virtually all tern poral variability data for Einstein stellar targets are restricted to the few thousand seconds of contiguous observing time; in only a few select cases (for example, for some flare stars and OB associations) are data spanning substantially longer intervals available - and virtually without exception, the longer data base showed evidence for substantial variability. It is evident that 'coronal' emission from stars is highly variable on a wide range of time scales, from seconds or less (flares), to days and weeks (viz. rotational modulation), to months and possibly longer time scales (cycles?). Such Variability in stellar 'activity' is seen by ground-based observers (e.g., in Ca Ii), but only in a very restricted portion of the KR diagram: in contrast, x-rays allow us to look for activity modulation (via rotation and/or cycles) in stars Independent of spectra! type or luminosity class, including stars in the spectral range 0 - A. T se observations therefore call for the ability to carry out e*-tended obs rvations (> 10 sec) which can be repeated on time scales rang ing from 10 sec to 10 sec. 3. Galactic Effects of Stellaf Activity. In addition tc being a major research area in its own right. stellar x-ray astronomy forms a new bridge between the stellar astronomers and the galactic and extragalactic astronomers: the radiative and particulate emission of active stars may well contribute signiflcantly to the radiation fleid and matter content of the ga!axy. For example, Rosner et a/. (1 981 ) have shown that latetype dwarf stars carr tribute a not insignificant amount of radiation to the galactic component of the diffuse soft x-ray background between - 0.28-1.0 keV. Because of Einstein's spectral coverage, the stellar contribution at lower energies is not well established (as It depends sensitively on the assumed 'coronal' temperature). Furthermore, the limited effective spectral resolution did not permit relative elemental abundance analysis, such analysis is of great interest for studies of the elemental composition of stellar mass input to the ISM (as it is known from solar observations that the photospheric and con>nal/wind abundances are not identical). The necessary observational capability to conduct such studies corresponds to that of 8 9.1 above. 4. Desired instrument Characteristics. In uder to place the instrument discussion on concrete grounds, we shall adopt the following desirata. (i) Energy Range: - 10-200 A; this range encompasses virtually all of the significant spectral lines of interest to coronal observations. (ii) Optics: Imaging, with spatial resolution better than - 1'; such resolution is required both for accurate target identification and for optimal signaktenoise. (iii) LhWctors: Photon-counting (i.e., IPC or HRi-like); this requirement follows from the necessity of observing flares on sub-second time scale; (so +hat, for exam pie, read-out ndse of integrating detectors becomes a significant liability). Note, however, tha: CCD detectors nay be preferable for extr6me:yhigh sen+ tivit) ..- Jervations for which big;, remporal resolutlon is not reqr*'red. (iv) Spectroscopy: High thrxqhput, with resolution in excesd of 100 (300 for OB star wind Jiagwgtics), calls for transmission 01 reflection grat,.igs. (v) Sensic,uity: Assuming that of order 1r30 pointings (with spectroscopy) are to be carried out on a time sca '!ears, with sufffcient sensitivity tu probe stars dowr to a level of . O ' o ~ ~ / s a : at distances of order M) pc (these numbers will yield a sufficientkt large samplq to carry out statisr.cal analysis of tuminac'ty functi ns throughout the K R diagr 1, one requires a wint source aansitivity @ 10 sec approximately 3-4 times that of €instsit,. order loeia erg/sec/cm s . ?? 'd 8 8 C. Instrumevt Chat8cteristlx~. I 373 ' Following the above considerations, we have recently developed two verslons of a trial payload (the STellar COronal Explorer) which meets in essentially all respects the observational deslrata. Perhaps the most crucial aspect of the design is that the Instru- ment is dediccrted to stellar observations. This point is not to be slighted: the character of stellar observlng programs (i.e., large numbers of very extended pointlngs, with sub- stantlal degree of repetition) is such that it severely conflicts with the requirements of, for example, extragalactic observations; in this circumstance, comblned missions may well be false aconomy as the principal scientific goals (which presumably underly such ven- tures) may not be reallzed. This emphasis on dedication, however, does not exclude the possibility of including nmstellar programs whose observing strategies do not overtly conflict with stellar observations (such as 0bserva:ions of galactic highluminosity x-ray sources). Figure 1 shows a schematic of the 'simpler' instrument; the optics consist of a nested pair of classical paraboloid-hyperboloid mirror assemblies, together with a movable objec tive transmission grating and an HRI and imaging proportional counter (PSPC) in the focal plane (mounted on a stage so that the entire dispersed spectrum may be explored); the more *complex* design simply calls for two additional nested mirror assemblies, which results in a rough doubling of the instrument effective area. The physical characteristics of the instruments and the relevant seiisitivities in both the imaging and spectroscopic modes are given in Tables 1 and 2; and graphs of the total effective area of the instrv ment In the high resolution mode and of the resolving power are shown in Figures 2 and 3. It Is to be noted that these results are not speculations, but rather reflect the current statsof-thsart in detector and transmission grating technology: no development is required to obtain such capabilities. In order to appreciate the power of such an instrument, we show first the range of spectral lines (and soutce temperatures) accessible to observatlon (Table 3): the STCOEX can observe coronal line emission from plasmc in the temperature range 6.6 < log T < 7.5; the lines chosen take advantage of the spectral sensitivity of the instrument, and so are not just marginally detectable. A more revealing analysis is to fully simulate a typical obserration. Consgijqan 9ervat ion of an RS CVn star at -60 pc, whose volume emission measure is -10 cm ; the source spectrum is shown in Figure 4 (using recent Raymond 1980 calculations with sdar abundances). We have simulated II STCOEX observation by folding this source spectrum through the Instrument response (unless other- wise stated, for the 2-mirror system), including the mirror reflectivities, the grating transmission, the detector sensitivities, and the instrument resolution (including mirror pfr and abberations fzii a coma-corrected grating). If detector noise is nd taken into account 4 (but photon statistics are), one obtains the spectrum shown in Figure 5 in 10 sec; adding detector noise (we assume a background of 1 ct/sec over the entire HRI) th n leads to the spectrum shown in Figure d. Comparing Figs. 4 and 6, we see that a 10 sec expo sure is sufficient to allow one to resolve most of the strong lines in the source spectrum; however, the limited count statistics would not permit sensible line ratlos to be calculated. If one goes to longer exposure times, &his restriction Is completely eliminated: Figure 7 shows what can be accomplished in 10 sec, using the 4-mirror system; again. both pho ton statistics and detector noise have been taken into account in the simulation. Spec- troscopy of such resolution and sensitivity, in combination with the capability for dedicated observations, will clearly allow the observational goals outlined above to be met. Support of the above studies by NASA grant NSG 7176 is acknowledged. 8 Bibliography. Ayres, T. R., Linsky, J. L., Vaiana, G. S., Golub, L., and Rosner, R. 1981, Ap. J. (in press). Pallavicini, R., Goiub, L., Rosner, R.. Vaiana, G. S., Ayres, T. R., and Linsky, J. L. 1981, Ab. J., 248, 278. 374 Rosner, R. et el. 1981, Ap. J. (Leners). 249, L5. Vaiana, G. S. et al. 1981, Ap. J., 246, 163. t.;,3f?iAL PkGE IS Of POOR QUALITY Figure 1: Schematic of the STCQEX, showing the principal instrumentation. Only the 2- mirror system is shown; an rdternatlve design calls for 2 additional concentric mirror assemblies. 375 Figure 2: Total effective area of the 2-mlrror STCOW design, including mirror reflcctivitfes, transmission grating performance, and detector sensitivity. We show results for the HRI with either a Beryllium (solid) or organic (dashed) filter, and for the imaging proportional counter (PSPC; solid/dashed). Note that the grating spec- tral .'esolution with the high-sensitivity PSPC would be comparable to or somewhat better than that of the Einstein SSS. 200, , . , . , . , , , .. l . , ' , . 1 . , . , 1 J ---. O J , . ( . , . ( , , , IO 20 30 40 50 60 70 , - , 80 90 I . ( . ( . 100 lI0 I20 WAVELENGTH Figure 3: Detailed spectral resolution of the trarumlssion grating for two different grating periods end the two possible focal plane detectors. 376 1 4 1 , . 6 . U . 1 6 - .I*. - #.I# _ . . . . Figure 4: Source spectrum for an 'active' latetypo ?tar, with T = logs8 K; the model is that of Raymond (1980). Figure 6: Convolution of the source spectrum of Fig. 4 with the totel instrumental re-,pgnZe of the 2-mirror deslgn, assuming a source volume emission measure of 10 ~ m ' ~ at a distance of 60 pc; we a sumed the Berylllum filter, use of the 2000 lpmm grating, an exposure time of 10% sec, end no detector noise (photon statistics were taken into account). 377 . . . . . . Figure 6: Same as Fig. 5, but now taking the detector noise into account; note that most of the strong llnes in the parent source spectrum are detected, but that line inten- sity ratios cannot be reliably calculated. 8 l C O - u .. nn I... ...... -1 nMm.0. .. w *.+- CICI, ..................... 1 . ~ 1 1 ~ 1 ........................... -t .... ._(.%-OS .1.l.rh!r ¶ .................. f 8 ..... . . . . . .- . . . . . . . . . . , .. 6 Figure 7: Same as FIG. 8, h J t *'-r tha 4-mirror design a t an exposure of 10 sec. In this case, the ;pectroscoplc goals discussed in the text are readily within reach; note that the 2-mirror design would lead to somewhat degraded signal-tenoise. . -. - _I 1 I 379 I 37 :m82* 2 6 0 7 4 A DIFFUSE S o n X-RAY SPECTROMETER L;m McCammon Department of Physics, University of Wisconsin, Madison i Abstract A desig: for a diffuse X-ray spectrometer u t i l i z i n g Bragg reflec- t i o n i s described. physical size and allows simultaneous observation a t a l l wavelengths w i t h i n i t s range. An implementation w i t h overall dimensions about 33 x 66 x 66 cm uses 1800 an reflector t o cover the wavelength range 42 to 84 Angstroms w i t h a throughput of about 0.01 un sr, including detector efficiency. Spectral resolving power i s about 35 to 50. A similar unit equipped with thallium acid pthallate crystals w i l l cover the 11 - 24 A range with about half th i s t h r o u g h p u t and twice the resolving power. The geometry has a very h i g h throughput for a given 2 of lead stearate multilayer Bragg 2 38c) I - I NT RODUCT I ON There i s now s t rong evidence i n d i c a t i n g tha t most o f the d i f f u s e X-ray background a t energies l ess than 1 keV i s l oca l t o the Galaxy (Sanders e t a l . 1977). D iscre te sources seem t o account f o r a t most a small f r a c t i o n o f t h i s , a t l e a s t below 0.3 keV (Vanderh i l l e t a l . 1975; Rosner e t a l . 1981). The lack o f acceptable non-thermal d i f f u s e emission mechanisms (Will iamson e t a l . o f t h i s X-ray f l u x i s due t o thermal emission from a ho t (1 ?r 10 tenuous component o f the i n t e r s t e l l a r medium. theo re t i ca l grounds t h a t such mater ia l occupies a l a rge f r a c t i o n o f t he volume o f the Galact ic d isk , w i t h a f i l l i n g fac to r perhaps as h i s h as 0.5 - 0.9 (Cox and Smith 1974; Smith 1977; McKee and Os t r i ke r 1977). I f t h i s i s t r u e (and the u b i q u i t y of 0 V I absorpt ion i n the u l t r a v i o l e t lendsconsiderable credence t o the idea: see r x i e w by Jenkins 1977), 1974) leads t o the conclusion t h a t most 6 K ) , I t has been argued on then the hot component must have a profound in f luence on the nature o f the ISM, and any comprehensive understanding o f Galact ic s t r u c t u r e and evo lu t ion w i l l r equ i re much more d e t a i l e d knowledge o f t h i s h igh tempera- t u r e mater ia l than we c u r r e n t l y possess (McCray and Snow 1979, and references there in ) . Our p i c tu re o f the I S M i s g e t t i n g so complicated tha t broad-band spectral measurements are no longer adequate f o r t e s t i n g the more de ta i l ed and complex models now being proposed. Fortunately, r a d i a t i o n from gas around 10 K w i t h any reasonable composition i s almost e n t i r e l y i n cha rac te r i s t i c l i n e s o f the heavier elements, and observations o f 6 these can i n p r i n c i p l e supply a wealth o f in format ion on phys ica l con- d i t i ons w l t h i n t h i s component, and even i t s past h i s t o r y . I n t e r p r e t a t i o n 38 1 of spectra w i l l admi t ted ly be compl icated considerably by m u l t i p l e temperatures, abundance v a r i a t i o n s and non-equi 1 i b r i urn cond i t ions o p t i c a l spectroscopy has had t o deal w i t h s i m i l a r o r even more severe compl icat ions and s t i l l managed t o make s i g n i f i c a n t con t r i bu t i ons t o ast rophys ica l understandi ng. bu t 11. OBSERVATIONAL APPROACHES The fo l low ing are poss ib le avenues t o observing i n t e r s t e l l a r gas a t 6 -10 K: 1. Opt ica l and U.V. forbidden l i n e s : These have been observed i n some h igh surface b r i ghtness supernova remnants, bc t seem hopelessly f a i n t from the general ISM. 2. Resonance absorpt ion l i n e s : I n t h i s temperature range, on ly the 0 V I l i n e s f a l l longward o f the Lyman l i m i t . l o c a l neutra l hydrogen dens i ty might a l l ow observat ion o f o ther l i n e s i n the XUV out t o a d is tance o f 100 pc o r so, and could provide much The anomalously low usefu l in format ion about the so la r neighborhood. Observation o f absorpt ion l i n e s i n the X-ray reg ion could be done against b r i g h t Galact ic X-ray sources w i t h AXAF-class inst rumentat ion and should g ive some very i n t e r e s t i n g r e s u l t s . few i n number and almost a l l a t l a r g e distances, so on ly a very However, su i t ab le sources are l i m i t e d amount o f in fo rmat ion can be obtained i n t h i s way. 3 . Photo ion izat ion s ignatures: One colrld conceivably look f o r evidence o f absorpt ion o f X-ray and XUV photons i n surrounding cooler ma te r ia l . However, none o f the access ib le r e s u l t i n g s tates seems s u f f i c i e n t l y unique t o make t h i s a p r a c t i c a l approach. 4. X-ray emission l i n e s : These represent the dominant coo l ing mechanism f o r the hot gas and have p o t e n t i a l l y t h e h ighest in format ion content. One o f the major observat ional d i f f i c u l t i e s i s the low surface br ightness. Since t h i s i s a t r u e d i f f u s e source, c o l l e c t i n g o r focussing op t i cs can serve only t o increase s p a t i a l resolut ion, which w i l l be des i rab le someday, bu t i n a f i r s t -genera t i on instrument the add i t i ona l i n e f f i c i e n c y in t roduced by these elements would exacerbate the major problem o f g e t t i n g enough photons. 111. TYPES OF SPECTROSCOPIC INSTRUMENTS Possible instrumental types f o r moderate-resolut ion spectroscopy below 1 keV inc lude: 1. S o l i d s t a t e detectors : These have wide acceptance angles, but a t the cur ren t s t a t e o f the a r t g ive l i t t l e improvement over propor t ional counters below 1 keV. 2. Re f lec t i on gra t ings : Al low observat ion o f a l l wavelengths simultaneously, but must be used near g raz ing incidence, making acceptance angles and throughput extremely small f o r d i f f u s e sources. 3 . Transmission gra t ings : S m a l l d ispers ions again requ i re small E f f i c i ency i s a l so lower than r e f l e c t i o n grat ings. acceptance angles. 4. Bragg c r y s t a l s and mu1 t i - l a y e r analyzers: These have r e l a t i v e l y h igh in tegra ted r e f l e c t i v i t i e s , can work a t l a r g e angles, and thus can have r e l a t i v e l y h igh throughput. c r y s t a l a r e a can be used for on l y one wavelength a t a time, i f a l l incoming X-rays a r e a r r i v i n g from the same d i r e c t i o n . Thei r major disadvantage i s t ha t a given e For studying the d i f f use background, we can get around the prob:em o f one wavelength a t a t ime when us ing Bragg r e f l e c t i o n by look ing a t many wavelengths simultaneously, but from d i f f e r e n t pa r t s o f t he sky. ,pat ia l coverage can e i t h e r be smeared ou t t o get an average spectrum o r sor ted ou t by s p a t i a l b ins as the instrument p o i n t i n g i s changed t o eventua l l y ob ta in a complete spectrum o f each s p a t i a l element wh i le r e t a i n i n g the spect ra l mu l t i p lex advantage. describes one geometry which has been worked out t o take advantage o f t h i s scheme. The The fo l l ow ing sec t ion I V . GEOMETRY FOR A DIFFUSE X-RAY BRAGG SPECTROMETER To ob ta in a f i n i t e count ing r a t e from a t r u l y d i f fuse source, the de tec tor must accept X-rays from a f i n i t e range o f angles. To keep the Bragg r e f l e c t i o n angle (and thus the observed wavelength! from changing as f a s t as the angle w i t h respect t o the de tec tor , the c r y s t a l can be curved t o compensate f o r the changing look d i r e c t i o n , keeping the r e f l e c - t i o n angle constant. For a one-dimensional de tec tor , the requi red curve i s an exponential s p i r a l . l a r g e as desired wi thout a f f e c t i n g the spec t ra l resol!:t ion, but i t works on ly f o r a s ing le wavelength. This a l lows the f i e l d o f view t o be made as Our approach i s t o extend a pos i t i on -sens i t i ve detector along the x - a x i s , and t o make the curvature o f the c r y s t a l above each po in t on the counter appropr ia te t o an exponential s p i r a l about t h a t po in t , subject t o the cons t ra in ts tha t the c r y s t a l be continuous and have a continuous slope. This al lows the use o f a plane detector w i t h a simply constructed c o l l i m a t o r d i rec ted perpendicular t o the counter face a t a l l 384 p o s i t i o n s . w i l l va ry w i t h p o s i t i o n on t h e x-axis, b u t a t any g iven p o s i t i o n t h e r e w i l l be no change i n t h i s angle t o f i r s t o r d e r as t h e c o l l i m a t i o n angle departs from pe rpend icu la r . The shape thus determined for t h e c r y s t a l t u r n s o u t t o be a c i r c u l a r c y l i n d e r , w i t h t h e d e t e c t o r l o c a t e d a long a diameter, as shown i n F i g u r e 1. a n a l y t i c express ions f o r parameters o f i n t e r e s t , which a r e g i ven i n Table 1 . expression cvs 0' = cos 0 cos &+,and the approx imat ion s i n 0'2 s i n 0 cos A $ , where 0 i s t h e r e f l e c t i o n angle f o r t h e ray pe rpend icu la r t o t h e d e t e c t o r , and 0 ' i s t h e r e f l e c t i o n angle f o r a r a y which i s a t an angle AI$ from per- pend icu la r i n t h e d i s p e r s i o n d i r e c t i o n o r A* from pe rpend icu la r i n t h e cross d i r e c t i o n . i n the d i s p e r s i o n d i r e c t i o n always increases t h e wavelength from t h e nominal va lue a t t h a t x - p o s i t i o n , w h i l e going o f f - a x i s i n t h e cross d i r e c t i o n always decreases i t . f i e l d o f view, and t h e FWHM o f the r e s u l t i n g p r o f i l e i s l e s s than h a l f t h e A X c a l c u l a t e d us ing t h e FWHM angle o f t h e c o l l i m a t o r . The r e f l e c t i o n angle for rays pe rpend icu la r t o t h e d e t e c t o r This makes i t s imp le t o c a l c u l a t e These f o l l o w from t h e Bragg c o n d i t i o n , X = 2d s i n 0', t h e exact As can be seen from these expressions, Coing o f f - a x i s These e f f e c t s p a r t i a l l y cancel over most o f t h e o f f - a x i s F i y u r e 2 shows a cross s e c t i o n o f an i ns t rumen t as i t m igh t be b u i l t us ing a l e a d s t e a r a t e (PbSt) Bragg r e f l e c t o r t o cover t h e 148-295 eV band. The i n h e r e n t r e s o l v i n g power o f PbSt v a r i e s from 40 t o 60 over t h i s range and the c o l l i m a t i o n ang!es on the d e t e c t o r can be opened up t o 20" x 20' FWHM be fo re t h i s i z degraded by a f ac to r o f pu te r model has been w r i t t e n t o p r e d i c t t h e response 3f such an inst rument t o an a r b i t r a r y i n p u t spectrum. E-. A r a y - t r a c i n g com- The program takes i n t o account o f f - a x i s 385 rays, v ignet t ing , c o l l i m a t o r transmission, and detector e f f i c i e n c y for a p r a c t i c a l p ropor t iona l counter, as w e l l as the f u l l rock ing curve and specular r e f l e c t i o n o f t he Bragg c rys ta l s . The l a t t e r were measured for PbSt a t the Tantalus synchrotron r a d i a t i o n f a c i l i t y of the Un ivers i ty o f Wisconsin's Physical Sciences Labaratory, using a g r a t i n g double monochrometer. s k i r t e d 0.5 A bandpass o f t h i s source allowed measurements o f the wings o f the rock i i lg curve down t o three orders o f magnitude beloh the peak, and specular r e f l e c t i v i t y out t o angles where i t dropped below l o ? These parameters are impor tant i n determining the response o f a Bragg spectrometer t o such quasi-continuous sources as ast rophys ica l plasmas. The h igh i n t e n s i t y , low scat tered l i g h t , and steep- Calcu lat ions f o r the geometry o f Figure 2 w i t h a de tec tor 23 cm wide i n the x - d i r e c t i o n and both de tec tor and c r y s t a l 53 cm long g ive a throughput as shown i n Figure 3 . t o a sky un i fo rm ly b r i g h t i n boron and carbon K-alpha emission. The natura l widths o f these l i n e s when they a r e exc i ted from elemental s c l i d s as published by Ho l l i day (1967) have been included; the response t o a monochromatic l i n e o f the same nominal wavelength i s shown by the dashed l i n e . exposed t o l a r g e a lpha -pa r t i c l e exc i ted targets o f boron and carbon which e n t i r e l y f i l l e d i t s f i e l d o f view. response. ( The ho r i zon ta l scale, f o r which nomi nal wave1 engths a r e given , i s l i n e a r i n x -pos i t i on on the detector . p ropor t ion counter i s about 1 -.un FWHM a t 70 eV pulse height , and does not con t r ibu te s i g n i f i c a n t l y t o the l i n e p r o f i l e widths.) We do not know why Figure 4 shows the ca lcu la ted response A spectrometer has been b u i l t t o the above spec i f i ca t i ons and The data po in ts i n Figure 4 show i t s The p o s i t i o n reso lu t i on o f the 386 the boron l ine appears s ign i f icant ly narrower than the published p r o f i l e , but this has a lso been noticed when tes t ing f l a t crystal samples i n a monochrometer w i t h an electron-excited boron source. The spreading of the profiles a t low ampli tudes (most noticeable on the long-wavelength wing of the carbon l i n e ) i s probably due t o imperfections i n the cyl indri- cal figure a t the edges o f the individual elements of the mosaic crystal panel. calculated throughput t o be t te r than 15% for both l ines . Absolute inte:isi ty cal ibrat ions of the sources agree w i t h the Figure Sa shows the response t o a two-temperature Raymond and S m i t h (1977) thermal spectrum which has been f i t t o three-band sky survey data for a typical region i n the southern hemisphere. the spectrum a r e a lso indicated as they appear before being folded through the instrumental response. The dashed l i n e shows the contribution due t o the cosmi c-ray background level predicted by laboratory measurements ( 6 x limits used i n in tegrat ing the signal have been varied w i t h posit ion on the counter t o roughly optimize the signal t o noise r a t i o . shows the same response integrated in to position b i n s for an assumed 5000-second observation, w i t h the resul t ing number of counts i n each b i n further subjected to a Poisson randomization. figures showing the PbSt bandpass, the collimation angles have been reduced t o 10' x 10" FWHM i n order t o preserve essent ia l ly the f u l l inherent resolving power o f the crys ta l s . The brighter lines i n counts cm -* s-' k e f ' ) . I t slopes because the pulse height Figure 5b For this and the following A similar geometry can be used w i t h other Bragg crys ta l s . Thallium a c i d pthall2te ( T A P ) shows the greatest promise for covering the in te res t ing 387 M band region ( .5 - 1 .O keV) where the number of important l i n e s i s not so large as a t lower energies and the physics should be eas ie r t o dis- entangle. means that the integrated r e f l e c t i v i t y i s lower than for PbSt, and this, coupled with the much lower fluxes, means t h a t l o n g observing times will be necessary. the same s ize as the PbSt spectrometer, b u t u s i n g a TAP crystal t o cover the 516 t o 1032 eV range. 15" x 15" i n order t o fncrease the throcghput, and as a resu l t the observzd l ine prof i le w i d t h s a re almost ent i re ly due t o e f fec ts o f the collimation angles. a re not u n i q u e , and Figures 7 and 8 show the resu l t of two d i f fe ren t m u l t i - component spectral f i t s t o another part of the sky. three-temperature f i t s with equilibrium models, a n d while non-equilibrium models would cer ta inly be d i f f e r e n t , they are unlikely t o be s igni f icant ly more complex. Unfortunately, the much higher inherent resolving power of TAP Figure 6 shows the predicted resu l t s from an instrument The collimation angle has been opened up t o Even thermal equilibrium f i t s t o the three-band sky survey data These are one- and V . OTHER BRAGG REFLECTORS No natural c rys ta l s have 2d spacings long enough t o cover the 150-300 eV region, and PbSt i s the best material tested so f a r for a Langmuir-Blodgett mu1 t i l a y e r analyzer. the highest integrated r e f l e c t i v i t y of commercially available c r y s t a l s . For the 500-1000 eV range, TAP has However, recent successful work w i t h a r t i f i c i a l multilayers i s very promising (Spi l le r e t a t . 1980; Barbee 1981 ; underwood and Barbee 1981). The rocking-curve width of PbSt i s actually rqther well matched t o the 3d8 resolving power which could be used on even a fa i r ly lengthy shuttle- based observation. probably n o t more t h a n a factor of two t o be gained by going t o an optimized a r t i f i c i a l multilayer. resolving power of abouz lo3, which i s much higher t h a n can be used with expected observing ti:: es and instrumental backgrounds, and we calculate a factor of five potential improvement for a r t i f i c i a l multilayers that seem feasible t o fabricate. The materials are also nearly optimum and there i s TAP, on the other hand, has an inherent V I . CONCLUSIONS Spectroscopy o f the G a l a c t i c component of the diffuse X-ray background i s necessary for an adequate understanding of the coronal phase of the inters te l lar medium. This i s especially important i f t h i s phase i s as pervasive as has been predicted. Whether this spectroscopy eventually i s done with an instrument similar t o the one described here, or with some more clever specialized design, or as a sideline by a much larger general-purpose instrument, two major characteristics must be kept in mind and designed in. i s simply h i g h t h r o u g h p u t . I t i s probably impractical to attempt these observations with less t h a n about 10% of the area-solid angle factor of the examples in this paper. of view, th i s implies very large areas. equipped with very high efficiency (50%) objective reflection gratings a n d having abou t the same physical s ize as the tvJo instruments described above p u t together has abou t three orders of magnitude less t h r o u g h p u t . The f i r s t of these For an imaging telescope with a small f ie ld For example, a n imaging instrument 389 The second v i t a l consideration i s a low-background detector. The Bragg spectrometers whose responses are shown i n Figures 5-8 have about aslarge a convergence angle a t the detector and as high an optical efficiency as i s practical t o design into an instrument, particularly a general-purpose imager, and so we expect that these simulations repre- sent an upper l imit t o the surface brightness of the spectrum a t the detector. background limited a t an assumed background rate o f 6 x cm -* s-l keV" between 70 eV and 1000 eV. Even so, i t can be seen that they are very nearly cosrnic-ray counts During a sounding-rocket f l i gh t , position-sensitive proportional counters showed a cosmic-ray back- ground rate (door-closed o r earth-looking) wh!ch was actually abou t one- third of t h i s , b u t w i t h the spectrometer field-of-view exposed to the sky an additional backgrciund due to electrons scattering off the crystal panel and penetrating the counter windows was observed. adequately w i t h magnetic f ie ld screens,but there i s also another sky- This can be reduced related background which may be due to a sof ter a n d mu-h more intense component of the electron flux producing fluorescence and bremmsstrahl ung X-rays as they s t r ike the crystal panel. This i s b rough t up to emphasize the care t h a t must be taken t o mini- mize - a l l extraneous sources o f background a t the detector, whether from cosmic rays, scattered electrons, or s t ray ultraviolet , i f there i s any intention o f making these interesting and potentially important measure- ments. 390 TABLE 1 . GEOMETRICAL CHARACTERISTICS x = d is tance from center o f curvature, 0. I f : I, = source br ightness i n photons an" s - l sterad-' and: (A*n), = spectrometer throughput i n an2 s terad Then count ing r a t e = I, (A*a),. (A*r;)x = aR RI(Xjhg A$ s i n OR().) cx. Where: II = l eng th of detector and c r y s t a l R = c r y s t a l rad ius o f curvature RI ( A ) = in tegra ted r e f l e c t i v i t y o f c r y s t a l A Q = c o l l i m a t i o n HWB i n d ispers ion d i r e c t i o n A$ = c o l l i m a t i o n HWB i n cross d i r e c t i o n o(x) = Bragg angle E = detector e f f i c i ency Contr ibut ions t o l i m i t e d reso lv ing power: x A) Co l l imat ion i n d ispers ion d i r e c t i o n : 2 -- x % -2 tan 0 A X 2 A Q 6 ) Col l imat ion i n cross-dispersion d i r e c t i o n : C ) Pos i t i on r e s o l u t i o n of detector : -.- A = s i n a tan e R- A x A X 0 ) F i n i t e width o f c r y s t a l Rocking c w v e : A = tan Q 1 A A A0 - - 39 1 REFERENCES Barke , Troy Y. Jr. 1981, Proceedings of the Topical Conference on Low- Energy X-ray Diagnostics, A.I.P., k n t e r e y , Calif. (P rep r in t ) . Cox, 0. P., and S m i t h , B. W . 1974, Ap. J . , - 189, L105. Hoil!day, J. E. 1967, Norelco Reporter, X I V , 84. J e n k i n s , E. 8. 1977, i n Topics i n I n t e r s t e l l a r Hat te r , H. van Woerden ed., Dordrecht , Rei de l . RCray, R., and Snow, T. P. Jr. 1979, - Ann. Rev. Ast. and Ap., ‘17, 213. McKee, C. F., and Ostriker, J. P. 1977, Ap. J., - 218, 148. Raymond, 3. C., and Smith, B. W . 1977, Ap.J.(Suppl .), - 35, 419. Rosner, R., Avni, Y . , Bookbinder, J . , Giacconi, R. , Golub, L . , Parnden, F. R. Jr., Maxson, C. W . , Topka, t f . , and Waiana, 6 . S. 1981, preprint ( t o be published i n ap.J. (Le t t e r s ) ) . Sanders, Y. T., Kraushaar, W . L., Nousek, J . A., and Fried, P . M. 1977, Ap.J., 217, L87. SRlith, B. W . 1977, G., 211, 404. Spi l l e r , Eberhard, and SegmOler, Armin 1980, Appl. Phys. Lett., - 37, 1048. Underwood, 3. H. , and Sarbee, T. W. Jr. 1981, Proceedings o f the Topical Conference on Low-Energy X-ray Diagnostics, A.I.P. , Monterey, Cal i f . (Prepr in t ) . Vandcrhill, H. J . , Borken, R . J . , Bunner, A. N . , Burstein, P. H . , and Kraushaar, W . L . 1975, Ap.J. (Le t t e r s ) , 197, L19. Uilliamson, F. O . , Sanders, W . T . , Kraushaar, W . L., McCammon, O . , 8orken, R . , and Bucner, A . N . 1974, Ap.J., - 193, L133. 392 F I GURE CAPT IONS F! gure 1 : The Bragg r e f l e c t o r i s on the inner surface o f t he c i r c u l a r c y l i n d e r o f rad ius R. p o r t i o n o f t h e x-ax is below the r e f l e c t o r . The detector occupies a Figure 2: Cross sec t i on o f an instrument t o cover the energy range 148-195 eV, showing sample ray t rac ings. Figure 3: Area-Sol id Angle product vs. photon energy f o r a PbSt spectrometer w i th a 23 cm x 53 cm propor t ional counter detector and 20" x 20" co l l ima t ion . Figure 4: Predic ted and actual responses o f a PbSt spectrometer wi th 20" x Z O O c o l l i m a t i o n t o uni form d i f f u s e i l l u m i n a t i o n o f the f i e l d o f view w i t h boron and carbon K-alpha r a d i a t i o n . The dashed curve shows the predic ted response t o a monochromatic l i n e o f the same nominal wavelengtb. Figure Sa: Predic ted response o f a PbSt spectrometer w i t h 10" x 10" c o l l i m a t i o n t o a twc-temperature equ i l i b r i um plasma model f i t t o observed broad-band count ing ra tes a t a t y p i c a l reg ion i n the southern Ga lac t i c hemisphere. shown before fo ld ing through the spectrometer response. r lght -hand scale cji;'es the i nd i v idua l l i n e i n t e n s i t i e s , the l e f t -hand scale g iver s p c t r u m surface b r a 'Itness on the de tec tor . The b r igh te r l i n e s a l s o are The 393 FIGURE CAPTIONS, c o n t . b: Tne above spectrum i n t e g r a t e d i n t o b i n s over an assumd 5000-second observat ion, w i th t h e counts i n each b i n subsequent ly m o d i f i e d by a P o i s s o n - d i s t r i b u t e d Monte- Cat l o procedure. F igure .pa: The spectrum o f f i g u r e 5 f o l d e d through a TAP spectrom- e t ? r w i t h 15" x 15" c o l l ' m a t i o n which covers the 515 - 1030 eV ratige. b: The above spectrum i n t e g r a t e d for 5000 seconds, binned and randomi red . F igure 7a: PbSt spectrum o f th,.ec-tlainpct-atcrre ' i t t o a p a r t o f the Ngrth Po lar Spur. b: TAP response t o above spectrum. Figure 8a: S i n g l e tcmperatsre p l u s power l a w f i t suggzstpd by Nagoya Univ. group f o r data from North Polar SFur. PbSt spec t r m e t e r . b : As abave, b u t TAP spectrnmeter response. ORlGlNAL PAGE IS OF POOR QUALW 0 0 M 0 m cu 0 0 cu 0 rr, - 0 0 - 0 ln 0 (D 0 b 0 a0 397 FIGURE 4 398 x IO-' 6 In \ N € 4 \o v) c 2 c) - - - 2 2 0 I50 Z iii 100 \ v) + 50 u 0 I 1 I I I 1 I 1 POSITION ACROSS COUNTER --* (23cm) FIGURE 5 399 FIGURE 6 400 ORIGINAL PAGE IS OF POOR QUALlrV I 1 I I I I I - ( b ) .- - - e .I - I - - c I I I I I I I I I I I I L 20 15 ANGSTROMS FIGURE 7 40 1 ORIGINAL PAGE IS OF POOR QUALITY 14 12 10 8 6 4 2 0 I .8 I .6 I .4 I .2 1.0 0.8 0.6 0.4 0.2 0.0 20 ANGSTROMS FIGURE 8 , . . . . ...*-.* I .,. , REQUIREMENTS FOR HIGH QUALITY X-RAY SPECTROSCOPY I N AN EXPLORER CLASS MISSION Webster C. Cash, Jr. Laboratory f o r Atmospheric and Space Physics Universi ty o f Colorado Boulder, Colorado 80309 ABSTRACT Through the use o f simulated x-ray spectra we address the question o f instrument requirements f o r a spectrograph t h a t could s i g n i f i c a n t l y adVdnCe x-ray astronomy. We conclude that resolut ion (A/M) i n the range o f 200-500, and e f f e c t i v e co l l ec t i ng crea i n excess o f 200 cm are required. We present one design, based on the object ive r e f l e c t i o n grat ing concept, which would meet these s t r ingent requirements i n an Explorer class mission. 2 4 04 I. INTRODUCTION I n the l a s t meeting o f t h i s nature, the Workshop on Concpact Ga lac t ic Sources, Rappaport (1979) gave a b r i e f overview o f x-ray astronomy i n terms of the threc major d isc ip l ines : E ins te in Observatory s t a r t i n g operations, x-ray imaging had reached a l e v e l of maturit! f a r beyond t h a t o f t im ing o r spectroscopy. The workshop concluded t h a t iin X-ray Timing Explorer was the next l o g i c a l step t o balancing the advance of x-ray astronomy. But, thG decis ion t o pursue t im ing a t the expense o f spec- troscopy wh3 made on technological and budgetary grounds; we know how t o b u i l d a t im ing experiment f a s t e r and cheaper than a spectroscopy experiment. w i t h XTE i n the Explorer quege, our a t ten t i on must t u r n t o spectroscopy i f new imaging, spectroscopy and timing. A t t h a t time, w i t h the Now, s c i e n t i f i c domains are t o be opened. The h i s t o r y o f astrophysics a lso supports the argument t h a t spectroscopy must be next. Opt ica l astronomy d i d no t evolve i n t o astrophysics u n t i l we l l i n t o t h i s century, when h igh q u a l i t y photographic spectroscopy was developed and the r e s u l t s i n te rp re ted throbgh modern physics. A t the t u r n of the century, o p t i c a l astronomy was i n a s i t u a t i o n very s i m i l a r t o the cur ren t s ta tus o f x-ray astronomy. The f i r s t concave g ra t i ng spectrograph o f the sun, obtained by Rowland (1882), showed an amazing wealth o f informat ion. The Solar Maximum Mission i s cu r ren t l y showing us a fabulous array o f x-ray spectrat features ; : a the sun. Yet, fo l low ing Rowlands so la r spectrum, there was a delay of more than twenty years before observator ies became s u f f i c i e n t l y sens i t i ve t o study s ta rs wi th h igh resolut ion. I n t h a t same per iod imaging became q u i t e advanced: The f i r s t photographic surveys d f the sky were completed, and ex t ragh lac t ic astronomy was born through the discovery o f vast numbers of nebulae. Simultane- ously, t im ing studies were advancing w i t h the quan t i t a t i ve studios o f e c l i p s i n g 405 variables and novae. The problem wi th spectroscopy was tha t the b r igh t sources i n the sky were, t o f i r s t order, simple blackbodies, while the objects wi th exotic spectra were too f a i n t t o observe. Today, we know from low resolut ion x-ray spectroscopy tha t the br ightest sources i n the sky hake, t o f i r s t order, featureless continua , while many of the fa in ter sources have qui te exot ic spectra. The pursu i t o f very high reso- l u t i on spectroscopy o f brigllc sources, and adequate resolut ion spectroscopy of fa in t sources, i s s t i l l the primary concern o f cp t ica l astronomers. Over ninety percent o f a l l observing t h e on the world's major telescopes i s dedicated t o spectroscopy. I th ink i n the future i t w i l l be the same f o r x-ray astronomy. In t h i s t a l k I w i l l address pr imar i ly the question o f what performance 1 w i l l f i n i s h w i th my level i s needed before x-ray spectroscopv w i l l mature. suggestion o f how the necessary performance level can be achieved. 11. A SIHULATED X-RAY SPECTRWI Ihe qrral i ty of a spectrum, and hence the qua l i t y o f science derived from it, depends on a var iety o f factors i n the spectrograph and i n the source. The performance of a spectrograph i s mainly determined by spectral resolut ion ai4d co l lect ing area, while bandpass and noise character ist ics play lesser roles. Since the spectrograph must be ab;e t o c lear ly detect the targeted spectral features, so the brightness o f the source, as well as the size and shape of i t s spectral features play a central ro le i n determining the spectrograph design. To investigate the int-erp:ay between spectrograph design and source char- ac te r is t i cs I have performed a simple computer simulation. With the help of flike $hul l , I have created a synthetic spectrum containing many o f the x-ray 406 spectral features we know should be present. Then, by smoothing the spectrum I could determine the resolut ion necessary t o locate, separate, and i d e n t i f y typ ica l features. By introducing varying amounts o f Poisson noise, I could gni- eve determine how many photons are needed t o make the features s t a t i s t i c a l l y s f icant. Through t h i s exercise I have reached some conclusions tha t I bel are convincing. I 1 I . RESOLUTIONS The i n i t i a l spectrum entered i n the computer was an array one thousand elements long, with each b i n o f 2eV width. Thus, the spectrum extended up t o 2 keW. By doing a simple boxcar smooth, I was able t o simulate lower reso?ut ion spectra. The series o f smoothings i s shown i n Figure 1. Part (a) shows the spectrum smoothed t o 280eV, typ ica l o f a proport ional counter. The spectrum i s devoid of features. Pulse height spectra such as these were obtained w i th HEAO-1, and through a clever inversion technique found the 0 absorption edge i n the Crab and Sco X - 1 (Kahn and B l i sse t t , Charles -- e t al . , Kahn -- e t al) . Moving t o Figure l ( b ) we see a smooth spectrum w i th a ledge a t the pos i t i on o f the oxygen edge. It has a resolut ion o f 180eV, s im i la r t o tha t o f the SSS on Einstein, although the SSS was not able t o observe below about 600eV. T r i p l i n g the resolut ion ( t o 64eV) i n l ( c ) produces a dramatic dif ference. The absorption d ip a t the 0 edge i s c lea r l y delineated, and an emission l i n e has appeared a t the pos i t i on o f the Fe L l ines. Figure l ( d ) doubles the resolu- t i o n again, w i th the e f f e c t o f sharpening ine emission l i n e and h in t i ng a t structure i n the 0 edge. I n Figure ] (e) , where the resolut ion i s 16eV (comparable t o the OCS on Einstein), the structure i n the 0 edge has c lea r l y become an absorption l i n e 407 a t the pos i t ion o f the 0 VI11 Lyman a line. structure. The i ron l i n e i s s t a r t i n g t o show The next doubling o f resolut ion, t o 8eV, brings with i t the resolut ion of the i ron l i n e i n t o a complex o f c losely spaced l ines. Also v i s i b l e f o r the f i r s t time i s the fact that the i n tens i t y drop across the 0 edge ;s not a s ingle step. Seweral ionized species o f oxygen are contr ibut ing t o the edge. A t a resolut ion o f 4eV i n Figure l ( g ) the l a s t hidden feature emerges. A weak absorption l i n e a t the pos i t i on o f the 0 VI11 Lyman a l i n e i s v i s ib le . The l a s t part , Figure l (h) , shows the o r ig ina l spectrum w i th f u l l 2eV resolut ion. l i n e complex i s f u l l y resolved. I t now shows the 0 VI11 absorption l i n e t o be saturated; the Fe With each improvement i n resolut ion we discovered new features i n the spectrum un t i 1, i n the l a s t two graphs, a1 1 o f the o r ig ina l features were appar- ent. Ry pushing the resolut ion beyond 2eV one could study emission and absorp- t i o n l i n e p ro f i l es , but t h i s would require resolut ion o f nearly 0.2eV. we current ly don't even know the gross spectral features o f x-ray sources, t a l k of observing l i n e p r o f i l e s seems a l i t t l e absurd - that should be l e f t f o r the fo'lowing generation o f spectroscopy. I n short, 2ew resolut ion w i l l be enough t o disccver and c l a s s i f y features, but l i t t l e extra w i l l be gained u n t i l resolut ion i s almost an order o f magnitude higher. Since Converting t o the more conventional spectroscopy notation, A - E 500eV - R = - = - 250. I V . POISSON NOISE 11; ;pectroscopy as i n imaging, Poisson noise i s the great enemy of the x-ray astronomer. Our experiment must co l l ec t enough photons i n each spectral 408 reso lu t i on b i n t o m a k the b in - to -b in va r ia t i ons (which ca r ry the i n f o m a t i o n ) s u f f i c i e n t l y small. To inves t iga te t h i s e f f e c t , I have taken the saw synthe t ic spectrum a t f u l l r eso lu t i on and run a Poisson randomizer over i t w i th var ious scale factors . Taking the f i g u r e p a r t by pa r t : The r e s u l t s are displayed i n Figure 2. a. b. C . d. e. f . This i s a minimal spectrum, t y p i c a l l y two photons per b in , w i th a t o t a l o f 1281 counts i n the spectrum. ove ra l l shape o f the curve, an exponential w i th an i n t e r s t e l l a r c u t o f f . With 10 c /b in we s t a r t t o see the 0 edge. A t 30 c /b in the 0 edge i s c lear . t i c a l ly s i g n i f i c a n t . With 100 c /b in the spectrum i s look ing f a i r l y good. Most o f the major features are v i s i b l e . The biggest problem i s t ha t , wh i l e i t i s c lea r tha t the features are present, quan t i t a t i ve statements about t h e i r s i r e w i 11 be inaccurate. This i s an exce l len t spectrum. With 1000 c / b i n a l l the features are c lear , and quan t i t a t i ve assessment i s c l e a r l y possible. A t 10 c h i n there i s v i r t u a l l y no d i f fe rence between the o r i g i n a l and the randomi zed spectra. Nothing i s v i s i b l e except the The 0 V I 1 1 Ly Q i s becoming s t a t i s - 4 From Figure 2 we have learned t h a t a spectrum w i t h less than 100 c /b in i s 4 inadequate w h i l e i n most cases 10 c /b in is more than i s requ red. A good number t o shoot f o r i s 1000 c!bin, and a bare ly adequate spectrum i s 100 c/b in . I recommend tha t the Explorer be able t o ob ta in 100-1000 c /b in per iod o f t ime or! a l l prime sources. n a reasonable The s i t u a t i o n i s d i f f e r e n t when emission l i n e sources are considered. Take the case o f the synthet ic emission l i n e spectrum shown i n Figure 3(a). When these l i n e s aI.e Poisson randomized we f i n d t h a t f a r fewer photons are 409 required. Figure 3(b) i z the same spectrum w i th a t o t a l of only 46 photons, rebinned i n t o 80 bins t o best show the data. I t i s a f a i r representation of the o r ig ina l spectrum. Fiyure 3(c) i s a s im i la r spectrum w i th 188 photons. I n 3(d) we have 830 photons and i n 3(e), 55,000 photons. It i s c lear tha t i n order t o re t r i eve the basic physics o f a l i n e source, only 100 photons per l i n e are needed. A t o t a l o f 10 photons should show a l l the major features i n a l i n e source. This compares t o the 10 t o 10 photons required f o r a good spec- trum o f a cont inwa source. s t e l l a r coronae and supernova remnants. 3 5 6 I t i s p a r t i c u l a r l y good news f o r the studies o f V. COLLECTINC AREA Start ing w i th the level o f s t a t i s t i c s recumended i n the previous section, With an Explorer, a I n I n t h i s amount o f time we require we can derive the needed spectrograph co l l ec t i ng area. typ ica l observation should be completed i n 2 x lo4 ;econds o f exposure. real terms t h i s requires an e n t i r e day. enough col l ec t i ng area t o observe a large number o f sources from many classes. This d i ve rs i t y o f appl icat ion i s necessary i f the Explorer i s t o make a major contr ibut ion t o astronomy. I n Table I we l i s t the four most prominent classes o f x-ray point sources: the x-ray binar ies, s t e l l a r coronae, Seyfert galaxies, and quasars. We would l i k e t o be able t o observe ten o r (Ifore objects i n each o f these classes, so we have l i s t e d the approximate f l u x ( i n HEAO-2 IPC counts) o f the tenth br ightest member of each class i n column 2. I n column 3 t h i s f l u x has been converted t o ph ctn-2s-1bin-1, oxcept i n the case of s t e l l a r coronae where i t i s i n un i ts o f ph cm s line-’. obtain colurrn 4 which shows the ph cm-2bin-1 collected. matter t o calculate column 5, the co l l ec t i ng area needed t o achieve the desired 1000 c/bin, or f o r coronae, the 100 c / l ine. - 2 -1 4 Mult ip ly ing by the maximum observing time (2 x 10 s) we Then, i t i s an easy 41 0 It i s apparent that , a t an absolute minimum, the spectrograph should have 2 50 ca of c o l l e c t i n g area. This would pertnit the c c l l e c t i o n o f good data on s t e l l a r coronae and x-ray b inar ies , bu t bare ly reach the Seyferts. I t would f a i l completely on the quasars. However, wi th 200 cm the Seyferts could be we l l studied, and the b r igh tes t quasars would be wi th in reach. This higher c o l l e c t i n g area valuable on the x-ray b ina r ies and coronae as we l l , s ince i t i s less l i k e l y tha t v a r i a b i l i t y could change the spcrtrum before the observation i s completed, and more ta rge ts could be observed. 2 I recommend t h a t the spectrograph have no less than 200 cm2 e f fec t i ve c o l l e c t i n g area. V I . SPECTROGRAPH DESIGN We are now faced w i t h a very large order: Design an Explorer c lass i ns t ru - 2 ment w i t h reso lu t i on o f 250 and c o l l e c t i n g area Jf 200 ca . be done, and i n t h i s sect ion 1 o u t l i n e one poss ib le design. 1 be l ieve i t can The f i r s t p o i n t t o recognize i s t h a t we must use d ispers ive spectroscopy coupled w i t h grazing incidence opt ics . A l l a l t e rna t i ves simply w i l l not produce t h e requi red performance. Crysta l spectrographs w i 11 not achieve the necessary c o l l e c t i n g area i n a reasonable s i z e payload because c r y s t a l s operate by absorb- i ng the great bu lk o f the inc ident photons. So l i d s ta te spectroscopy has low reso lu t i on (180eV versus the 2eV required), and there i s no foreseeable chance tha t i t w i l l achieve h igh reso lu t i on dur ing t h i s generation. The only ether a1 te rna t i ve i s d ispers ive spectroscopy. I n the s o f t x-rays we have the choice o f e i t h e r transmission grat ings or There are arguments i n favor I w i l l merely say t h a t 1 r e f l e c t i o n grat ings f o r the d ispers ive element. o f each o f these which I do not w i s h t o pursue here. 41 1 persanal l y bel ieve t h a t i n the fu ture x-ray astronomers * 1 i ke op t i ca l astrono- mers, w i l l perform v i r t u a l l y a l l t h e i r spectroscopy w i th r e f l e c t i o n gratings. Reflect ion gratings o f f e r higher e f f ic iency, greater s t a b i l i t y and higher dispersion - a1 1 propert ies o f central importance t o achieving high spectral reso 1 u t i on. I have been invest igat ing i n the laboratory the leve l o f e f f i c i ency one Figure 4 shows the r e s u l t obtained on a can expect from r e f l e c t i o n gratings. gold coated grat ing a t 13 from a 6000 Plmn, 21' blaze master, was ru led by Hyperfine, Inc. t o make an (Cash and Kohnert, 1981). This grat ing, rep l icated opt ica l element f o r the shows a peak e f f i c i ency would have been closer 2 achieving the 200 cm . High Resolution Spectrograph on Space Telescope. It o f 40% i n f i r s t order. With a n icke l coating t h i s t o 50%. Such very high e f f i c i ency i s c r i t i c a l t o I n addit ion, i t i s necessary t o keep the number of elements i n the op t i ca l t r a i n t o ail absolute minimum. concept of the Objective Ref lect ion Grating Spectrograph (ORGS), which, co inc i - dental ly, maximizes spectral resolut ion. I t i s qu i te simple, requi r ing only that an array o f r e f l e c t i o n gratings be mounted i n f ron t o f an x-ray telescope such that the telescope views the surface o f the gratings. With t h i s i s mind I have been working on the I have jo ined i n a col laborat ive e f f o r t w i t h D r . Richard Catura o f the Lockheed Palo A l to Research Laboratory and Dr. J . 1 . Culhane o f the Mullard Space Sciences Laboratory t o b u i l d an ORGS. We are modifying t h e i r e x i s t i n g telescope t o hold an array o f r e f l e c t i o n gratings as shown i n Figure 5. I t i s an Aries class telescope, and we hope t o obtain our f i r s t spectrum from a rocket f 1 i g h t i n one t o two years. I t can be shown that the resolut ion (A/M) of an ORGS i s given by 41 2 a$z1 where n i s the order number, A the wavelength, d the grat ing r u l i n g spacing, and B the telescope resolut ion i n radians. We take the best telescope resolu- t i o n consistent w i th an Explorer mission and a proportional counter detector t o be 20'' (e = The highest r u l i n g density current ly avai lable i s about 10 U m l a o r d = lC00 f t . Thus, i f R = 250 we must have 4 nh = 25 f l . A t energies below O.SkeV we can achieve f u l l resolut ion and co l lec t ing area, while above t h i s energy we have the choice o f e i ther f u l l co l lec t ing area and reduced resolut ion (1st order), or f u l l resolut ion and reduced co l lec t ing area (higher orders). This i s not an ideal s i tuat ion, but i t i s adequate. The telescope needed t o support t h i s concept i s of reasonable size. To 2 achieve a 200 cm spectrograph, we need a telescope with an e f fec t i ve area of 400 cm . A telescope the same size as ROSA1 w i l l su f f ice, but the mirrors need only perform a t the 20" leve l . A proportional counter w i th .125 nun resolu- t i o n i s adequate, although one might hope tha t some other e f f i c i en t , imaging detector might become available. 2 We conclude by point ing out t ha t object ive gratings cannot perform w e l l on extended sources. There i s a possible solution. A 1 x 30 arcminute collima- t o r mator could be mounted i n f ron t of the gratings, allowing study o f extended objects w i th R = 80. V11. SUMMARY I have demonstrated tha t i n order f o r x-ray spectroscopy t o become a serious too l of astronomy, we need t o b u i l d 3 spectrograph with resolut ion i n excess 2 250 and co l lec t ing area i n excess 20C cm . I have presented the ORGS concept, which I believe i s the easiest way t o achieve these str ingent speci f icat ions. I wish t o thank R. Catura and F1. Shull f o r help i n preparing th is ta l k . I acknowledge support from NASA grant NAG5-96. TABLE 1: TARGET FLUXES FLUX i n 2 x 104s cm2 needed for 10th BRIGHTEST ph cm-zs-'bin-' ph cm-2bin-' 1000 ph/bin or (IPC COUNTS) (R = 200) 100 ph/ l i n e X-RAY B I N A R I E S 30 1.5 30 30 SEYFERTS 3 1 . 5 3 300 QUASARS 0 . 2 1 I C - ~ 0.2 5000 STELLAR CORONAE 1 1 x 2 50 ph c n ~ - ~ s - ' l ine - ' ph cm-'i ine 41 4 REFERENCES Cash, W. and Kohnert, R. 1981 AJ&. m. , in press. Charles, P . A . , e t a l . , 1979, AJ. 1 . ILetters), 230, L83. Kahn, S.M. and B1 i s s e t t , R. 3 . 1980, AJ. - - 3. , 238, 417. Kahn, S.M. , Charles, P.A. , Bowyer, S. and Bl i s se t t , R.J. 1981, AJ. - J. , in press. Rappaport, S. 1979, in "Compact Galactic X-ray Sources," ed. F. Lamb and Rowland, H.A. 1882 - Phil. Mag., 13, 469. 0. Pines, p . 181. 41 5 FIGURE CAPTIONS FIGURE 1: A syn the t ic x-ray spectrum i s shown smoothed t o var ious resolu- t ions: a) 280eV; b) 180; c) 64; d) 32; e) 16; f) 8; g) 4; h) 2eV. See t e x t f o r discussion. FIGURE 2: The syn the t ic spectrum w i t h 2eV i s shown randomized by Poisson s t a t i s t i s w i i b increasf i iy number o f photons. FIGU!?E 3: The e f f e c t o f Poisson s t a t i s t i c s dn a l i n e spectrum i s i n v e s t i - gated. FIGURE 4: 13A e f f i c i e n c y o f a 6000 U m m r e f l e c t i o n g r a t i n g i s shown a, a func t i on o f graze angle. FIGURE 5: Schematic of an Object ive Ref 1 e c t i on Gra t i ng Spectrograph 41 6 2.00 1.00 - 0.50 v) I- 0.00 3 > < t- K ” 1.00 > t- 0.50 z W k 0.00 z a 2.00 a m a 1.50 - - 2.00 1 .so 1.00 0.50 0.00 - / 0 FIGURE 1 2000 eV 2.00 1.60 1.00 0.50 0.00 2.00 1.50 1.00 0.50 0,oo 2.00 1 .so 1 .oo 0.60 0.00 0 / I I I I ' I 2000 eV 41 7 418 2 .OO 1.58 1 .oo 0.60 0.00 3.00 2.00 ORIGINAL PAGE 1s OF POOR QUALlW . I D I I h 1 .oo 0.00 0 2000 eV 42 1 1.20 0.80 0.40 0.00 2 E 6.0 33 t- z 4.0 ~~ 3 0 0 2.0 0.0 20.0 16.0 10.0 6.0 0.0 b 1 FIGURE 3 422 40.0 30.0 20.0 = - 10.0 m \ cn 0.0 I- z 3 600 0 400 200 0 d e 423 I I I I I I I1 I I I I I I 0 0 0 0 0 0 b Qp la t cr) a4 (lN33U3d) A3N3131Jd3 I 0 P ORlGlML OF POOR f ! i PAGE IS QUALm PZP FUTURE PROSPECTS FOR HIGH RESOLUTION X-RAY SPECTROMETERS Claude R. Canizares Center f o r Space Research, M.I.T. and I n s t i t u t e of Astronomy, University of Cambridge I. INTRODUCTION The t a sk I set myself f o r t h i s meeting was t o review and compare the c a p a b i l i t i e s of the kinds of X-ray spectroscopy payloads I knew about, t o compare those with some of my own est imates of the an t i c ipa t ed c a p a b i l i t i e s of AXAF, and t o do t h i s i n the context of the science we want t o achieve. Let me preface a l l t h i s by echoing i n general terms what many of yesterdays speakers has demonstrated the tremendous s c i e n t i f i c power of spectroscopy t o probe deeply the astrophysics of a l l types of c e l e s t i a l X-ray source. However, i ts l imi t a t ions of s e n s i t i v i t y and r e so lu t ion have i n most cases permitted u s only t o whet our appe t i t e s . spectroscopic instruments can and w i l l provide the banquet. s a i d i n d e t a i l ; namely t h a t E ins t e in The next generation of XI. PARAMETERS OF X-RAY SPECTROMETERS The comparison of a l t e r n a t i v e types of X-ray spectrcmeter i s extremely d i f f i c u l t because so many orthogonal parameters must be considered and weighed aga ins t one another. l ist of these includes the energy range, the s e n s i t i v i t y or throughput (which i s usual ly a s t rong funct ion of energy, E), the degree of background r e j e c t i o n , the resolving power (E/*, a l s o o f t en a funct ion A probably incomplete 426 of E), the degree of spatial and spectral multiplex advantage (ability to observe multiple spatial/spectralelements simultaneously), the effectiveness on extended sources (often involving trade-offs of throughput and resolution with field-of-view), the technical difficulty of the instrument and, not least, the size and weight. a) General Considerations 1 will only consider instruments capable of moderate to high spectral on discrete celestial sources (but see McCamnon's resolution (EIAE > 100) contribution) as the Einstein results demonstrate that this is what is needed for the plasm diagnostics of a source (e.g. Winicler et al. 1981). The state-of-the art then lin5ts us to dispersive instruments. clear requirement is that any instrument has some degree of signal concentration so that the detector area (AD is << the effective collecting area (AC). sources have fluxes d 10 czn-2s-1, instruments generally have peak efficiencies s 20% and a more 01 less irreducible particle induced background rate for low-background detectors (achieved in flight by the FPCS and IPC as well as the Wisconsin rocket payload, for example) is % 1 x 10 an s keV . One needs a concentration factor AC/% just to achieve a signal-to-noise ratio > 1. Of course, with sufficient concentration an instrument becomes signal limited. Another This is simply because the lines from all but the strongest -3 -3 -2 -1 -1 Dispersive spectrameters divide into two distinct classes: (i) Bragg spectrometers that use crystal diffractors (like the Einstein FPCS; see Giacconieal 1979, Canizares et al. 1979) and (ii) spectrometers that use gratings in either transmission (like the Einstein OCS, see Schnopper et al. 1977) or reflection. The Bragg instruments are capable of high spectral resolution, but .hey suffer the tremendous shortcoming of having no inherent spectral 427 I , multiplex advantage. plasma diagnostics require a t least 4 - 6 l ine strengths. For Bragg instruments X-rays from a given direct ion incident on a given an2 of Bragg crys ta l , are ref lected only i n a narrow passband near the Bragg energy - a l l others are absorbed. of a Bragg spectrometer is hF = PbROJ n Rc/AO where ApW is the projected area of the d i f f rac tor (or telescope i n a focal plane instruntent), n is the t o t a l efficiency of a l l elements other than the d i f f r ac to r , % is the inherent "integrated re f lec t iv i ty" of the c rys ta l , and A0 i s the to t a l "acceptance angle" of the c rys ta l (the range of incident angles on the c rys ta l i f it is bent o r thu range through which it is The latter point is crucial , since meaningful The e f fec t ive area at a given energy rocked). approximately * 9, where W is the "rocking curve width" of the c rys ta l and (AEFF) is ,btained i f A0 I, W. In pract ice t h i s may not be possible or desirable fo r various reasons; for example one may want A0 t o be large enough so the corresponding AE covers adjacent l ines such as the He-like t r i p l e t s . Bu t ultimately a l l B r a g g spectrometers are limited by the limited ava i lab i l i ty of d i f f rac tor materials and so they share the same relevant inherent parameters such as I$ or I$ and h/AF. fo r point sources. reference the Einstein FPCS had %T % 2 - 3 cm near 1 keV and up t o 10 times less a t sane energies. backgro*nd is obtained e i the r with a telescope, or by sui tably curving the c rys ta l so i t ac ts as a concentratorldiffractor. I assume a uniform area or exposure across 80. % i ? I, is i ts "peak ref lect ivi ty" , so a near-optimum efficiency MAX Selection of a par t icular A0 may be one of the design trade-offs. For 2 The concentration required t o reduce Crating instruments have the spectral m l t i p l e x advantage over a t leas t a sizeable part of the energy spectrum, inevitably performed with a telescope. I am familiar w i t h c a l l fo r objective gratings in transmission o r Here concentration is The only t ru ly viable designs 428 r e f l ec t ion . (see W. Cash's paper a t t h i s meeting). g ra t ings have been discussed a t var ious times i n the p a s t , t he re appears t o be no good so lu t ion ye t t o the fundemental l i v i t a t i o n of t he mismatch between the flnumbers of the grazing incidence te lescopes and r e f l e c t i o n grat ings; telescopes produce r ay bundles with angular divergencrs of % 10' , which i s many times l a rge r than the acceptance angle of standard X-ray r e f l e c t i o n g ra t ing spectrometers. The l a t t e r i s a r e l a t i v e l y new concept f o r X-ray astronomy Although f o c a l plane r e f l e c t i o n b) Bragg Spectrometer Desip ( i ) AXAF Focal Plane Crystal Spectrometer (AXAF FPCS) &re I assume a scaled up vers ion of the E ins t e in FPCS. The telescope provic'es concentration by a f a c t o r of % 1000 f o r a point source. The instrument operates i n a scanning mode with se l ec t ab le d i f f r a t o r s t o cover the f u l l energy range. of its astigmatic imaging p rope r t i e s , although an aperture should be used t o l i m i t the f i e l d of view, e.g., t o 3 x 30 arc min f o r t he E ins t e in FPCS. l i m i t t h e t i m e r e so lu t ion f o r s t u d i e s of adjacent l i n e s because the event r a t e is so low. For AEFF % 10 - 50 and l i n e f luxes % 10 cm s there are 20 - 100s per event, and reasonable scan times have neg l ig ib l e It can handle extended sources by v i r t u e ( I do want t o note i n passing t h a t scanning general ly does not -3 -2 -1 e f f e c t on the achievable time resolut ion.) (ii) Conical Crystal Spectrometer (CCS) This design i s associated largely with Bruce Woodgate and co l l abora to r s (Woodgate e t a l . 1973). concentration f a c t o r of % 100 t o a line focus. rocked t o cover a moderate energy range ( ?, 50%). Larger ranges must be covered by independent instruments. Extended sources must be coll imated A c r y s t a l panel is conical ly curved t o give a A given panel can be 1 t 429 t o Q, 10' t o give E/A8 Q, 100. ( i i i ) Spherical Crystal Spectrometer (SCS) This design is due t o Schnopper and colleagues (Schnopper and Taylor 1980) and is described f u l l y by G r i f f i t h s a t t h i s meeting (see a l s o Culhane's presentat ion) . CCS is t h a t the bent c r y s t a l concentrator second bend which introduces a pseudo-multiplex advantage a t the expense of peak o f f e c t i v e area. (e.g. A8 is >> W by design). Thus the SCS need not be scanned (saving mechanical complexity), and the re i s a f u r t h e r advantage i n the imaging c a p a b i l i t y (a nore s u b t l e point is t h a t the instrument can b e n e f i t from the high r e f l e c t i v i t y of mosaic c r y s t a l s without necessa r i ly s u f f e r i n g f u l l y the usual ly accomp&iying degradation of energy resolut ion.) f ixed panel w i l l cover a AE/E of 2r 10 - 20%. area t o bear on a s i n g l e l i n e nor avcid devoting area t o what may be un in te re s t ing port iogs of "-e spectrum f o r some sources. Again, mult iple energy ranges require mult iple instruments c a p a b i l i t i e s a r e very good: resolving powers af 2 100 can be achieved over a f u l l io f i e l d . The main d i f f e rence between t h i s and the (again Ac/\ % 100) has a The p r i c e one pays is i n f l e x i b i l i t y . A given One cannot b r ing a l l the But the extended source ( iv ) Imaging X-ray Spectraneter This is a "barn-door'' object ive c r y s t a l spectrometer such as the one described by Angel and Weisskopf (1970). with Ken Pounds and co l l abora to r s f o r spacelab. A f l a t c r y s t a l panel is followed by a mOdeYite r e so lu t ion te lescope, which gives a concentration f a c t o r of Q, 10 Thus t h i s instrumeid has a s i g n i f i c a n t s p a t i a l / s p e c t r a l multiplex advantage. I proposed such an instrument 5 f o r a point source md f u l l f i e l d imaging over near ly lo. 430 A t a s ingle s e t t i n g one obtains a f u l l image i n a narrow band whose c e n t r a l energy var ies @.long one dimension . up a s p a t i a l l y resolved spectrum over thc whole image. panel can y i e l d higher resolut ions than bent c r y s t a l spectrometers over i t s spectral range ( l i k e the SCS it can get high reso lu t ion with mosaic c r y s t a l s ) , and unlike the other instruments i ts reso lu t ion is not degraded by source extent . The c r y s t a l must be rocked t o bui ld The f l a t c r y s t a l c ) Grating Spectrometer Designs ( i ) AXAF Transmission Gratings Spectrometer (AXAF TGS) Although r e s u l t s from the Einstein OGS have been slow i n coming, the instrument did acquire some remarkable spectra , and s i g n i f i c a n t improvements have been made i n gra t ing fabr ica t ion s ince the HEAO program a t Utrecht, MF'I and MIT. f o r rcher missions (e.g. ROSAT r e f l i g h t ) . I w i l l concentrate on the parameters of a possible AXAF OGS, but any similar system would share many of the same propert ies . a point source i s lo6 - 10 because of the mall image s ize . EXOSAT has a gra t ing system and gra t ings are under consideration The concentration f a c t o r of such a s y C . e m f o r 7 A s a s l i g h t digression l e t me describe some of the improvements i n i s grat ing fabr ica t ion being made a t MXT. our E l e c t r i c a l Engineering Department, with whom I and Mark Schnttenburg a re col laborat ing t o per fec t g ra t ings of high s p a t i a l frequency. been par t icu lar ly in te res ted i n making thick gra t ings f o r use around the 6 keV iron l i n e s using thc technique of s o f t X-ray lithography perfected by Smith and h i s coworkers a t MIT and Lincoln Lab. the calculated, one-sided f i r s t order e f f ic iency f o r an 0 .9 um thick grating. u l t i m a t e peak two-sided e f f i c i e n c i e s of 50%. feas ib le is shown i n Figure 2 , which i s an e lec t ron micrograph of a This,work of Prof. Henry Smith of We have Figure one shows This a c t s as a phased gra t ing i n the region of i n t e r e s t giving That such gra t ings a re 431 3000 lpmn gold g ra t ing of thickuess 0.6 m. mounted on polyimide s u b s t r a t e s (taken i n t o account i n Fig. 1) t h a t make them extremely rugged. be free-etanding. present ly pushing f o r increasing the thickness t o the desired value. The gratingo a re a l l Of course a t lower energies the grat ings should A 5000 l p m g ra t ing has been made a d w e are Transmission g ra t ings i n coma-corrected mountings (Beuerman e t al . 1978) could give very high throughput and re solutioo (WAE /L 100 - 600) f o r point sources. most of the f l u x is i n a few emission l i nes . This i s indeed the case f o r many supernova remnants. su i t ed t o t h i s because of t h e i r small s i z e , and cooling cores i n galaxy c l u s t e r s may be s i m i l a r l y accessible . They would even be usrable on extended sources i n which The remants i n the LMC are p a r t i c u l a r l y well ( i i ) Objective Reflect ion Grating Spectrometer (ORGS) This c lever design is discussed i n d e t a i l by Cash ( t h i s meeting). 5 The concentration f a c t o r i s % 10 one needs r a t h e r f i n e coll imation (1' x 20 ' ) t o avoid degradation of the r e so lu t ion t o below E/AE - 100, and t h i s w i l l both add ca > l e x i t y and l i m i t the s i g n a l on extended sources such as SNR's and c l u s t e r s . ORGS has g rea t promise, but as the youngest of the instruments mentioned here it na tu ra l ly has the highest degree of undemonstrated technology at t h i s time. f o r p o i x sources. For extended sources The 111. COMPARATIVE ANALYSIS A comparison of the perforsance of the various spectrometer designs i s extremely d i f f i c u l t and fraught with p i t f a l l s . Each design contains enough f r e e parameters t h a t i t is nearly always possible t o improve one c h a r a c t e r i s t i c a t the expense of another (e.g. , AEFF at some energy a t the exptnse of energy range). compared i n any d e t a i l . Neverthelese, i t is i n s t r u c t i v e t o analyze the Thus only hard and f a s t designs can be 432 relative merits of several "strawman'' instrunents. to the various proponents of each instnxwnt and with the warning that the parameters listed belaw can change by factors of up to 10 ae the designs are modified. I do so with apologies I have made various assumptions in canputing Tables 1 and 2. AXAF parcaseten are estimated fran various AXBF working group papers. For the crystal instruments (CCS, SCS, IXS) I assume crystal projected areas of 10 cm and diffractor properties appropriate to TAP (1 keV) or LiF (6.? keV). For the ORGS and IXS I take telescope effective areas of 2 SO0 cm at 1 keV I assme all detector efticiencies are 1.0. Collimator 4- transmissian is taken as 0.7 for the CCS and 0.5 for the ORGS. The SCS is assumed to have % 150 spectral resolutim ele~~ents that span the 5 lines in question (e.g. it has pseudo spectral multiplex advantage). the weak source limit in which the background is all of noa-X-ray origin with flux l!~-~csl-~s-\eV-? measure this background simultaneously with the signal in unilliminated The 3 2 (and zero at 6.7 keV) I take I assume that the CCS like the FPCS can pertions of the position sensitive detector (as we have done with the Eirstein FPCS). The 20' source is assinned to have unifow surface brightness, so that apertures reduce the flux to the instrumnt. The listed 3a flux limits are for each of five lines (to allow 5 sone plasma diagnostics) and a tctal exposure of 5 x 10 s. the multiplexed instruments will get information about the entire spectrum simultaneously. there are many more lines near the former energy. Of course, This is of greater value near 1 keV than near 7 keV, as Except where indicated the instruments are signal limited and so the number of detectcd photons is very smell. IV. CONCLUSIONS One imwdiate conclusion is that each of the strawman instrments 433 has a .sensit ivity t h a t is a t lekst an order of magnitude b e t t e r than t h a t of tSe E ins t e in FPCS. instruments is sure t o provide the s p e c t r a l banquet I r e fe r r ed t o i n 0 1 . For i l l u s t r a t i o n I show i n Table 3 some crude est imates of the typical l i n e s t r eng ths f o r a given class of ob jec t s ( i .e . , not j u s t the b r i g h t e s t one or two). With l imi t ing f l u e s of lo-' t o 10 cm 8 one w i l l have many hundreds of g a l a c t i c and e x t r a g a l a c t i c ob jec t s t o This means t h a t the next generat ion of -6 -2 -1 study. A second conclusion is t h a t e&-): instrument ifivolves trade-offs and compranises t h a t a u s t be weighed with g rea t care. ana lys i s shows how strongly the r e l a t i v e merits of a given design depend an the d e t a i l s of the object ive (e.g. extended vs. point sources). I have not even addressed important d e t a i l s such as how the r e so lu t ion degrades w i t h source ex ten t o r how s e n s i t i v e the instrument may be t o spacecraf t point ing unce r t a in t i e s . Even my crude A t h i r d conclusion is t h a t AXAF promises t o have a powerful s p e c t r a l capab i l i t y when i t is eventual ly launched. Probably the g r e a t e s t weakness of the two AXAF instruments considered here i s the a t least p a r t i a l breakdown of the multiplexed TGS f o r sources I t would seem t h a t t h i s leaves a major hole f o r some o the r fu tu re mission, e spec ia l ly around the Fe l i n e where the e f f e c t i v e area of the FPCS is severely l imited by the telescope e f f i c i ency . I am g r a t e f u l t o the I n s t i t u t e of Astronomy f o r t h e i r h o s g i t s l i t v , 20" i n extent . t o the Royal Society f o r t h e i r support through a V i s i t i n g Research Fellowship, and t o NASA f o r par t ia l support under contract IUS-8-30752. I thank Andy Hawrylick, Mark Schattenburg, and Henry Smith f o r Figures 1 and 2 . I thank the Alfred P. Sloan Foundation for a Research Fellowship 4 34 REFEKENCES Angel, J.R.P. and Weisskopf, M . C . 1970, Astron.J., E, 231. Beuerman, K.P. et al. 1978, Applied Optics, - 17, 2304. Canizares, C.R. et al. 1979, Ap.J., - 234, L33. Giacconi, R. -- et al. 1979, Ap.J., - 230, 540. Schnopper, H.W. -- et al. 1977, Applied Optics, 16, 1088. Schnopper, H.W. and Taylor, P.O. 1980, Applied Optics, - 19, 3306. Winkler, P.F. -- et al. 1981, Ap.J., 246, L27. Woodgate, B.E. -- et al. 1473, Applied Optics, - 12, 2759. -- I* 435 TABLE I WEAK SoUElcE FLUX LIMIT FOR DETECTION OF 5 LINES AT I keV IN 5 x 10 8 . 4 2 INSTRUMENT (cm AXAF FPCS 3 W a ) ccs 240 (a) scs 2 Ixs 170 AXAF n;s 200 ORGS 75 AETW 2 (cm I 10 0.02 0.005 I o4 0.005 FOV (arc min) 3' x 30' 10' x so 30' x so l o x lo 1 ' x 20' Notes : 4 4 a. b. Sensitivity is background limited at 10 s. (C) Extended source capability for sources No multiplex advantage; bIN assumes 10 s each on 5 lines. < 20" and strong emission line sources. Deconvolution possible on other sources. 436 TABLE 2 WEAK SOURCE J%VX LIMIT FOR DETECTION OF 5 LINES AT 7 keV IN 5 x 104s -2 -1 hIN (cm s 1 POINT 20' Fo" (arc min) MAX 2 (cm 1 INSTRUMENT %FF -2 (cm 1 - AXAF FPCS (a) 20 1 3' x 30' 5 x 10-5 4 x 10*(b) CCS (a) 140 10 10' x so 2 10+(b) 4 scs 4 .02 30' x 5' 5 5 AXAF TGS 50 lo+ 1 ' x 20' 4 x IO* (C 1 Notes: 4 assumes 10 s each on 5 lines. IMIN (a) No spectral multiplex advantage; (b) Sensitivity is background limited. (c) Extended source capability for sources < 20" and strong emission line sources. Deconvolution possible on other sources. 437 TABLE 3 ESTIMATED LINE FLUXES FOR CLASSES OF ASTROPHYSICAL OBJECTS SIZE LINE FLUX Io-' BINARIES (Fe LINES) IO-^ LMC SNR SNR COOLING CLUSTER CLUSTER Fe LINES CORES S T E L M CORONAE IO-^ I o3 IO4 GAS IN GALAXIES (e.g. M86) * CLUSTERS @ Z - (3.5 4 38 FIGURE CAPTIONS Figure 1: Calculated one-sided first-order efficiency for the illustrattu gold transmission grating on a polyimide substrate. Figure 2: Electron micrograph of a 3000 lpmm gold grating on a polyimide substrate. The grating thickness is 0.6 urn plus 1.0 urn of polyimide. fabricated by a muitistep process involving both holographic exposure and soft X-ray lithography. This grating was 439 ORIGINAL PAGE Is OF POOR QUALITY - u z J 0 t3 d (3 0 0 u 0 - 441 3 2 2- p. 'N82 26011 BROAD BAND X-RAY TELESCOPE (BBXRT) P. J . Serl emi tsos Laboratory f o r High Energy Astrophysics NASA/Goddard Space F1 i g h t Center Greenbelt, MD 26771 ABSTRACT An approach i s presented a1 ong w i t h corroborat ing measurements f o r s i g n i f i c a n t l y enehancing the po ten t ia l o f S i ( L i ) spectrophotometry i n X-ray astronomy. The key new element i s an unconventional X-ray m i r r o r t h a t meets q u a l i f i c a t i o n s o f lov~ cost, l f g h t weight, and la rge throughput over a broad energy band (i 7 keV) a t moderate angular resolut ion. The po ten t i a l f o r o ther appl i c a t i ons i s a1 so d i scussed. I. INTRODUCTION I w i l l be repor t ing on a 0.5-10 keV non-dispersive spectroscopy experiment proposed by the Goddard X-ray group some three years ago i n response t o NASA's Spacelab AO. telescopes w i t h cooled S i ( L i ) spectrometers a t each focus. follow-up e, oeriment t o the s o l i d s ta te spectrometer cont r ibu ted by the Goddard group and flown aboard the E ins te in Observatory (experiment 8-5; Joyce e t a l . 1978). were no t accessible wi th the previous instrument. It consis ts o f 2 co-aligned grazing incidence BBXRT i s a It represents a major e f f o r t t o tap new research areas t h a t Asslgned t o NASA's OSS-2 442 mission, i t has been funded for definition and some limited hardware development, OSS-2 i s not as yet on NASA's approved f l i g h t program. Some prel imi nary test resul t s i n v o l v i n g prototype telescope and detector hardware will be presented and discussed. We will also consider some specific observations from a shuttle mission t h a t l i n k the instrument's potential t o several o f the items t h a t have surfaced i n this meeting from the theoretical reviews a s well as t o potential goals of an explorer mission. I I . OBJECTIVES AND IMPLEMENTATION BBXRT was conceived and is presently under development t o meet the fol 1 owi ng objectives: 1. Expand t h e energy band of S i ( L i ) spectrophotometry t o adequately include i ron K-shell transitions. Recover some measure of the spatial information t h a t telescopes provide b u t which i s lost i n single-pixel photometry. Reduce detector background sufficiently t o make possible the study of the many new sources identified i n Einstein IPC and H R I fields. I n spite of obvious limitations with resolution, focal plane S i ( L i ) spectroscopy h a s a potential t h a t we have only begun t o exploit w i t h the instrument aboard Einstein. One direction t o go for a predictably large impact on future observations is t o extend the response of grazing incidepce optics t o about 10 keV. This would make possible the study of iron K-shell transitions which are the dominant transitions whether col1 isionally excited i n hot plasmas w i t h kT > a few keV or due t o the fluorescence process. Even when the spectrum i s featureless, a broad band-pass will result i n better determination of the shape of the continuum and of any detectable absorption 2. 3 . 44 3 e f f e c t s a t the low end o f the spectrum. E f f i c i e n t r e f l e c t i o n a t the higher 7 keV) energies w i l l only occur from surfaces a t small ( $ 0.5 degrees) angles t o the inc ident beam. cy1 i n d r i c a l telescope geometry, the instrument focal length determines the I n a maximum mi r ro r aperture that s a t i s f i e s the above condit ion. The high energy througnput w i l l depend on how e f f e c t i v e l y we u t i l i z e the area w i t h i n t h i s 1 i m i t i n g aperture. When nest ing confocal surf ices i t i s imperative t h a t we maximize the r a t i o o f the r e f l e c t i n g t o dead areas, the l a t t e r made up o f mi r ro r w a l l s , gaps and supports. The BBXRT m i r ro r design opts f o r maximum high energy response a t the a expense o f high reso lu t ion imaging. The l a t t e r i s compromized by a d ras t i c reduction i n m i r ro r wal l thickness and by the use o f an approximate geometry: both the paraboloid and the hyperboloid i n the Wolter type I geometry are replaced w i t h t i g h t l y nested confocal t h i n f o i l cones. Note t h a t the approximatihzn improves f o r small grazing angles and shor t cone segments. We hab2 chosen a cone length which resu l t s i n an i n t r i n s i c spat ia l reso lu t ion < 0.5 a r c minute h a l f power radius (i.e. if we neglect m i r ro r d is to rs ions and assembly tolerances). The f o f l r e f l ec to rs are formed from .005-inc+, high q u a l i t y but commercially ava i lab le aluminum f o i l overcoated w i t h l o p o f a c r y l l i c lacquer on wh:ch * 500 A o f gold i s vacuum deposited. This simple 0 surface preparation replaces the customary po l i sh ing which, aside from cost, would be q u i t e imprat ica l i n t h i s case. As we sha l l show, i t produces a surface smoothness comparable w i t h t h a t o f h igh ly pol ished surfaces. The parameters f o r each o f the 2 BBXRT mi r ro rs are l i s t e d i n Table 1. Note i n p a r t i c u l a r t h a t the e f f i c iency f o r u t i l i z i n g the telescope aperture i s 0.7 (0.6 i f the small cent ra l vo id i s added t o the dead area) which i s n3t too d i f f e r e n t from the transmission e f f i c i ency o f the co l l ima t ion t o a l a rge area 444 gas propor t ional counter. for each m i r r o r as a func t ion o f the angle f o r an o f f - a x i s i nc iden t beam. wish t o p o i n t ou t t h a t as the e f f e c t i v e area i s reduced due t o v ignet t ing, the image q u a l i t y i s independent o f o f f -ax i s pos i t i on f o r t h i s type o f geometry. I n Figure 1 we have p l o t t e d the ne t geometric area We We have used the 150 f t long X-ray c a l i b r a t i o n f a c i l i t y a t Goddard t o c a l i b r a t e a telescope segment (quadrant) equipped w i t h only the inner most 50 r e f l e c t o r s due t o beam s ize ? im i ta t i ons . This segment i s shown i n Figure 2. The beam divergence r e s u l t s i n considerable v igne t t i ng which was taken i n t o considerat ion when computing the r e f l e c t i o n e f f i c i ency . Furthermore, i t increases the grazing angies a t the f r o n t m i r r o r by about 30% which would tend t o reduce the measured response a t h igh energies. t h i s e f fec t . complement o f the BBXRT c i r r o r ( n e t geometric area 220 c d ) using bremsstrahlung o f f a be ry l l i um ta rge t i n te rcep t ing a 20 keV e lec t ron beam as we l l as cha rac te r i s t i c l i n e s from the various other ta rge ts l i s t e d on the f igure. 6 arc minutes i n radius. Our measurements d i d not extend below * 1.2 keV because o f a bery l1 ium detector cover. are i n good agreement w i t h those compiled by Seward (1977) f o r gold-coated mechanical l y pol ished mir rors . Assuming t h a t t h i s agrement holds for the e n t i r e range o f grazing angles o f the f u l l telescope complement we have estimated the combined e f f e c t i v e area o f the 2 BBXRT telescopes as shown i r Figure 4. Tor a rough comparison, we have included i n the f i g u r e the measured response o f the E ins te in s o l i d s ta te spectrometer. We have no t corrected f o r Figure 3 shows the measured response f o r the 50 r e f l e c t o r We used a t the focus a cooled S i ( L i ) detector Ref lect ion e f f i c i e n c i e s i n f e r r e d from these r e s u l t s Wz have no t as y e t been able t o f u l l y evaluate the imaging po ten t i a l o f t h i s telescope. Ray t rac ing ind ica tes t h a t the approximate geometry y i e l d s images w i t h < 0.5 arc minutes h a l f power rad ius (HPR). Aside from the 445 geometry however, the image qua l i t y w i l l depend on other factors as w e l l sucn as distort ions o f the f o i l re f lectors whether inherent i n the f o i l o r introduced i n the assembly process and, possibly, surface roughness. Prel iminary resul ts from our cal i b ra t i on o f the prototype telescope segment indicate an image w i t h * 3 aic minutes HPR independent o f energy. independence n i l es out surface roughness as the major cause o f image broadening. Other evidence seem t o indicate t h a t errors i n the r e f l e c t o r assembly may be mostly responsible for the observed degraded image. A goal o f * 2 arc minutes HPR, consistent with the s i t e o f the BBXRT detectors, scloul d be achievable. This energy Coming next t o our second objective o f recovering some o f the spat ia l information while s t i l l maintaining the S i ( L i ) energy resolut ion, we have developed a 5-element SC ( L i 1 detector spec i f ica l ly f o r t h i s appl icat ion by segmenting a s ingle s i l i c o n crysta l as shown i n Figure 5. BBXRT mirror, the central element extends t o 3.7 arc minutes radius whereas the outer pixel perimeter i s a t 8.6 arc minutes. A t the focus o f the The pa r t i t i on ing grooves are weak S. 0.5 arc minutes wide. This type o f detector allows the study o f isolated point sources while simultaneously monitoricg backgrouna i n adjacent pixe For dif fuse sources with several arc minutes extant i t increases the efficiency f o r generating rough spectral maps. I n preliminary tests we f ident ical performance f o r a l l p ixe ls wi th an energy resolut ion comparable that of the Einstein instrument. nd t o Our t h i r d objective stems from the f a c t t ha t v i r t u a l l y notCling i s known about the X-ray spectra o f sources a t f' leve ls ergs ~ n - ~ s ' l . The sample includes the majori ty o f the n w sources discovered w i th the imaging detectors aboard the Einstein observatory from stars t o d is tant c lusters o f galaxies and quasars. What prevented the S i ( L i ) spectrometer aboard Einstein - 446 from supplying some o f t h i s information was a high background ra te t o t a l l y unrelated t o the telescope throughput. Background reduction schemes relevant t o t h i s instrument must necessarily deal w i th the rad iat ion environment and i t s e f fec ts as contrasted, f o r example, w i th reducing the size o f pixels. d i rec t interact ions i n the detector sensit ive volume, intercat ions near the detector edges resu l t i ng i n p a r t i a l charge col lect ion, e lectronic cross t a l k from large events i n the outer unused detector perimeter, enhanced noise associated w i th electronic base1 ine d is tor t ions caused by large ion iz ing events, etc. The 5-pixel detector offers us the opportunity t o apply pixel- to-pixel anticoincidence which i s a proven technique f o r el iminat ing a large f rac t i on o f the above-mentimed background. incorporated a charged p a r t i c l e guard i n the form o f a CsI(Tr) cup placed aroitnd the detector and i t s cold f inger assembly as shown i n Figure 6. The size o f t h i s cup and, therefore, the cost and complexity associated with i t have been kept t o a min imum by the use o f a cooled S i ( L i ) detector f o r detecting the l i g h t pulses from the CsI . coupling through a p o r t a t the inner surface o f the base of the crystal . The threshold f o r anticoincidence i s set a t a 300 keV, well below the ion izat ion losses o f s ingly chxged minimum ion iz ing part ic les. A1 though t h i s r e l a t i v e l y small c rysta l would not be very e f fec t i ve i n act ive ly guarding the detector from y-rays as well , the 0.5 inch high-Z wal ls form an e f f i c i e n t passive shield up t o y-ray energies o f * 200 keV. Background events may r e s u l t from I n addition, we have The detector views the CsI without We have comleted an i n i t i a l evaluation of the BBXRT detector background using a prototype cryostat assembled i n the manner shown i n Figure 6. This was done i n an enhanced rad iat ion environment from a Co60 source as well as a t ambient ground leve l (no source) condit iors. We d i d confirm that, a t ground 447 level, the dominant background reduction scheme i s the pixel-to-pixel anticoincidence rather than the CsI guard. Figure 7 shows the background reduction effected when the Co60 source was i n the proximity o f the cryostat. Note the r e l a t i v e l y small improvement obtained near the threshold energy o f 0.5 keV as compared wi th the substantial gains a t higher energies. Ground level detector performance i n the absence o f sources i s s ign i f i can t i n that i t i s of ten a r e l i a b l e measure o f the performance t o be expected i n space: our group displayed i n space a background ra te t y p i c a l l y w i th in a factor o f - 2 o f t h e i r ground level background. observed BBXRT ground level background along w i th a measure o f the gain real ized i n a d i r e c t comparison t o s o l i d s ta te spectrometer aboard Einstein. Again we point out the r e l a t i v e l y modest improvement a t the lower energies i n rough agreement w i th the Co60 results. i n the few keV range, gas proportional counters flown by I n Table 2 we have tabulated the I n summary, we have demonstrated tha t the background reduction techniques we have incorporated r e s u l t i n substantia? gains especial ly a t energies > 1 keV. a t the very l o w energies and these have t o be deal t separately. It i s apparent that some addit ional e f fec ts dominate the background ra te 111. OBSERVATIONS FROM THE SPACE SHUTTLE BBXRT i s wel l suited t o carry ing out observations with typ ica l exposures o f a few x l o3 s as would be the case wi th a regard t o extragi i lact ic objects we may single out quasars and c lusters o f galaxies as the two classes o f sources where substantial gains could be made with t h i s instrument. involv ing a quasar wi th a spectrum as i n 3C273 (Worrall e t a l . 1979) a t a f l u x 7 day shut t le mission. With I n Table 3 we indicate what could be a typ ica l example l eve l o f not general ly come i n t o play f o r a t l e a s t one order o f magnitude below t h i s s e n s i t i v i t y . We have assumed a 2 - fo ld increase i n instrument background over what was measured a t ground leve l . We note t h a t such an observation would be essent ia l l y photon l i m i t e d over most o f the spectral rarige and that a 2000 s exposure would be q u i t e adequate f o r deducing the shape o f the spectrum f o r such an object. ergs ~ r n - ~ s-'. Fcr our f i e l d o f view, source confusion would S imi la r arguments apply t o the s e n s i t i v i t y f o r measuring the spectral shape and, therefore, the temperature o f d i s t a n t c lus te rs o f galaxies. other hand, the poss ib le detection of Fe l i n e s from these sources from which other parameters such as the r e d s h i f t may be i n f e r r e d i s considerably more demanding. BBXRT w i l l have t y p i c a l l y 200 eV FWHM reso lu t i on a t 7 keV. I n a 2000 s ob:ervation the prominant Fe l i n e end from a Perseus-like c l u s t e r a t 2 - 2 w i l l be detected a t the 3a l eve l . t o r the nearby c lus te rs i t w i l l be possible t o make accurate Fe abundance determinations and t o search f o r possible temperature and Fe abundance grandients w i t h distance from the c l u s t e r center. For the Virgo c l u s t e r we w i l l be studying the emission associated w i th ind iv idua l galaxies such as M86. An observational program w i l l , o f course, inc lude other classes o f ext ragalact ic sources as well. The two-component spectra o f BL Lac objects and the l a rge column densi t ies and i r o n absorption edges associated w i t h some of the Seyferts are spectral charac ter is t i cs t h a t can best be studied w i t h t h i s broad-band instrument. On the Po ten t i a l l y i n te res t i ng observations o f ga iac t ic sources w i l l include the temperature c l a s s i f i c a t i o n of the various types o f stars. equivalent widths should help us determine the nature o f the high energy component associated w i th some systems such as the RS CVn b inar ies (Swank e t a l . 1981). I ron l i n e I r o n l i n e emissfon and absorption features invar iab ly appear i n 449 the spectra o f the more luminous binaries and X-ray p&lsar; (Prsvdo 1978). The type o f questions tha t could be answered w i th t h i s instrument deal wi th 1 ine broadening, mu1 t i p l e components, Doppler sh i f ts , fluorescence, phase dependence, etc. instrument are r i c h i n l i n e features (Becker e t a l . 1980). The addi t ion o f the higher energy 1 i nes and continuum should he1 p separate non-equil i b r i um e f fec ts from bonafide abundance anomalies. Spectra o f supernova remnants obtained w i th the Einstein I V. FUTURE PROSPECTS We bel ieve tha t a l l aspects o f the experiment we have described may wel l have a bearing on a fu ture Explorer mission. We have demonstrated the f e a s i b i l i t y o f segmenting a large S ( L i ) detector i n t o smaller elements f o r focal plane use. We have also out1 ned techniques f o r suppressing the background f o r such an ar ray i n order t o make i t e f fec t i ve i n the study o f weak sources. rad ica l ly new way o f rea l i z i ng a broad-band, large throughput instrument a t moderate angular resolution. Key elements are a simple r e f l e c t o r surface preparation and an approximate telescope geometry which great ly simp1 i f i e s re f l ec to r preparation and assembly. The fabr icat ion o f such a telescope i s actual ly not too d i f f e r e n t from what i s involved i n the assembly o f a large area gas proportional counter. To the extent t ha t large throughput a t l o w cost i s a necessary ingredient o f a fu ture Explorer !nission, we bel ieve tha t our approach deserves seriou: consideration. For example we can achieve l o 4 cn? o f e f fec t i ve area i n 16 modules inside a 2 M envelope. The corresponding area a t 7 keV would be 1600 c d . However we view as the s ingle most important contr ibut ion the The BBXRT spat ia l r e s o l u t i m i s probably inadequate for an Ehplorer 450 instrument although i t suff ices f o r the instrument we are developing. We wish t o enphasize t h a t the cause o f the broader image does not l i e i n the approximate geometry. re f lectors we are using we could eventually improve on our image qua l i t y through bet ter r e f l e c t o r preparation and assembly. matter o f seeking a be t te r balance between throughput and r e f l e c t o r thickness. The potent ia l f o r 0.5 arc minute HPR image i s simply there. It i s qui te conceivable t h a t even w i th the t h i n f o i l I f not, i t would be a I wish t o acknowledge major contr ibutions t o t h i s pro ject by C. Glasser, F. Birsa and 0. Arbogast. 45 1 REFERENCES Becker, R.H., Hol t, S.S., Smith, B.W., White, N.E., Bo1 dt, E.A., Mushotzky , R.F., and Serlemitsos, P.J. 1980, Ap. 3 . (Le t te rs ) - 235, L5. Joyce, R.N., Becker, R.H., Birsa, F.B., Holt, S.S., and Noordzy, M.P. 1978, IEEE Trans. Nuc. Sci 25, 453. - Pravdo, S.H. 1978, Proceedings o f the X X I COSPAR/IAU Symposium i n X-Ray Astronomy, Pergamon Press, Ltd., Oxford. Seward, F.D. 1977, Lawrence Livermore Lab., UCID-17505. Swank, J.H., White, N.E., Holt, S.S., and Becker, R.H. 1981, Ap. J. - 246, 208. Worrall, D.M., Mushotzky, R.F., Boldt, E.A., Holt, S.S., and Serlemimtsos, P.J. 1979, Ap. J. 232, 683. - 452 TABLE 1 Max (Min) Dia. Focal Length Focal Plane Scale Spat ia l Resolut ion I n tri ns i c Goal M i r r o r Mater ia l Number o f Nested H i r r o r s BBXXT TELESCOPE PARAMETERS 43 (17.8) c d 3.8 M 0.91 arc min/mm Grazing Angles Aperture U t i 1 i z a t i o n < 0.5 arc min HPR 2 arc min HPR .005 i nch A1 f o i l (Gold) 101 (4- inch cones) 0.34 - 0.75 deg 0.6 Weight 10 kg 453 E(keV) .5 - 1.0 1.0 - 1.5 1.5 - 2.0 2.0 - 3.0 3.0 - 11.0 TABLE 2 BBXRT PROTOTYPE GROUND LEVEL DETECTOR BACKGROUND 3C273* dF!/dE = .0ZZE'1*55 N" < 2.2 x 1021 cm-2 COUNTS s-l keVol GAIN OVER SSS 2.3 x 9 4.1 .x 10-3 20 1.8 x 5.4 10-4 2.4 10-4 33 89 -- TABLE 3 QUASAR SPECTRP; 2000 s EXPOSURE EXPECTEU COUNTS ENERGY S I N S (keV) 1-2( 1000 CII?) 3-4(450) 6-7 (200 ) > 19700 2820 484 Same spectrum renormal i zed t o > 180 ergs/cm2s (2-10 keV) D i f f u s e Background 17.4 Detector Background 24 25 4.4 2.4 0.4 2 2 * Worra l l e t a t . 1979 I." 454 ORIGINAL PAGE IS OF POOR QUALm cu z 0 700 600 500 400 300 200 100 0 0 BBXRT NET GEOMETRIC A R E A VS. OFF - A X I S BEAM DIRECTION MAX. GRAZING A N G L E S ARE .41° RESPECTIVELY 25 REFLECTORS 7 .75O 1 . 5 8 REFLECTORS 50 REFLECTORS , 5 IO A R C MINUTE Figure 1 456 200 100 n 50 Q:=E u U 20 IO I I I I 1- BBXRT NET A R E A PER TELESCOPE (50 REFLECTVRS) - B e TARGET t A i , K , T i , C r , F e , N i , C u D io-- - (I ORIGINAL PAGE IS OF POOR QUALITY t t t + + t t t + + I 2 3 4 5 6 7 8 9 IO I I I I 0 60 I20 180 240 CHAN. NO. Figure 3 1000 800 600 400 200 100 80 60 40 20 0 ORIGINAL PAS': Ei OF POOR QUALln I I I I 1 I I EFFECTIVE BBXRT AREA 2 TELESCOPES \ (ESTIMATE FOR 1 L 4 6 8 10 E ( k e V ) 457 Figure 4 458 ORlGlNAL SA@ IS OF POOR QUALIN Figure 5 459 e 460 100 50 20 R I O 5 2 - B B X R T PROTOTYPE - Co6* INDUCED BACK- GROUND - R = - - - - RAW CTS NET ZTS - 0 2 4 6 8 IO E ( k e V ) Figure 7 SOHE OBSERVATIONAL TESTS OF X-RAY PULSAR EMISSION MODELS’ NASAIGoddard Space F l ight Center Laboratory for High Energy Astrophysics Code 665 Greenbelt, Mary1 and 20771 *h iso a t the University o f Maryland. On leave of absence from Max-P1 anck-Insti t u t fu r Physik und Astrophysik MPA, Garching. ‘Paper presented a t the NASAIGSFC Symposium on X-ray Astronsnly, S.S. Holt , ed., Greenbelt, MD, 5-7 October 1981. 462 Abstract Our understanding of neutron s ta r physics r e l i e s heavily on knowing values o f t h e i r mass, radius and magnetic f i e l d strength, and our only information about these canes irom the surface radfation. There are however s t i l l major uncertainties about haw t h i s arises, concerning the nature o f the mass exchange and the accretion flcrs, the magnetoiialrse structure, the i n f a l l deceleration, the actual pulsation mechanism and the atmosphere geometry. We discuss the alternatives, and point out several possible observatianal tests. ORIGINAL PAGE 1s OF POOR QUALITY I . Some Outstanding Cruc ia l Ambiguit ies Out of the many unsolved questions about X-ray pulsars, there art? three, connected w i t h the actual emission mechanism, which are p a r t i c u l a r l y irksome t o the theoret ic ian, bu t which appear t o be w i t h i n reasonable reach of observational so lu t ion. These are a) Is t he mass exchange mechanism due t o Roche lobe overflow, or due t o (X-ray induced or spontaneous) wind t ransfer , and i f the l a t t e r , w i l l a d isk form o r not? ( l , 2, 3 ) . beaming a t the pn la r caps ( 4 ) , or t o occu l ta t i on a t the Al fven surface ( 5 , '1, and if due t o bcth, i n what propor t ions? c) Is the decelerat ion o f the i n i d l l i n g matter b) Is the pu lsa t ion mechanism due t o i n t r i n s i c due t o d c o l l i s i o n l e s s s+cck (7,8), or t o binary coulo?b encounters (','I (besides rad ia t i on pressure cont r ibu t io l rs ) , t h a t i s , does the emi t t i ng po la r cap atmosphere s t i c k ou t s i g n i f i c a n t l y , e r l i t t ' - q both across the f i e l d as we l l as along it, o r i s i t mostly a ; a g it? These questions, i t i s seen, t i e i n t o each other ra ther c lose ly , and the reason why they a r e importan; i s t h a t t h e i r answer can have a s i g n i f i c a n t e f f e c t on the determination of a t l e a s t two c r u c i a l n e u t v ~ m s t a r parameters, namely, the surface g rav i ta t i ona l r e d s h i f t and the mag? - t i c f i e l d strength. F, leads o f cows2 : 3 d i f f e r e n t est imate o f Lqe accre t ion e f f i c i e n c y GM,R,-~C-~, depending on the degree o f beaming, t h a t i s eve i i f we know M, from doppler delay curves and ecl ipses, t h value of R, deduced depends c r u c i a l l y on the pu lsa t i on mechanism. The observed f l t i x a t ear th TI, i i s o f i n t e r e s t a lso f o r our understanding o f the dynamics o f the neutron s t a r i n t e r i o r , v i a i t s e f f e c t on the ca l cu la t i on o f the moment o f i n e r t i a (lo). The magnetic f i e l d strength, f o r i t c par t , i s e i t h e r deduced from i t s e f f e c t s on the r a t e o f change o f the pu lsa t i on periad, -p/p , fo r which the Al fven geometry and t W mass exchange mode i s important, or i s d i r e c t l y measured v i a c;flctron 1 ines, the frequencj o f which i s g r a v i t a t i o n a l l y redsh i f ted and therPf,re a f fec ted by whether t t m e i s a shock which stands of f above R, or not. Spectroscopic 464 observations with a resolut ion €/A€ CI > 100, as wel l as studies of the corre la t fon o f pulse shapes wi th the X-ray luminosity i n d is ferent bands, may resolve these questions. 11. Mass Exchange Mechanism and Disk Question In the X - r a y domain, t h i s bo i l s dawn t o resolv ing the dynamics of the f l o w a t the AlfvEn surface, located t y p i c a l l y a t RA(sphl = (1 for spherical i n f a l l , and a t -16/69 18/69 3x107 (8/10 l2 G) 40/69(R ns /10 cm) 120/69(~~0)- ’3/69(i/1.3 IO gs a n 18 -1) f o r disk accretion, where a i s the v iscos i ty parameter of the disk (a < 1). The Keplerian (- f ree fa1 1) and thermal ve loc i t i es are The Fe l l n e observed I n many X-ray pulsars very probably ar ises from t h i s region (11, 12), and i t could be used t o trace the temperature and f l a w st.rvcture a t the i n te r face between tne inf lawing gas and the magnetic f fe id , somewhat as the galact ic structure i s studied i n rad io astronoqy. I f one could resolve the blue and red r o t a t i o n components o f the Fe l i ne , t h i s would confirm the existence o f a magnetopause. Analysis o f the outer shoulders of the pulse p ro f i l es could reveal whether most o f the emi t t ing matter i s a t a p a r t i c u l a r distance from the axis o f rotat ion, which would argue f o r a disk, o r whether i t i s spread out over a range of ax ia l distances smaller than the maximum, which might i nd i ca te e i the r a very wel l covered Alfven surface, fed from a disk, if the minimum veloc i ty component i s r e l a t i v e l y very strong, o r a more o r less symetrical i n f a l l from a l l direct ions, i f the emission strength a t d i f f e r e n t ve loc i t i es changes only very gradually. The strength and thermal width of the l i n e might also give some clues as t o the width o f the Alfven shel l , providing valuable information about the v iscos i ty a t the f i e l d - p l asma interface, and possible i n s t a b i l i t i e s which might cause penetrat ion o f plasma i n t o the magnetic cavi ty. The presence o f several d iscrete ve loc i ty components might ind icate e i t h e r penetrat ion o f diamagnetic blobs, or strong non-di pol e f i e l d csmponents. the best f o r t h i s , but E/AE LI > 50 would already be extremely useful. I I I . Pul sat1 on Mechani sm Spectroscopic observations w i t h E/AE - > 300 waul d be In order t o t e s t the possible contr ibut ion of the Alfven shel l as an occul t ing scteen, one would wdnt t o know f i r s t the geometry and dynamics o f the fiagnetopause f low, as described i n 11 This should be complemented by long term studies o f the corre1at:on o f the X-ray lum(nos4ty w i th pulse shape i n a t l e a s t four bands, - 2-6 , ET - 8-12 , ET' - 15-25 keV E, 5 0.5 , €ptl (4 i i Here E, i s character is t ic o f screen reprocessing, Eph i s the band where photoabsorbtion may dominate; and ET. ET' are bands where scatter ing dominates. A t l e a s t two 3f the l a t t e r are helpful, because the scatter ing opecfty ebove 8 keV 466 should be frequency independent a t the A l f v t h shel l and the pure screen mechanism would predict s imi lar shapes a t dif feFent energies. (The in tens i ty Xu, or ig inat ing a t the surface o f the star, could o f course be dif ferent). If the screen mechanism dominates pulse formation, one would expect that a decreese i n Lx (i.e. M ) would decrease screen opaci t y and theref ore decrease the modul a r i on factor ( pul se throughs should f i l l in) , while wi th increasing L,, the pulse throughs should go down towards zero, i.e. . Screen modulation = increasing function o f (L,) On the other hand i f the i n t r i n s i c beaming dominates pulse formation, very l i t t l e change of the modulation wi th L, i s expected f o r e i ther pencil o r fan beams. For i n t r i n s i c fan beams, a change o f the duty cycle or pulse width i s expected t o be correlated with L, while for i n t r i n s i c pencil beams the pulse m u l t i p l i c i t y i s expected t o vary i n a def in i te fashion with frequency, but not wi th luminosity, as discussed i n the next section. I V . Matter Deceleration a7d Geometry o f the Emission R- For luminosities L, 2 erg s-l rad iat ion pressure plays a major ro le i n the dekeleratian o f the i n f a l l i n g matter a t the polar caps, creating a smoothed-out tadiatiot: shock ( 1 3 ) . about ds 2 0.1 R, t o many times Re, increasin? approximately as ds - Lxm, wi th m - 2/3. The mi t t i n g atmosphere i s the accb et ion funnel below the shock, sticlring out above the s t e l l a r surfice. The i n t r i n s i c beaming w i l l be preferent ia l ly fan type, especially a t the higher L , when the sf :e surface largely exceeds the top surface of the columm. The Tap beam pulse width WI 11 be i n f l b e v o d by the amount o f curvature of the side surface being savled, t h i s increasing w i t b ds and L,. f l a t s,rface emits a narrow in t r ins 'c barn, one can roughly estimate the angular I n t h i s case, the shock standoff distance varies from I f a 467 width o f the actual beam from dS'l - d& where we used a = a curved surface as A#- channel widthlheight - dS3l2 '0 [ (R* + ds)/R*]312 for the width o f a dipole channel and took ds >> R*. For ds 5 R, the dependence i s not as strong, though s t i l l noticeable. Since i n a rad ia t ion shock ds - L,2I3, we expect A+ ( i n t r i n s i c pulse) N L, 113 , (6) f o r L, 2 erg s-'. Thus, i f the luminosity o f the pulsar varies i n time, so should the pulse width. Another quanti ty t h a t may vary wi th L, i s the frequency o f the cyclotron l i n e feature, observed i n some objects, if t h i s arises near the shock. I f the f i e l d i s a dipole one would expect 213 1-3 - (1 + const L, -3 u,, - (R* + dS1 S For luminosit ies L, N < erg s-', rad iat ion pressure loses i t s importance, and one o f the ke j uncer ta in t ie . i n deducing an atmosphere structure i s whether a co l l i s ion less shock occurs, o r whether the i n f a l l i n g protons are decelerated i n the denser pa r t o f the stat ionary atmosphere by binary (coulomb or nuclear) encounters. (if present) exchange length. The co l l i s ion less shock would thus lead (14), by a reasoning s imi la r t o the high L, case, t o I n the absence o f rad iat ion pressure, the co l l i s ion less shock height go as ds - ne" - L,-', being given by the cool ing or the coulomb for L, << 103' erg sol. This d i f f e r s from the high L, case treated before i n equation (6 ) . S i m i l a r l y if the cyclotron feature arises near the shock, one would get 468 -1 )'3 - (1 + const Lx -3 % - (R* + ds) 9 ( 9 ) f o r L, << 1037 erg s-1. If no co l l i s i on less shock occurs (and theoret ica l ly , the physics o f these shocks i s la rge ly unknown, so t h i s i s a d i c t i n c t p c s s i b i l i t y ) the stopping by binary encounters requires yo - 5 - 50 gm/cn?, occurring i n the dense p a r t o f the atmosphere. I n t h i s case the emi t t ing region does not s t i ck out s ign i f i can t l y above the surface, and the beaming i s essent ia l ly i n a penci l pattern. f i e l d B - lo1* gauss point ing perpendicular t o the surface, the strong angular and frequency dependence o f the cross sections leads t o a very d i s t i n c t i v e frequency behaviour o f the pvlse shapes. atmospheres ind icate a rough r a l e o f thumb f o r the pulse m u l t i p l i c i t y i n d i f f e r e n t frequency ranges (q = cylcotron frequency), namely I n a magnetic Detai led calculat ions (15) f o r homogeneous single pulse: IO-1 - < 4% 5 1 -1 t r i p l e pulse: few x 10" 5 w / y I 5 IO doubt e pul se : 2 w/% 5 few x 10- , (10) f o r pencil beam systems i n which the magnetic axis i s not too close t o 90° away from the l i n e o f sight. These ca lcu lat ions also ind icate tha t the spectrum i s harder a t midpulse, which obserbationally has been known t o occur i n some objects. Unl ike i n the shock cases, very detai led theoret ical pl;lse p ro f i les have been calculated for the penci l beam models (15), which would m i s e from binary p a r t i c l e deceleration. A refinement i n progess now (16) consists i n ca lcu lat ing the temperature and density p r o f i l e o f the cciul omb decelerating atmosphere, which should provide more rea? i s t i c spectra and pulse shapes t o compare wi th observations. I 46 9 References 1. Rappaport, S., Joss, P. C., 1977, Nature, - 266, 683. 2. Ghosh, P., Lamb, F. K., 1979, Ap. J. - 234, 296. 3. Kolykhalov, P. I., Sunyaev, R. A., 1979, Sov. A. 3. Le t t . - 5, 180. 4. Gnedin, Yu. N., Sunyaev, R. A., 1974, Astron. Astrophys. - 36, 379. 5. McCray, R., Lamb, F. K., 1976, Ap. J. - 204, L115. 6. Basko, M. M., Sunyaev, R. A., 1976, Sov. A. J. - 20, 53;. 7. Zeldovich, Ya. B., Shakura, N. I . , 1969, Sov. A. J. - 13, 175. 8. Langer, S., Rappaport, S., 1981, Ap. J. i n press. 9. Basko, M. M., Sunyaev, R. A. 1975, Astron. Astrophys. - 42, 311. 10. Pines, 3 . - 1979, i n Procs. Wkshp. on Compact Ga lac t ic X-Ray Sourccs, F. Lamb and D. Pines, eds., U. I l l i n o i s , Urbana. 11. ROSS, R., Weaver, R., McCray, R., 1978, Ap. J. - 219, 297. 12. Basko, M. M., 1980, Astron. Astrdphys - 87, 330. 13. Basko, M. M., Sunyaev, R. A. 1976, M.N.R.A.S. - 175, 395. 14. Meszaros, P., 1981, i n Procs. Neutron Star Symposium, 5 th European Physic81 Society GeneTal Meeting, Is tanbul , Turkey. 15: Meszaros, P., Bonazrola, S., 1981, Ap. 3. i n press. 16. Harding, A. K., Mesrdros, P., ). irk, 3 . G., Galloway, D., 1981, i n preparation. ~ " - " . 17 PRECEDING PAGE BLANK NOT FICMC?) OBSERVABILITY OF ATOMIC LINE FEATURES I N STRONG MAGNET I c FI ELDS G , WUNNER, H, RUDER, All'? H , HEROLD Institut f5r Theoretische Physik, Universitat Erlangen-Niirnberg West Germany J , TROMPER Max-Planck-Institut fiir Physik und Astrophysik, Institut filr Extraterrestrische Physik, Garching, West-Germany ABSTRACT As an application of our comprehensive investigaticis of the physical properties of atoms in superstrong magnetic fields B % 10l3 - 10 l3 G, characteristic of ne'itron stars, we discuss the possibility of detecting magnetically strongly shifted atomic lines in the spectra of magnetized X-ray pulsars. Careful esti- mates r f the relevant parameters lead us to the conclusion that it would be profitable to look for magnetically strongly shifted Fe XXVI Lyman lines in rotating neutron stars of not too high lu- minosity using spectrometers working in the energy range 10 - 20 keV, with sensitivities 2 tion E:AE 1, 10-100. photons per cm2 and second, and resolu- The observation of cyclotron ?eatures in the X-ray spectra of accreting neutron stars has confirmed the existence of superstrong magnetic fields of the order of B % 10l1 - 10 1977). The discovery of further lines in the spectra of atomic origin for :xample, would provide an independent check on both the strengths of the magnetic fields and the assumptions of the physi- cal conditions prevailing there. From the source temperatures kT % 'Research supported in part by the Deutsche Forschungsgemeinschaft (DFG) . 13 C (cf. TrUmper et a]. 472 10 - 30 keV, derived from the observed spectra, it follows that the emitting matter is almost completely ionized, with a small fraction of hydrogen-like ions. The cosmic abundance of elements then implies that the lines with the largest possible energies and the highest intensities are members of the Lyman-series of 20 Fig.1. Level scheme for Fe X X V I and the photon spectrum resulting from the drawn transitions for B = 5*10L2G in comparison with B = 0 (dashed lines). The states are labelled by the usual field-free quantum numbers N,l,m, and by the quantum numbers n,m,U of the adiabatic approximation. The intensities have been obtained assuming an average occupation number of one electron in the excited state. The onset of the continua is marked by hatching. I 473 j FE xxvl 1 3Po-lSo 0 0 3 ~ 2 2Po-1so 0 0 1 ~ 3 2P-l-lSO O-%OOo 4 2so --2P-l 002-0-(0 5 361-2P-l o-n*o-10 62SO-2PO 002-001 1s 20 fiw in keV 2s i t I I t 1012 . L I t I - 30 Fig.2. Energies and intensities of Fe XXVI lines as continuous functions of B in the range 1 0 1 1 - 1013 G . The dasired lines represent the energetic posi- tions of the lines, the widths of the corresponding corridors are proportional to the logarithms of the intensities in units of lo'* W. B in Gauss 0 10l1 lo1' 5 - 1 0 l 2 5 . 1 0 ~ ~ Energies of y ' s in keV 6.9 7.0 7.9 9.3 15.9 2 1 . 0 39.7 5 2 . 2 Numbers of y ' s in 1015 S - 1 0.28 0 . 3 1 0 . 5 4 0 . 8 2 2 . 5 8 4 . 2 2 1 2 . 5 1 8 . 3 T a b . 1 . Energies and numbers of y-quanta of the transition corresponding to L in the field-free case for Fe XXVI assuming one excited atom on the time average. a 474 Using the scaling laws (Surmelian and O'Connell 1974, Wunner et al. 1981) of the one-electron energies Em, Em(Z,B) = Z2Em(Z=l,B/Z2), (Z nuclear charge, m eigenvalue of the component of the angular momentum parallel to the magnetic field), and of the oscillator strengths, fTT, (Z,B) = fTT, (Z=l,B/Z2', the energy levels, oscillator strengths,transition probabilities, etc. of Fe XXVI in magnetic fields of arbitrary strength can immediatc.ly be derived from our comprehensive calculations (Wunner and Ruder 1980, Wunner et al. 1981) of the continuous B-dependence of these quantities for the H atom. Fig.1 shows the level schemes of Fe XXVI at B = 0 and 5*1012G and the resulting line spectra. The magnetic field dependence of both the energies and intensities of these lines over the whole regime of interesting field strengths can be seen from Fig.2. It is clearly exhibited that from B z 10llG onward for 2 = 26 the maqnetic forces begin to outweigh the Coulomb forces, to the effect thht the structure of the spectrum changes from the field-free form to the magnetic-field-dominated shape. Furthermore one recognizes that La (asymptotically 001 + 000) remains the strongest transition. In Tab.1 we therefore provide, for this tran- sition, the numerical values of the energies, and the numbers of y-quanta emitted per second. The observability of iron lines in the X-ray spectra of accre- ting magnetized neutron stars depends on a number of parameters such as plasma temperature and density, abundance of iron, source dimensions, magnetic field strength, and, of course, source dis- tance. In addition, to be detectable the lines should be suffi- cieiltly narrow (AE/E s 0.1). Doppler broadening should be negli- gible for iron lines at the typical temperatures of these sources (kT % 10 keV), and collision broadening effects can be estimated to be less than 1 keV at the prevailing temperatures and densi- ties. However, a nontrivial conditian is that the Thomson optical depth of the emitting plasma should be T 5 1 , in order to avoid too large line smearing by Comptonization. The appropriate conditions may be found in accreting magnetic neutron stars of low luminosity, such as 413900-40. Here one can expect that the interaction between the infalling gas and the radiation produced via Thomson scattering is small, and the mate- rial remains in free fall down to the stellar surface. I f it is stopped there by Coulomb or nuclear collisions (Basko and Sunyaev 1975, Pavlov and Yakovlev 1976) the heated layer will have a thick- ness of 50 g/cm’, viz., T % 20. However, recent computations by Kirk and Galloway (1981) show that plasma effects will considera- bly reduce the stopping length to values of the order of a few glcm’, which means T % 1 for the heated layer. To arrive at a quantitative estimate, let us consider a radia- ting hot spot of area lolo cm’ and a thickness 2.5 glcm’ sponding to a total number of protons 1.5=10~~), a temperature of 10 K, and B = 5=1012G. Starting from the Saha equation in a mag- netic field (Gnedin et al. 1974), for these conditions the frac- tion of Fe XXVI ions in the first excited m = 0 state is calcula- ted as 1-10-4, which leads, together with the cosmic Fe abundance 3=1O-’, to 4.5=10’5 excited Fe ions on the time average. Using (corre- 8 476 Tab.1 and assuming a s o u r c e d i s t a n c e ->f 1 kpc, w e arrive a t a l i n e f l u x of i r o n La-photons o f 1.0-10-3 cm'2s'1, which s h o u l d be de- tectable w i t h X-ray spectrometers a v a i l a b l e a t p r e s e n t . A c t u a l l y , t h e i r o n l i n e f l u x from t h e h o t p o l a r s p o t may be much h i g h e r t h a n t h e above estimate, i f t h e r a d i a t i n g plasma c o n t a i n s a s u b s t a n t i a l f r a c t i o n o f "stirred up" s u r f a c e material, which s h o u l d c o n s i s t of pu re i r o n . Of c o u r s e t h e d e t a i l e d shape and wid th of t h e l i n e f e a t u r e are s e n s i t i v e l y a f f e c t e d by t h e p h y s i c a l c o n d i t i o n s i n t h e a c c r e t i o n column ( c f . Yahel 1981) . For example it c a n be imagined t h a t due t o re sonance s c a t t e r i n g l i n e t r a p p i n g o c c u r s , and a p o s s i b l e e m i s s i o n f e a t u r e above t h e continuum background is changed i n t o an a b s o r p t i o n f e a t u r e . A t any rate, w e conc lude t h a t it would be p r o f i t a b l e i n f u t u r e X-ray e x p l o r e r m i s s i o n s t o look f o r magnet i - c a l l y s h i f t e d i r o n l i n e f e a t u r e s i n magnet ic n e u t r o n s t a r s , u s i n g s p e c t r o m e t e r s working i n t h e ene rgy r ange 10 - 20 keV w i t h s e n s i - t i v i t i e s o f 10-4cm-2s-1 and energy r e s o l u t i o n s EIAE % 10-100. REFERENCES Basko, M.M., and Sunyaev, R.A. 1975, Astron.Astrophys.42,311 Gnedin, Yu.N., Pavlov, G.G. , and Tsygan, A.I. 1974, SocPhys. JETP 39,20 Kirk, J.G., and Galloway, D.J. 1981, Mon.Not.R.astr.Soc. 195,45P Pavlov, C.G., and Yakovlev, D.G. 1976, Sov.Phys. JETP 43,389 Surmelian, G.L. , and O'Connell, R.F. 1974, Ap.J. 190,741 Trbper, J., Pietsch,W., Reppin, C . , Sacco, B., Kendziorra, E., and Staubert,R. Wunner, G., ar.d Ruder, H . 198cAp.J. 242,928 Wunner, G., Ruder, H . , and Herold, € I . , 1981, Ap.J, 247,374 Yahel, li. 1981, presented at the Sth General Conference of the EPS, Istanbul - - - 1977, Ann. N.Y. Acad. S c i . 302,538 - - ! A NEW METHOD FOR FOCUSLXG AND IMAGING X-RAYS AND --RAYS WITH DIFFRACTION CRYSTALS 1 Robert K. Smither Argonne Na t iona l Laboratory Received ABSTRACT The new method can f o c u s monochromatic r a d i a t i o n from a po in t source o r p a r a l l e l beam down to a p o i n t image. Conversely, r ad ia - t i o n from a point source can be focused i n t o an outgoing p a r a l l e l beam. The focus ing t o and from a p a r a l l e l beam has never been achieved before wi th d i f f r a c t i o n c r y s t a l s . The method makes use of bent d i f f r a c t i o n c r y s t a l s i n which t h e i n t e r c r y s t a l l i n e - p l a n e spacing i s v a r i e d as a f u n c t i o n of p o s i t i o n i n t h e c r y s t a l . The Bragg ang le f o r d i f f r a c t i o n of monochromatic r a d i a t i o n w i l l now vary as a f u n c t i o n of p o s i t i o n i n t h e c r y s t a l and t h i s new degree of freedom is used t o o b t a i n focus ing and imaging of the d i f f r a c t e d beam. This new approach to focus ing and imaging i s a p p l i e d t o t h e design of a l a rge -a rea , h igh r e s o l u t i o n x-ray t e l e s c o p e t h a t is a l s o a high r e s o l u t i o n monochromator w i th the unique f e a t u r e t h a t t he width of t h e energy increment t h a t is d i f f r a c t e d can be c o n t r o l l e d and va r i ed du r ing t h e o p e r a t i o n of t h e instrument . This makes i t p o s s i b l e t o f i r s t scan t h e f i e l d with a wide energy increment ant? ther switch t o a narrow energy increment f o r t h e high r e s o l u t i o n work. The new focus ing s y s t e m can be used with both t r ansmiss ion and r e f l e c t i o n type d i f f r a c t i o n c r y s t a l s and is a p p l i c a b l e t o photons wi th e n e r g i e s from a few keV t o 500 keV. High r e s o l t i o n imaging (few arc s e c ) is p o s s i b l e even a t the h ighe r photon e n e r g i e s . 478 ORlGlMAl PAGE IS OF POOR QUALW 1. INTRODUCTION Dif f rac t ion c r y s t a l s have been used f o r many years t o focus a point o r l i n e source t o a l i n e image (Johnson, 1931) and i n some spec ia l cases t o focus a point source to a point image (Berreman, et al., 1954 and Berreman', 1955). A schematic drawing of the bas ic geometry used I n these instruments is shown i n Figure 1. The source, image, and bent c r y s t a l a l l l i e on a foca l circle whose BENT CRYSTAL Fig. 1. Classic ref lect ion-type focusing c r y s t a l - d i f f r a c t i o n spectrometer. 9 is the Bragg angles , "A" i s the rad ius of the foca l c i r c l e and "2A" is the rad ius of curva ture of the bent-crystal . and t h e source d i s t a n c e , r e spec t ive ly D 1 and D2 are the image d i s t ance diameter i s equal t o the rad ius of curvature (2A) of the bent-crystal . the image d i s t ance i n equal t o D2 the object d i s tance . I f the d i f f r a c t i o n crysal i s bent t o a c y l i n d r i c a l shape the r ad ia t ion w i l l be focused t o a l i n e image. t o t h e d f f f r a c t i o n plane ( the d i f f r a c t i o n plane is the plane containing t h e incoming and outgoing rays) then it is possible t o focus the r ad ia t ion from a point source t o a point image. The f i r s t type of focusing system works q u i t e w e l l i n the l abora to ry when used with reasonable values fo r the diameter of the f o c a l circle. A mult ip le crystal vers ion of t h i s type of instrument was used i n conjunction with the grazing incidence x-ray te lescope i n the Elns t ien Ob- se rva to ry t o do s p e c t r s l ana lys i s of the x-ray image (Canisares, e t al. 1977). D 1 , I f the d i f f r a c t i o n c r y s t a l is a h 0 bent in the d i r e c t i o n perpendicular i : I . . . . 479 ORIGINAL PAGE 1s OF POOR QUALIW This type of instrument c a n not be extended f o r u s e w i t h a point source at i n f i n i t e because as D2, t h e sou rce d i s t a n c e i n c r e a s e s the v a l u e of D 1 , t h e image d i s t a n c e inc rease as well and i n t h e l i m i t when D2 + w , D1 a l s o ap- proaches i n f i n i t y and t h e bent c r y s t a l becomes a f l a t c r y s t a l (2A + 0 ~ ) . Thus t h e incoming p a r a l l e l beam is d i f f r a c t e d as a p a r a l l e l outgoing beam with no focusing i n t h e d i f f r a c t i o n p l ane . The use of a t r ansmiss ion type d i f f r a c t i o n spectrometer does not h e l p ( s e e Figure 2 ) . Here the d i f f r a c t e d beam i s always Fig. 2. Transmission-type cr) . a l - d i f f r a c t i o n spec t romete r . "A" i s the r a d i u s of t h e f o c a l c i rc le and "2A" i s the r a d i u s of c u r v e t u r e of the bent crystal defccused and approaches a p a r a l l e l beam d i f f r a c t e d by a f l a t c r y s t a l when t h e source d i s t a n c e approaches i n f i n i t y . I n t h i s mode of o p e r a t i o n t h e d i f f r a c t i o n c r y s t a l can be viewed as a n e x c e l l e n t monochromatic c o l l i m a t o r . Although t h i s type of instrument fa not very u s e f u l f o r imaging i t could be used t o make a s p e c t r a a n a l y s i s of a l a r g e g a s cloud or o t h e r extended s o u r c e s . wavelengths would be focused a t d i f f e r e n t p l aces on t h e f o c a l c i r c l e so t h e f u l l spectrum could be recorded a t t he same time with a p o s i t i o n s e n s i t i v e d e t e c t o r The d i f f e r e n t 480 ORIGINAL PAGE OF POOR QUAL^ 11. NEW METHOD OF FOCUSING X-RAYS The new focus ing method Is d i scussed i n d e t a i l i n a r e c e n t ar t ic le i n R.S.I. by R. K. Smither (1982). The b a s i c approach le i l l u s t r a t e d i n Figure 3. Here a f l a t crystal is used t o focus monochromatic r a d i a t i o n from a p o l n t or l i n e source t o a l i n e image by varying t h e c r y s t a l "d" spac ing i n t h e d l f f r a c - t i o n c r y s t a l as 13 f u n c t i o n of t he d i s t a n c e , "x", from t h e b a s e l i n e ( l i n e from source t o image). A thermal g r a d i e n t is used i n t h i s example t o change the "d" spacing so t h a t t he Bragg cond i t ion : is s a t i s f i e d over t h e whole c r y s t a l s u r f a c e f o r monochromatic r a d i a t i o n . I f D 1 = D2 t h i s w i l l a u t o m a t i c a l l y s a t i s f y t h e focusing requirements as w e l l . CRYSTAL WITH THERMAL GRADIENT Fig. 3. New t ransmission type of c r y s t a l d i f f r a c t i o n spectrometer u s ing an unbent c r y s t a l with a thermal g rad ien t appl ied t o t h e c r y s t a l perpendicular t o t h e d i f f r a c t ion planes wi th dec reac ing temper- a tures as x i n c r e a s e s . The image d i s t a n c e , D 1 , e q u a l s the source d i s t a n c e , D i . D 1 must be equal t o D2 fo r t h e Bragg d i f f r a c t i o n c o n d i t i o n t o be s a t i s f i e d The f r a c t i o n a l change i n t h e "d" spacing must match the f r a c t i o n change i n t h e value of s i n 8 . Thus: Ad/d = h s i n B/sIn 8 = A 8 c o t 0 ( 2 ) where e is the Bragg ang le and d t h e "d" spacing i n tile c r y s t a l . I n Eq. ( l ) , x is the wavelength and "n" t h e d i f f r a c t i o n o rde r . 481 The thermal g r a d i e n t needed t o achieve t h i s d i f f r a c t i o n focus ing is given by Eq* ( 3 ) A t 1 Ad -cos2 0 n - . - I - ax Ax ad Ax ( 3 ) where a is the c o e f f i c i e n t o f thermal expansion and Adld = aAt. 0 t h a t can be focused is determined by t h e temperature d i f f e r e n t i a l , A t , that is a p p l i e d t o t h e c r y s t a l . I f t h e d i f f r a c t i o n p rocess uses the p l anes i n q u a r t z t h a t are 9e rpend icu la r t o t h e o p t i c a l ax is , a is 1.34 x 10'5/0C and assuming a A t of 200°C t h i s g i v e s Ad/d I f 0 = 20" then A0 = t h e rocking cu rve o r acceptance ang le of a good c r y s t a l without the presence of a thermal g r a d i e n t . Th i s r e s u l t s i n an improvement of a f a c t o r of 100 f o r t h e d i f f r a c t i o n i n t e n s i t y , while a t t he same time ob ta in ing good convergence of t h e beem. The width of t h e image w i l l be equal t o t h e width of t he source and w i l l not r e f l e c t t h e mosaic s t r u c t u r e of t h e c r y s t a l . l e a d s t o much b e t t e r imaging than one might expect and comes about because most high q u a l i t y crystals are composed of many small c r y s t a l l i t e s t r u c t u r e s whose rocking cu rve o r d i f f r a c t i o n r e s o l u t i o n is much b e t t e r than the width of t h e mosaic s t r u c t u r e which is a measure of the r e l a t i v e misalignment of t h e s e c r y s t a l l i t e s . The change i n 2.7 x r a d i a n s o r 200 arc s e c . Th i s c a n be compared with the two arc sec f o r This l a s t e f f e c t The more g e n e r a l case where D 1 is n o t equal t o D2 is shown i n Figure 4. Both the t ransmission case (Figure 4a) and t h e r e f l e c t i o n case (Figure 4b) l e a d t o focus ing so both l a r g e and small va lues of 6 can be handled with ease. t r ansmiss ion case acts l i k e a c y l i n d r i c a l l e n s ( t h e f i r s t p r a c t i c a l l e n s f o r high energy x-rays) and t h e r e f l e c t i o n case l i k e a c y l i n d r i c a l m i r r o r . Both cwses r e q u i r e t h a t the d i f f r a c t i o n c r y s t a l be ben t . The r a d i u s of c u r v a t u r e , R c , which c o n t r o l s t he rate of change i n the c r y s t a l p l ane tilt ang le , @, is given by Eq. (4) f o r t h e t r ansmiss ion case, The and Eq. ( 5 ) f o r t h e r e f l e c t i o n case where R 1 and R2 are t h e d i s t a n c e from t h e image t o t h e c r y s t a l and from t h e source t o t h e c r y s t a l , r e s p e c t i v e l y , 4 is t h e ang le between t h e c rys ta l p l anes and t h e b a s e l i n e . For a d i s t a n t source Dg and R2 approach i n f i n i t y as be fo re but D 1 and R 1 remain f i n i t e and can be sca l ed up o r down as one wishes so we now have two systems t h a t w i l l f ocus a p a r a l l e l beam of r a d i a t i o n t o a l i n e image. f o c a l d i s t a n c e , D 1 , i n both cases is given by Eq. ( 6 ) The p a r a l l e l beam focusing case is shown i n Figure 5 . The 482 ORIGINAL PAGE IS OF POOR OUAlITY BENT-CRYSTAL (b) Fig. 4. ( a ) Transmission type and (b ) r e f l e c t i o n type d i f f r a c t i o n spectrometer dsing thermal grad ien ts and bent crystals f a r the genera l case when the image d i s t a n c e , D l , I s not equal to the source d i s t a n c e , D2. 4 is the angle of tilt of the c r y s t a l plane, x is the height G f the crystal plane above the base l ine , and R1 and R2 are the d i s t ances from the point of d i f f r a c t i o n t o the image and source, respect ively. 483 ORIGINAL PAQE 18 OF POOR QUALITY (a) COLD/ PARALLEL BEAM Fig . 5 . Special case for paral le l beans (source at i n i f i n i t y ) for the (a) transmission case and (b) ref lect ion case of the n e w focusing method using bent crys ta l s and thermal gradients. Rl, and R2 are the same as in Fig. 4 8 , x, D1, D2, 484 I f a p i p : focus is required as it might i n an israging te lescope then a second c r y s t a l d i f f r a c t i o n element w i l l be required to focus the b e a m i n the plane at r igh t angles to the f i r s t focusinq plane. "his r equ i r e s t ha t the second crystal element be rotated by 90' around t h e axis of the b e a m as is shown i n Figure 6 . POINT FOCUS POINT HOT Fig. 6. Special case of double focusing of parallel bears for the (a) t r ans r i s s ion case and (b) r e f l e c t i o n case using the new bent c r p t a / t h e w a l gradient method 485 A mort elegant but more complicated method can be fount t o obtain double foruelng w i t h the r e f l ec t ion type d i f f r a c t i o n crystal (Berromen, 1953). Thf8 approach I s shown in Pigure 7 where the d i f f r a c t i o n c rys t a l ia also bent in (I accond dfrestion, i n t h e plane perpendicular to the d i f f r a c t i o n ptenc. E i t h e r type of double focueing opcna up the posslbiLi ty of doing pod i r~rg ing wlth a apace releecope that u8cs some Port of posi t ion s e n s i t i v e foca l p l a in de tec tor . UNBENT CRYSTAL CY L INOR I CAL SHAPE BENT CRYSTAL Pig. 7. Double fWUSing With a SfWlC Cryat81 ,lernent Chat b8 k e n npecia l ly ground and shlrped before bending and the appl ica t ion of a thermal uradient In the psrallcl beam focusing case D2 and R2 approach infinity. This s impl i f i e s the axprcsaiono for I€,., A t / A L or Ad/Al where A L ie the d is tance along the surface of the c r y s t a l i n the d i f f r a c t i o n plane . tranomieeton case l a given by Eq. ( 7 ) R, in the 436 and Eq. ( 8 ) i n t h e r e f l e c t i o n case Also = given by 8 and t h e f r a c t i o n a l change needed Eq. ( 9 ) f o r t h e t ransmiss ion case, Ad cos2 0 D -- - d 2R1 s i n 8 and f o r t h e r e f l a c t i o n case, Eq. (10) Ad cos 0 E- - d 2% i n A I A I The corresponding thermal g r a d i e n t needed ( 8 ) the c r y s t a l spac ing Ad/d is (9) f o r t h e t ransmiss ion I s A t cos2 8 cos 8 s i n 2 8 A I 2aR1 s i n 8 ax 2 s i n 8 E E-* - and i n t h e r e f l e c t i o n case A t cos e cos e s i n 2 0 A l l 2aR1 ax 2 - a- --.- For small va lues of 0 the thermal grad ien t approaches a cons tan t (l/ax) f o r the t ransmission case. I n t h e r e f l e c t i o n case the thermal g r a d i e n t goes to zera a s 8 approaches z e r o or 90". Note t h a t t h e "d" spacing of the crystal does not e n t e r i n t o t h e s e equat ions so t h e y are q u i t e genera l . Also, t h e minimum value R 1 or x t h a t c a i be used is set by t h e maximum value al lowable f o r A t / A l where A t is the l e n g t h along t h e c r y s t a l s u r f a c e i n t h e d i f f r a c t i o n plane. Thus the minimum s i z e of the spectrometer is set by t h e maximum per- m i s s i b l e va lue of the thermal g r a d i e n t , A t l A l l . (Quartz) and A t / A I 200°C/cm, Xmin = 329.5 cm i n t h e t ransmission caRe and xmin - 112.7 c m i n t h e r e f l e c t i o n case. For a A t / A l of lOO"C/cm t h e y w i l l be twice as big, etc. This means t h a t a spectrometer t h a t relys on a thermal g r a d i e n t to change the "d" spacing w i l l be l a r g e , e s p e c i a l l y for small va lues For 0 = 20" and a - 1.34 x 10-5 of e. 487 111. DIFFERENT METHODS FOR CHANGING THE CRYSTAL PLANE SPACING The above d i s c u s s i o n has used a thermal g r a d i e n t t o v a r y t h e "d" spacing i n t h e d i f f r a c t i o n crystals. The range of Bragg a n g l e s o v e r which the crystal w i l l focus monochromatic r a d i a t i o n is l i m i t e d by how large a temperature d i f - ferenc, can be app l i ed t o t h e crystal . In p r i n c i p l e one could a p p l y a A t of 1000°C t o a q u a r t z crystal which would correspond t o a Ad/d - 1.34 x 10-2 or 1.34%. I n p r a c t i c e a A t o f 30OOC (-100°C to +2OO0C) is a l l one would l i k e to cons ide r . This would g ive a Adld - 4 x or 0.4%. A s d i s c u s s e d i n t h e p rev ios s e c t i o n , t h e minimum va lue of x , t h e d i s t a n c e of t h e crystal from t h e a x i s or b a s e l i n e is determined by t' maximum va lue a t t a i n a b l e for A t / A L [ s e e Eq. (11) and ( 1 2 ) ) and sets a minimum s i z e f o r t h e spectrometer . An a l t e r n a t e method f o r changing t h e "d" spac ing i s t o grow a crystal composed of two types of atomic l i k e S i and B or S i and G e (Pearson, 1967), (Pea r son , 1972). and (Smi the r , 1982) and change the r e l a t i v e f r a c t i o n of each type of atom as a f u n c t i o n of p o s i t i o n i n t h e crystal . The l a t t i c e spac ing i n a Si-& mixed- element c r y s t a l can be changed from a value of d = 5.434 f o r pure S i t o a v a l u e of d - 5.657 f o r pure G e . Th i s change i n "d" of 4.1% corresponds t o a A t = 104"C f o r t he thermal g r a d i e n t method f o r va ry ing "d". advantage of g i v i n g l a r g e v a l u e s of Ad/d and A9 and a t the same time not re- q u i r i n g t h e power needed t o ma in ta in t h e thermal g r a d i e n t s needed i n t h e examples c i t e d , p rev ious ly . The main disadvantage of t h i s approach is t h a t i t does not have the a b i l i t y t o change the spac ing g r a d i e n t d u r i n g t h e experiment and a d j u s t for t h e focus ing of d i f f e r e n t wave l e n g t h s or t n e a b i l i t y t o scan over a l i n e i n t h e spectrum by j u s t changing t h e average temperature of t h e d i f f r a c t i o n crystal . I f t h e two approaches are combined, both the l a r g e A 9 and t h e v a r i a b l e Ad/AL can be r e t a i n e d . The use of a t least some thermal gra- d i e n t is a l s o important because i t r e t a i n s the a b i l i t y to c o n t r o l and change t h e increment of wavelength, A X , t h a t is focused a t one p o s i t i o n on t h e f o c a l plane. The n a r r o w s t va lue of A X is ob ta ined when t h e Ad/AL is matched t o t h e changing Bragg a n g l e needed f o r a p e r f e c t focus. I f t he system is detuned by us ing a n i n c o r r e c t va lue f o r t h e AdlAL then a range of wavelengths w i l l be focused a t the image s p o t . Th i s w i l l a l l o w a s e a r c h mode t o be used i n t h e instrument i f t he energy of t he l i n e is no t w e l l known. Af t e r t h e l i n e is loca ted then the v a l u e of AA can be ad jus t ed t o g i v e the b e s t s e n s i t i v i t y and lo r r e s o l u t i o n . Th i s approach has t h e I V . APPLICATION TO A X-RAY TELESCOPE A number of t h e s p e c i a l f e a t u r e s of t he new method, focus ing monochro- mat ic r a d i a t i o n , imaging, c o n t r o l of A X , l i n e scanning, e tc . , could be q u i t e u s e f u l i n the d e s i g n of a n x-ray t e l e s c o p e . Although the method could be a p p l i e d t o t h e low energy x-ray r eg ion ( E r < 5 keV) the c o n s i d e r a b l e succes8 of the g raz ing i n c i d e n t t e l e s c o p e l i k e the one flown on t h e E i n s t e i n Obser- v a t o r y i n t h i s energy r eg ion make it more i n t e r e s t i n g t o look a t a t e l e scope designed t o look a t r a d i a t i o n i n t h e 10 keV t o 200 keV range and t h e poss- i b i l i t y of ex tend ing i t t o t h e 200-600 keV range. The f l u x e s a t these h ighe r e n e r g l e s are q u i t e low so t h e c o l l e c t i o n area must be l a r g e , t h e o rde r of square meters. Th i s means t h a t a l a r g e number of c r y s t a l e lements w i l l be needed t h a t a l l c o n t r i b u t e t o t h e same f o c a l s p o t . Figure 7 is a schematic 488 ORIGWIL PAGE IS Of POOR QUALITY drawing o f a large area t e l e scope where t h e i n d i v i d u a l d i f f r a c t i o n c r y s t a l s are l o c a t e d on a l a r g e r i n g s t r u c t u r e . (Rad ia l d i r e c t i o n ) and 0.5 cm wide mounted on a r i n g 20 m i n diameter and assuming a 20% l o s s i n area f o r t h e support s t r u c t u r e then t h i s one r i n g w i l l have an en t r ance a p e r a t u r e f o r t h e t e l e scope of 3.0 x lo4 x c o s 8 (an2) f o r t h e t ransmission case and 3.0 x lo4 x s i n 8 ( c m 2 ) f o r t h e r e f l e c t i o n case. For small va lues o f 8 the t ransmission case or l a r g e values of e i n t h e re- f l e c t i o n case the en t r ance a p e r a t u r e would be about 2.5 t o 2.2 m2 and never need be lower than 2.1 m2. Assuming c r y s t a l s of 6 cm l e n g t h I f 5 such c r y s t a l r i n g s are used then t h e en t r ance CIRCULAR SLIT - SYSTE DETECTOR - I- OF CRYSTALS Fig. 8. Schematic drawing of a space t e l e scope wi th a r i n g of d i f f r a c t i o n c r y s t a l s focusing a p a r a l l e l beam i n t o a c i r c u l a r s l i t . D e t a i l s of t he c i r c u l a r s l i t and d e t e c t o r are shown i n t h e upper l e f t a p e r a t u r e w i l l i n c r e a s e t o 10.5 to 15 squa re meters or d o 5 cm2. high q u a l i t y c r y s t a l s then the throuah put of t h i s system can be quite large due t o t h e high r e f l e c t i v i t y of t h e d i f f r a c t i o n p rocess . I f one u s e s t r ans - mission c rys ta l s it is p o s s i b l e t o select a th i ckness f o r t h e c r y s t a l s t h a t I f one u s e s I 489 maximizes the product o f r e f l e c t i o n of the c r y s t a l times the t r ansmiss ion through the c r y s t a l so t h a t 30 t o 40% of the i n c i d e n t monochromatic r a d i a t i o n is focused on t h e f o c a l spo t f o r any s e l e c t e d x-ray energy from 20 keV t o 200 keV. A t 500 keV t h e through put drops t o 13%. Table I g i v e s r e f l e c - t i v i t y and t r ansmiss ion c o e f f i c i e n t s f o r optimum th i ckness Quar t z c r y s t a l based on measurements made on high q u a l i t y qua r t z c r y s t a l s a t ANL. I n p r i n c i p l e , t h i s Table 1. R e f l e c t i v i t y and t r ansmiss ion c o e f f i c i e n t s f o r h ighe r q u a l i t y qua r t z c r y s t a l p l a t e s when used as t ransmission type d i f f r a c t i o n c r y s t a l s . The th i ckness of each p l a t e has been optimized a t each t ransmission photon energy ( ) f o r t h e h i g h e s t product of t he r e f l e c t i v i t y x EY Thickness ( O p t . ) R e f l e c t i v i t y Transmission Ref. x Trans. (keV) 10. 20. 50. 100. 250. 500 1000. (mm) 0.04 0.22 1.45 4.50 20.0 30.0 50.0 48.2 48.5 46.6 42.7 25.7 13 .O -~ (I&) .82 5a .872 .897 .829 -670 .499 .430 ( X I 37.6 42 .O 43.5 38.6 28.6 12.8 5.6 aValues e x t r a p o l a t e d from d a t a a t higher e n e r g i e s . approach could be extended down t o 5 keV but the optimuu t h i c k n e s s a t these low e n e r g i e s becomes so t h i n t h a t the c r y s t a l s become d i f f i c u l t t o handle and t h e use of r e f l e c t i o n type c r y s t a l s is more p r a c t i c a l . A similar se t of va lues can be obtained f o r t h e r e f l e c t i o n case.Thus the e f f e c t i v e c o l l e c t i o n a rea of the t e l e scope f o r monochromatic r a d i a t i o n can be nade the o rde r of one t h i r d of t h e en t r ance a p e r a t u r e at any x-ray energy below 200 keV by choosing t h e r i g h t c r y s t a l s and c r y s t a l t h i c k n e s s f o r t he t r ansmiss ion case o r by us ing t h e r i g h t c r y s t a l s i n t h e r e f l e c t i o n case fo r x-rays wi th e n e r g i e s below 20 keV. Th i s g i v e s an e f f e c t i v e c o l l e c t i o n a r e a of 1.c) m 2 t o 0 .7 tu2 ( lo4 t o 7 x l o 3 cm2) f o r one r i n g and 5 m2 t o 3.5 m2 f o r t h e 5-ring assemble. e f f e c t i v e areas and assume s i n g l e crystal d i f f r a c t i o n elements. If the cry- s t a l s are bent i n only one d i r e c t i o n a8 i n Figure 5 , t hen a l i n e focus w i l l be obtained and t h e focused r a d i a t i o n from a l a r g e r i n g (F igu re 8 ) would be spread over an image spo t whose d i ame te r is equal t o t h e width of the i n d i v i d u a l d i f - f r a c t i o n crystals . For LL f o c a l d i s t a n c e ( D y ) of 10 m t h i s corresponds t o a spo t whose angular diameter is 100 arc sec. These are l a r g e A 0.5 cm width g i v e s a n area of t h e image spo t = 0.2 cm2. 490 Better a n g u l a r r e s o l u t i o n ( 4 0 arc sec) and e n e r g y r e s o l u t i o n can be o b t a i n e d by f o c u s i n g t h e crystals on a c i r c u l a r s l i t as shown i n t h e uppe r l e f t o f F igu re 8. T h i s approach has t h e d i sadvan tage i n t h a t it w i l l not allow imaging o f ex tended s o u r c e s . I f b e t t e r imaging is d e s i r e d t h a n was o b t a i n a b l e w i t h t h e c y l i n d r i c a l l y ben t c rys t a l s d i s c u s s e d above then a doub ly bent crystal element (see F igure 7) w j l l have t o be used or two s ingly-bent c r y s t a l s (see F igure 6 ) . I f t h e l a te r is used t h e maximum through put w i l l be reduced from (30% t o 40%) to (10% t o 16%). The much b e t t e r f o c u s however w i l l g r e a t l y i n c r e a s e t h e photonslcm2 and t h e s i g n a l t o background ra t io . The s p o t size w i l l now depend on t h e q u a l i t y o f t h e d i f f r a c t i o n crystals used and could have a d i a m e t e r of a few arc sec which c o r r e s p o n d s to an image d i a - meter of 0.2 mm f o r a f o c a l l e n g t h o f 10 m and a n a n g u l a r w id th o f 4 arc sec. The p r i c e t h a t must be p a i d f o r t h i s h igh r e s o l u t i o n is t h a t t h e in s t rumen t w i l l be h i g h l y monochromatic and o n l y a v e r y nar row band of wavelengths w i l l be d i f f r a c t e d a t one t i m e . wid th and a 2.57 f r a c t i o n wid th o f 0.3 ev . or a wid th o f 30 ev. The d o p p l e r broadened l i n e wid th (FWHM) of 6 keV l i n e i n a plasma w i t h T = 107 a would be 2.2 e v and f o r a 60 keV l i n e t h e broad- ened l i n e wid th would be 22 ev. The 60 keV case is match i n AE t t h e d o p p l e r broadening q u i t e w e l l and even i f t h e plasma t empera tu re was 108 P ~ 4 t h t h e i n c r e a s e d dopp le r wid th A% - 70 e v , t h e loss would be less than a f a c t o r of 2. The r e s o l u t i o n , AE, would a l so be t h e r i g h t o r d e r of magnitude to measure t h e t empera tu re of t h e s o u r c e by measur ing t h e l i n e wid th . The 6 keV l i n e Is no t matched v e r y w e l l t o t h e d o p p l e r broadening and i t may h e l p t o d e t u n e t h e d i f f r a c t i o n crystal by changing t h e thermal g r a d i e n t so t h a t t h e A8 is 16 arc sec r a t h e r t h a n 4 arc sec. T h i s would g i v e a AF. f o r t h e d i f f r a c t i o n p r o c z s s of 1.2 e v which would be a better match t o t h e d o p p l e r broadened l i n e . The t o t a l peak c o u n t i n g ra te would however remain t h e same because the g a i n In r e f l e c t i v i t y a t one ene rgy i s ba lanced by t h e loss a t a n o t h e r . The impor tan t f e a t u r e of t h i s type of AE c o n t r o l i s t h a t t h e r e is no loss i n r e s o l u t i o n e i t h e r i n t h e d i f f r a c t i o n phase or i n t h e p l a n e p e r p e n d i c u l a r t o t h e d i f - f r a c t i o n p l a n e . If AE is i n c r e a s e d by u s i n g a crystal w i t h a l a r g e mosaic s t r u c t u r e t h e l o s s o c c u r s I n bo th p l a n e s . I f t h e mosaic s t r u c t u r e of t h e d i f f r a c t i o n c r y s t a l i s i n c r e a s e d from 4 sec t o 16 sec t h e n t h e r e f l e c t i v i t y o f t h e crystal for a s i n g l e wavelength i s d e c r e a s e d by a f a c t o r of 4 as i t was b e f o r e but t h e r e is a l s o a n i n c r e a s e i n t h e image s p o t s i z e i n t h e d i - r e c t i o n p e r p e n d i c u l a r t o t h e d i f f r a c t i o n p l ane . T h i s second expans ion i n t h e p l ane p e r p e n d i c u l a r t o t h e d i f f r a c t i o n p l a n e is o f t e n n e g l e c t e d i n e f f i c i e n c y c a l c u l a t i o n but i t c a n s e r i o u s l y r educc the s i g n a l t o background r a t i o i n a f o c a l p l a n e d e t e c t o r . The expans ion of t h e image i n t h i s s ideways d i r e c t i o n is g i v e n by Eq. (13) The v a l v e of E /AEy - 2 x lo4 a t 6 keV f o r a 4 sec "d" s p a c i n g ( q u a r t z , 116). This c o r r e s p o n d s to a d i f - A t 60 kpV, t h e above example g i v e s E/AE = 2 x lo3 A ~ A * A& t a n 2 0 where A% is t h e wid th o f t h e mosaic s t r u c t u r e . ( E V 6 . 4 keV i n t h e above example) i t becomes e q u a l t o t h e mosaic s t r u c t u r e wid th . In t h e above example t h i s e f f e c t would expand t h e image s i t e f u r t h e r and d e c r e a s e t h e photons per a n 2 i n t h e Image s p o t b y a n a d d i t i o n a l f a c t o r of 4. T h i s a d d i t i o n a l s p r e a d i n g of t h e image w i l l no t o n l y r educe t h e s i g n a l t o backgrond r a t i o and t h u s t h e s e n s i t i v i t y o f t h e i n s t r u - For v a l u e s of 8 0 22' 491 ment but a l s o s e r i o u s l y i n t e r f e r e with the r e s o l u t i o n of two nearby images. This image broadening e f f e c t is d i scussed more throughly i n t h e next s e c t i o n . I V . IMAGING The g r e a t success o f t h e g r a z i n g i n c i d e n t t e l e scope flown on t h e E i n s t i e n Observatory i n g e n e r a t i n g h igh - re so lu t ion , two-dimensional images of extended sources has made everyone aware of t he importance of imaging i n x-ray a s t r o - nomy. I t is, t h e r e f o r e , of interest to see i f t h e high r e s o l u t i o n , monochro- matic, x-ray t e l e s c o p e d i scussed above can be used t o imege extended sources . I n most c a s e s a d i f f r a c t i o n c r y s t a l can be d e s c r i b e d as a c o l l e c t i o n of small nearly-perfect crystall i tes whose rocking cu rves or d i f f r a c t i o n wid ths are much narrower than t h e width of t h e mosaic s t r u c t u r e . I n high q u a l i t y q u a r t z t h e d i f f r a c t i o n width of t hese small crystal-.ites can be the o rde r of 0.1 a r c s e c f o r 50 keV r a d i a t i o n . The mosaic s t r u c t u r e width is thus a measure of t h e misa: gnment of t hese small c rys ta l l i t es and f o r v e r y monochromatic r a d i a t i o n from a d i s t a n t point sou rce o n l y a small f r a c t i o n of t hese crygtall i tes w i l l be a v a i l a b l e for d i f f r a c t i n g t h e incoming beam. The rest w i l l be o r i e n t a t e d a t ang le s t h a t do not s a t i s f y t h e Bragg c o n d i t i o n . I f t he d i f f r a c t i o n width of t he c r y s t a l l i t e s were the o n l y t h i n g c o n t r i b u t i n g t o s i te of t h e image on the f o c a l p l ane and t h e crystal had a n o v e r a l l mosaic s t r u c t u r e (Ah) of 10 a r c sec. , then an image would be formed wi th in t h i s LO a r c s e c range wi th 0.1 arc sec r e s o l u t i o n . Unfo r tuna te ly t h e c r y s t a l l i t e d i f f r a c t i o n width is not t he o n l y c o n t r i b u t o r t o t h e image spo t s i z e . As was mentioned in t h e p rev ious s e c t i o n t h e o v e r a l l mosaic s t r u c t u r e of the c r y s t a l s p r e a d s o u t t he image spo t i n t h e d i r e c t i o n p repend icu la r to t h e d i f f r a c t i o n plane. The amount of spreading is given by Eq. ( 1 3 ) . where b e of 9 t h i s s p r e a d i q w i l l be n o t i c e a b l e , ( e = S o , A 8 = 10 x t a n 10" = 1.8 a r c s e c ) and for 8 - 20" (e - 22.5', A0 - 10 x t a n 45" - 10 arc s e c ) or l a r g e r , t he spread w i l l be equal t o or l a r g e r than the mosaic s t r u c t u r e of t he whole c r y s t a l and make i t more d i f f i c u l t t o r e s o l v e images i n t h i s d i r e c t i o n . Th i s e f f e c t is shown i n Figure 9. The above examples assumed ve ry monochromatic r a d i a t i o n . Most as t ronomical l l n e sou rces have a f i n i t e l i n e width e i t h e r due t o t h e l i f e t i m e of t he x-ray or due t o doppler broadening. I n t h i s c a s e d i f - f e r e n t s u b s e t s of c r y s t a l l i t e s w i l l reflect d i f f e r e n t wavelengths a t s l i g h t l y d i f f e r e n t Bragg a n g l e s . This r e s u l t s i n a sp read ing of t h e image spo t i n t h e d i f f r a c t i o n plane. If t he l i n e is v e r y broad then t h e s p r e a d i r g i n t h i s d i r e c t i o n w i l l approach the mosaic s t r u c t u r e width. The image spo t s i ze f o r a range of va lues for @ is shown i n Figure 10. Off-axis sou rces w i l l a l s o be imaged on the f o c a l plane but w i t h some d i s t o r t i o n . For a s i n g l e element fo- cus ing crystal like t he one shown i n Figure 7 . The f i e l d of view is q u i t e wide i n t h e d i r e c t i o n pe rpend icu la r t o t h e d i f f r a c t i o n p l ane (p l ane con ta in ing t h e s o u r c e , c e n t e r of the c r y s t a l and c e n t e r of a n axis image). For monocro- matic r a d i a t i o n imaging i n t h e d i f f r a c t i o n p l ane is l i m i t e d t o an angular width similar t o t h e mosaic s t r u c t u r e . Images o f sou rces w i t h broad s p e c t r a l l i n e s w i l l be formed i n t h i s o f f - a x i s d i r e c t i o n w i t h i n t h e rang? of a n g l e s t h a t cor- respond t o t h e s p e c t r a l l i n e width. I f the t e l e s c o p e c o n s i s t s o f a l a r g e ring of focusing crystals then Rome p a r t of the r i n g w i l l always be a v a i l a b l e for o f f - a x i s d i f f r a c t i o n so l a r g e angular areas (10 min x 10 min) can be imaged wi th dec reas ing s e n s i t i v i t y as one moves f a r t h e r o f f - ax i s . The d i s t o r t i o n of t h e s e images w i l l not be too g r e a t even i f l a r g e d i ame te r circular rings are used. For t h e case of a 20 m diameter r i n g , 8 - IO", and an o f f - ax i s d i s t a n c e - A% t a n 28. Even f o r small va lues . 492 ORIGINAL PAGE IS Of POOR QUALITY 8- 2 O 5 O 1 oo - 0.14" - H 1 *' H 1 " 0.73" 3.69" I 2 oo L f 1.68" 1 O0 2 oo 1 I 8.36" - Q Fig. 9 . Image spot s i z e as a funct ion of e - 2 " . 5 " , 10". and 20' and mosaic s tructure width A ~ M - 2 sec and 10 sec for a double focusing s i n g l e c r y s t a l system for a point source and mono- chromatic rad ia t ion . The s o l i d f i l l e d area in the 50% height contour while the open l i n e is the 10% height contour -- 8 2 O 1 PMOTON/CM 0.14" I+= 0.36" 1 O0 b,j 0.73" Pig. 10. Image spot s i z e as a function of 9 = 2". 5 " . 10'. and 20' for a s ingle crystal system with a mosaic structure w i d t h A% = 2 sec when imagjng continuum or very broad l ine sources from a distant point nource. The shaded area is the 50% height contour w h i l e the outer ring i s the 10% height contour. of 20 arc sec t h e d i s t o r t i o n of the image, AJ,D, is only 2.5 a r c sec. I n general A$D given by Eq. (14) A$D = JI s i n 28 tan 29 (14) where J, is the off-axis angle of the source. Note tha t the angular d i s t o r t i o n is not a func t ion of the size of the instrument o r the foca l length and the r a t i o of A$Dl$D is only a func t ion of the Brag8 angle 9. The use of a l a r g e c i r c u l a r r ing of c r y s t a l s generates an image spot t h a t is a composite of the super imposed images from each c r y s t a l . This composite image can be generated from the s i n g l e crystal images i n Figures 9 and 10 by r o t a t i n g these images through 360" aroung t h e i r cen ters . The off-axis images are generated i n a similar way except t ha t as the image rotates around i ts cen te r , the center moves around a circle whose angular diameter (A$D) is given by Eq. (14). This d i s t o r t i o n tends t o disappear when the off-axis angle is l a rge because the c r y s t a l elements responsible f o r t he d i s t o r t i o n a r e no longer a t the r igh t angle t o d i f f r a c t the beam. In many of the cases shown i n Figures 9 and 10 the image is much l a r g e r i n one dimension than the o ther and t h i s r o t a t i o n w i l l generate an image with an appreciable l a r g e r area than the s ing le c r y s t a l image. This w i l l reduce the s e n s i t i v e of the sys tem and can considerably degrade the a rgu la r reso lu t ion . This image degradat ion can be p a r t i a l l y eliminated by using narrow pie-shaped s e c t o r s covered with c r y s t a l s r a the r than a continuous r i n g and focus each sec to r on a separa te pos i t i on s e n s i t i v e de t ec to r . This would requi re the cons t ruc t ion of sets of c r y s t a l s with d i f f e r e n t average c r y s t a l spacings but it could improve the imaging a t least in one d i r e c t i o n by a f a c t o r of 5 t o 10 depending on the angular width of the s e c t o r s (12" to 6") . As mentioned above, i f one increases A& i n order t o increase the range of wavelengths focused a spreading of the image i n the d i r e c t i o n perpendicular t o the d i f f r a c t i o n , A ~ A , a l s o occurs. This l o s s in reso lu t ion and s e n s i t i v i t y can be avoided i f one grows a ref lect ion-type c r y s t a l so t h a t the "d" spacing va r i e s with depth in to the c r y s t a l a s w e l l a s with the pos i t i on along the sur- face of the c r y s t a l . This v a r i a t i o n of "d" with d is tance in to the crystal has been used t o increase the i n t r e g a l r e f l e c t i v i t y of a c l a s s i c a l type instrument ( see Figure 1) by varying the amount of Boron added t o a s i l i c o n c r y s t a l g r o w e p i t a x i a l l y on a s i l i c o n c r y s t a l subs t r a t e (Fukuhara and Takano, 1980). The width of the d i f f r a c t i o n peak was increased from a few a r c sec f o r the undcw? Si c r y s t a l t o 80 sec f o r the e p i t a x i a l l ayer with graded Boron concentrat ion t h a t was 28 um th ick . This ccrresponds t o f r a c t i o n a l change Ad/d = -10-3 and an enhancement of the rocking curve f o r the Cu K, x-ray by a f ac to r of 4 when using the 220 d i f f r a c t i o n planes. estimated t o be 1.2 x The sur face concentrat ion of Boron was atoms/m3 (0.24%). V. SUMMARY The above d iscuss ion should not be considered as a formal proposal f o r an x-ray te lescope but r a t h e r an i l l u s t r a t i o n of the p r i n c i p a l s of the new focusing system and how i t might be appl ied t o t h e focueing and imaging of x-rays from a d i s t a n t astronomical source. The real breakthrough f o r t h i s app l i ca t ion is the a b i l i t y t o image p a r a l l e l beams ( d i s t a n t sources) of x-rays and t h e l a rge con- 495 vergency ang le s t h a t allow one t o c o l l e c t high energy x-rays from l a r g e areas without u s ing impossibly long f o c a l l eng ths . The abi1, ty t o change the width of t he wavelength increment, Al, being d i f f r a c t e d without changing t h e image size and t h e a b i l i t y t o s c a n the l i ae i n wavelength without changing a n y of t he p h y s i c a l dimensions of the i n s t r u - ment add t o t h e a t t r a c t i v e n e s s of t h i s approach. The main des ign problems are how t o apply thermal g r a d i e n t s t o t h e bent crystals i n a unlform and con t ro l - l a b l e manner and how t o p o s i t i o n and a l i g n a l a r g e a r r a y o r r i n g of crystals t o o b t a i n t h e good r e s o l u t i o n t h a t is t h e o r e t i c a l l y p red ic t ed . REFERENCES 1. Berreman, D. W., DuMond,J. W. M., and Marmier, P. E . , 1 9 5 4 , R.S. I., - 25, 1219. 2. Berreman, D. W., 1955, R . S . I . , - 26, 1048. 3 . C m i z a r e s , C. R., Cla rk , G. W., Bardar , D . , and Markert , 1977, SPIE, - 106, 154. 4. Fukahara, A. and Takamo, Y., 1980, J. Appl. C r y s t . , - 13, 391. 5 . Johnson, H. H., 1931, 2. Phys., - 6 9 , 185. 6. Pearson, W. B. , 1967, A Handbook of Lattice Spacings and S t r u c t u r e of Metals and Al loys , (Pergamon, New York, Vol. 1,-1958 and Vol. 2 , 1967). 7 . Pearson, W. B., The Crystal Chemistry and Physics of Metals and Alloys, ( U l l e y I n t e r s c i e n c e , New York, 1972). 8. Smither, R . K., 1982, R.S.I., ( t o be publ ished i n the Feb. 1982 i s s u e ) . ORIGINAL PAGE 1s of BOQR QUALm DJ? 497 P P S ’ I H A C T Yethotis c f re so lv inq t h e iron lines, t t e m o s t cormon X-ray emissior: l i n e s i n ootr g a l a c t i c and extragalactic X-ray liourcesp arc b r i e f l y revleh’ed. P s e l f - f c c u s s l n Q c r y s t a l spectrometer is shorn t c b e t h e most suitacle irstrument, nrd tbe opt laur r crystal qeometry I s spDerical (Pcr‘roFper and l a y l c r 1 9 8 0 ) . The r r l n c i r l e s o f oFeratlor? c f SUCP a spher ica l crystal l rni .gfn9 spectroneter (SCIS) a r e c t l e t l y reviewed, ard i t s s e n s i t i v i t y 1 s shown t o be two orders o f Raynftude bet ter t h a n a f o c a l olene c r y s t a l scectroireter on A X b F . P SFacelali version of t h t s payload w o r ~ l d b e v e r y c o s t - e f f e c t i v e . rHEC€UliG PAGE BLANK NOT FlLMED 498 Introduction The e m b a s t s i n this pacer will be 09 hiqh-resolution > i o a i of E spectroscony (the X-ray astroroieros definition: *I;E the iron lines in the 6-7 keW rencae. The first, and generally the stronqest lines cetccted 11, Fropartioral counter Spectra of botn qslactic and ertrasalactlc X-ray sources have becn the iron lines in the 6-7 keV ranqe. The reason for t h i s line strenoth in optically thin plasmas is trofold: a ) the high relative abundance of iron; and t l t k e ionization talence of elelpents at the temperatures typical c t X-ray sources, i . e . , 107 -10' K. Figure 1 1s plctted frcu the tabulations of Rsyrond 3nd S u i t h (1978) and shows theoretical eaiculatlocs of the relative emissivities of t h e stronqest irac l i n e s between 3.10.~and 1 0 f K (not 1nClUdlnQ satellite lines) and illustrates the reason that the Fe X X V I X X V I covplex Is so doainant in X-ray spectre - o t h e r elements are larcreljr striFFed cut at lower terperaturcs, In particular, the eftective temperature of intracluqter gar in galaxy clusters, typically 8 x 107K (see CIushotZkv et sl. 19781, is such that the ircn c o v l e x forms the mcst useful diagnostic of cluster Faremeters. The most combon lioe ctscrved in binary X-ray sourC?s i s t h e tluorcsccnt iron line, for rea8or.s of hiqh flucrcscecct yield ( 8 f a c t o r of 10 hiohet than ary c t r e t element - see lilhle I , taken from Pr8VdO 19781 , coupled b i t h the h i a n relcrtive abundance. , 1 499 24 25 26 27 3 -I erg cm s Fe XKV F i c t u t c 1 Relative cmtssiultles of t h e s t r o r a c s t i r o n Lines lrr a \nu-density p 1 4 s n d at temperatures betbeen be it,' and 1 C h (not l n c l i r d i r g sateAl*t.c l l n e s ) , 1 500 T A B L E 1 FfUGPFbCEhCF EMISSION Si 3 . c . c 4 1 .74 0 , 0 3 5 1.6 IOe5 c . c 9 2 , 3 1 0 . 0 3 Ar 8 . 10-G 0 * 1 3 2.96 0,OJ o-5 Ca 2. 10-c; 0.16 3 . 6 9 0.01 Fe 4 . 10-5- 0.33 6.40 1 ,oo i 2 . lo-l- 0 . 4 1 7,4? 4.07 1 50 1 tjob can the i r o n l l r e erercy he reached f a r sDectrdScCav, and no* can the ircn llres t: resolved fcr seasurements of tenperdture, abundance, derslty end Ionizatlcn calance? Figure 2 illustrates tnc resolutlor typical Of eperiy-disoersive devlces ( d a s r c d lines) ard tkose e € dlsoerslve devices (Solid lines). !dor-disrerslve devices have teen used almost exclusively tkus far i n t h e aeasurenents of iron line strenqths at 6-7 keV, t u t Pave lnsufficiert rebclution to serarate tne l ines cf Fe X X V frorr, Fe XXVI. Cas crcporticnal sclncillatlon counters have taice the resclvinq Dower of the conventional q a s Frororticrsl counters, kut this is still insufflcler,t to clearly serarate Fe 3 X V fron X X V I , o r t c resclve tne Fe X X V triplet lines t r o t e t P a t lines of F E X X V , I s z - l s 2 0 and l s z - l s n c kcre recently resclved DV a GPSC i r the spectrus of Cas A - Andreset et el. 19811. Tre solid-state detectcr, ot the klrd florn on i iEAC-2 , ( i i o l t , 1 9 7 6 1 , does have t h e resolution t o separate Fe X X V trow XXVI, and sa, i r principle, does a hlqh-spatial resolution array of sclld-state devices, 1.rOI the charge-caurled device. Corsiderable proqtess has been made in the developrect of a CCr: k i t r sufficient derletion deFtP for the 6 keV ransc ( G r l f t l t h s et al. 1981: Peckerar et al. 19P11 , b u t solid-state pertornance ccuivalent t o tnat from d conventlcnal Q1II.i) detector, has yet tc k e dcnonstrated fralr 8 f u l l CCc a r r a y at this energy, althourrk tnc lowest CCD r o i s e levels are 502 ORIGINAL PAGE 1s OF POOR QUALlW F l g u r c 2 Enerqy resoluttcr c f ncn-dlsczrsive d e v i c e s ( d a s h e d lfncs) apd disrerslve devices (Solid llnes). 503 or outru conparable to or beloa that cf the Si(L1) detec , EOth the conventioral solid-state detector and tre CCC have to rely on grazing-lncidencc optics fcr large collection areas, and in the case of HEAU-2, this resulted i n nc collection area at the 6 keV iron lines. For the CroFcsed mirror a e s i Q n for AXAF, the effective collectinr) area at 6-7 keV rill be atout 2 0 0 CIFa (Zorbeck 1981) . To separate the Fe X X V triplet lines, the necessary resolving power is about 3C0, asi this can tr? provided t y crystal spectroscopy, lithium f l u o r i d e having a suitable 2d spacing ( 41' Brag9 angle for the atcnic planes of LiF 2201. 1n order to achieve high sensitivity, the Particle backgrcurd in the X-ray detector has to be effectively eliminated, so that the spectrorcter is photon-linited for a larqe number of ap~iicatlons. Prev ious satellite-korne crystal svectroueters, on Ariel V , ANS, OSO-8 have been limited i n sensitivity by their partlcle-induced backgroucd l e v e l s , conccsitent an the flat crystal Panels employed in each case. For larqe crystal areas and small detector sizes, some form of self-focussing is obviously requlred, and varicus geo'netries have been proposed, and some instrurents flobn, i n recent years. The conicel-segrent spectroweter (Fig. 3a) has first proposed by Mocdqate et 4 1 . (19731 , and needs tc be scanned over the required energy ranqc. The convex version cf tPls, described by eetthelsdorf e t a l . 1976 (Fio, 3b) is enerqy-dispersive a long the detector a x i s , and has been employed on the solar maxtmum n i S S i o l r ( R a ~ l e y et 504 b t. ci3SiCirJAL PAGE IS OF: POOR QUALIW 505 ORIGINAL PAGE IS OF POOR QUALITY d e a r a d e d perfortrance fcr off-axis or extended sourccs, however, and the o p t i m u m ceornetry is tt-e spherical case, fully desCrlhe4 b y Schnoprer and Taylor ( i 9 ( 1 1 ) 8 and shown in Fiqure 3c. A brief description cf the Ferfcrmance of this srectroreter w i l l be repeated here. SDharlcal Crystal Inaqinq Scectronetey The 13x15 of the srectroneter, along which the X-ray detector lies, is pointed at the target X-ray source. The operatior of the instrument can be understcod by reference to Fiqura 4, wkicn S ~ O M S the principle of oreration in mcre detail. X-rays of eneray E , , incident in a cylindrical sheet on the tor of the crystal panel, are focussed cnto t5e detector at the point Z,, end X-rays of enerqy Eg, ircidert in a similar sheet on t h e bot tom of tPe papel, are fccussed on the other end of the detector at 2,. For a sprere radius of 2 , 5 1 8 usinq litt-ium fluoride for the ircn lires, the dispersion along tke a x i s is about 0.4 mm rer ev, o r 2.4 ma for a 6 eV crystal=limlted resolution element. Positicn sensitivity of 0 .S PW in the Droportional detector is therefore rutficlent f o r the encrqy-dispersive lint? fccus. ?he position-sensitive detector reads out each X-ray event 8s it occurs, end the e p t i r c spectrum i s thus reccrded in a carallel fashior, b i t n no mechanical Scanninq. The total enerCy rence covered bh the Srectrcxeter i s I\ 506 ONE RESOLUTION ELEMENT \ SPHERICAL CRYSTALL SURFACE \ t I TO SOURCE \CENTER OF SPHERE IMAGING PROPORTIONAL COUNTER c i i u r e 4 R a s i c o p e r a t i c r \ c f t h e s c h e r l c a l s r e c t r o n e t e r . ORIGINAL PAGE IS OF POOR QUALITY deterwined by the ranact o f Frarlg anules for X-revs ircident at the extrewe ePds of the spt?ericeJ section, A real, imperfect crystal consists of a nosaic of individual Perfect crystal blocks, on a microscopic scale, with a resultmt increase in inteqrated reflectivity t3eses 19481. For the spherical sfectrometer, the rocking curve w i d t h of tPe scanned flat crystal panel beCOKeS a correspondinq crystal-limited resolution element alons the Fcsition-sensitive detector. For a era99 anclle of 4 s C , the erergy resoluticn in the spherical spcctroreter Case is the sane as that for the scanned flat c r y s t a l panel. For Eragc) anyles much less War! 45'. there is Sore qaln in the effective energy resolution b i t h the spherical qeometry, but tbe effective area of the crystal panel is reduced and the geometry does not use the ava:lable vcluse as efficiently 4s the 45' case. An extended X-ray source, C f -;racecraft attitude error f o r a paint source, r i l l result in some sliqht degradation in Ferformance, tut i t can easily te shown that the effects are almost neclliqlble l n terrrs of sphctroscoFic rcsoluticn. These effects are most eclsily UCderStoOd by ccnsfderino spacecraft attitude errors and a F a i n t X-ray source. I t is obvious that, Is the spacecraft and spectrasetcr rotate slightly arourd t h e center of the sphere, the sDherical syssetry quaiJntees that the X-rays w i l l still be focussed on t h e line joining the cecter to the X-ray source, w i t h a s n a i l chanqe in tke ercrsy rarye of the 508 ORIGINAL PAGE 1s Of POOR QUALIW & ---a- X-RAYS FROM SOURCE 4 Q --- - -- ----- CENTER O f CURVATUfiE AYll I_ T IMAGE ON OETECTOR - t A3; I ___.,- PORTION OF CRYSTAL OIFFRACTING X: 2d sin 8 Flcure 5 Effect of off-axis X-rays: a ) i n the F l a n e of dlscersion; arc t ) orthoqona l t o tP,e r l a n e o f djsrcrslon. . ~ . . I 509 PT.'.C;T:r"IC F&GE IS CF POOR QUALITY l i n e t o c u s , bowever. I t is most i n s t r c c t l v e t o segeretc t b c Pointins e r r o r i n t o two CoRFccerts, one or t rogor+al t o t h e P lane of d i s p e r s i o r ( F i g . 5 t ) and one i n t h e d i s p e r s i o n ~ l a n t . F o r t h e o r thogona l p o i n t i n a e r r o r , a stiqmatic f o c u s is f o r r e d on t h e d e t e c t o r , aloncr an arc a t c u t t h e c e n t e r of t h e sphere, where t h i s a r c is dissr laced l n t h e navncr sho*n ( < 1 C R t o r a 0:s o o l n t l n a e r r o r ) , A Doint ina e r r o r in t h e r l a n e of dispersion ( F i q , 5s ) r e s u l t s in a l i n e focus 1 Y i n Q s l i g h t l y a t o v e o r below t h e d e t e c t o r anode, and t h e irraqe a t any p a r t i c u l a r enerqy is formed i n a h y p e r b c l l c a r c on t k e d e t e c t o r , where t h e c u r v a t u r e of t h e a r c causes an i n s i q n l f i c a r t loss of enerqy r e s o l u t i o n , T h e p o s t - f a c t a s p a c e c r a f t o r l r s t r u m e n t aspect s o l u t i o n can be u s e d t o c o r r e c t t cr t h e l n s t a n t e n e o u s size and p o s l t l o P of t h e h y p e r b o l i c a r c , so t r a t t h e r e su l t l nq s p e c t r u r is no t s i s n l f l c a n t l y degraded. Tbe p o i n t i n q r e a u l r e R e n t s f o r t h e s p h e r i c a l s b e c t r o n e t e r a r e t P . u s mlr+lmal, o f t h e o r d e r of ~ 0 ~ 1 , 0 T h e irraqlnq of an e x t e n d e l sou rce can kest b e cons ide red i n terms o f p o i n t sou rce corFoI?ents , where each o f f = a x l s p o i n t sou rce is imaqed into a s h c r t , hype rbo l i c a r c , as d e s c r i b e d above, T h e energy r e s o l u t l r p of t h e s p e c t r c r e t e r i n the P lane o f d i s p e r s i o n is s l i q h t l y degraded by s o u r c e e x t e n t , b u t t P i s e t f e c t becomes impor tan t on ly f o r s o u r c e s g r e a t e r thar 0':2 i n d i a m e t e r , ;Isin? l i t n i u w f l u o r i d e p a n e l s , In t h e p lane irthoqonel t o t h e d i s o e r s i o n o l a p e , t h e e r q u l a r r e s o l u t i a r , l a llmited by t h e c r y s t a l r o c k i r g cu rve w i d t h c f a b o u t S a r c minutes, ORIGINAL FAOE IS Of POOR QUALITY 0 a3 LL I 0 0 0 0 tu 1 - - 0 I N WllJ ‘NOIlISOd U0133130 51 1 ORIGINAL PACE IS OF POOR QUALITY Projected SCIS Performance The cor2iratjon of spectral and s p a t i a l resolution is illustrated i n Figure 6, which shows an idealized trnaqe cf B source about 0:s in extent, es i t t f r !q lines frorr bydroqer-like and heliur-like ircn. Fcr t h i s extreme source s i z e (eg., the Virga cluster), the heliu@-like t-iplet is slightly blended tut still usefully resclved, T h e resolution elctrent for d rcint )(=ray source emitting narrow lines is a l s o shoan in Fiqurc! 6 for cowparison PurFoses, and ir. t h i s case the helium-like triplet is fully resolved, The lron spectral features tlhlch can b e covered by a 4 ' ranqe i n Pragcl angle csirq lithiun fluoride are surrrarized in Table 2, and extend frolr the neutral FeI absorption edoe at 7.1 keV throliclrl the Fe X X V and X X V I transitions to a 10% red-shifted Fe X X V I line, The enerqy resclution of the L , i F crystals is 6 c J . The FlasPa diagnostics u t ? l c h can be pcrformed k i t h this enerqy ranqe and rescluticr, are ineicated i n T a b l e 3, a ilSt of necessary sensitivities and resolvinrl powers for the iron lines, taken from tne work of Pahcall and Sarazin (19781 , These include the dielectronic satellite lines of helium=like iron, which may be at comoarakle fluxes to l i r e s o f t h e bore usual transitions. Geperally, It can be Seen t r o r the table that the resolvina power cf t h e crystal spec troac ter descr1k:ed here 1s necessary f o r these plaSTa dlaQnOStiCS. 51 2 t'e I K ilt-s. edcc 7 .13 keU Cc I KJ Fe X X V I 1 s - 2 ~ Fe X X V 1s - 1 ~ 2 ~ 1s -1s2p 3 P', lS - I S Z ~ ~ S Fe 1 K < !'e X X I I 16% rcdshlfted i 7 . 0 5 6.93 3. I L C o b & b.64 6.3Y 6.30 q. Takle 2 l r c n SrectraA f e a t u r e s In the b-7 k e b range OXGi ibL PAGE IS of POOR QUALITY 51 3 Y lo-'* Ficiite 7 frirruw c'etectdtle e o u l v a l e n t k i o t n s and i i ~ e strenqths. 51 4 ORIGINAL PAGE IS Of POOR QUALlrV The s:-t-erlcal c t y s t i r e c i n c s r e c t r c n e t e r achieves r i m s e n s l t i v i t y b y e s o l w i ~ c i a 1drc.e c r y s t a l area and a l d r a e coFcentrat ion f a c t o r i r t c a l or bdchqrcutid de t ec to r ( t n e concentrat ior l f a c t o r is a t c u t 100, r e l a t i v e t c t h e f l a t c r y s t a l case) . I s e c l ir , ir -ur c e t e c t a b l c ar lulvaler t h i d t h s and l i n e s t r e r lq t r s , f c r an i r s t r c r e c t w l t n 0.4 J * c ro jec t ed area , a r e sn0r.r: l r t ioure 7 , rrh lc t c e r c r s t r d t e s t n a t tbese s e n s t i v i t t e s a r e c t inc t io t ] c t contlnLcr f l u x for the s t r c n c e r sources, b r i t the SPoctroreter i s photon-limited, l ee . , l in l ted by l i n e c o u n t s t a t i s t i c s , f o r most scc rces o f l r t e r e s t ir: okservat ion times of 6 1 0 s. r i t h l o w e r o t s e r v a t l o r t i r e s , '10 s or lonqer, the l n s t r u i e n t s e n s i t i v i t k b e c c c c s 1 lT i t ed by de tec to r backiround. Fiaure 7 also i nd lca t e s s c i e c f tne l i n e strenotr!s and eauivdicn t + ; d t h S O f Iron l i n e t e a t c r e s a l ready detected by carcrorticnal callnters or. rCCket ar\d s a t e l l i t e exne r i r en t s . In t kcse cases enere t h e scurce i s e x t e n d e a , t h e obscrved Strenqths a r e srown cer s r a t i a l - s p e c t r a l r e s c l u t i o n element o t the r toaosea 1 n s t t u T e F t ( 5 a r c mir' x h eV). I t can t e r ead i ly seen f r o - tnese s e n s i t i v i t y curves t r a t t r c l n s t r u a e r t i s s e n s i t i v e t o any o t tne c l o t t e d Sources i n an observat ion t i n e o f 10 6, and € o r s o n e , d n observat ion t i r e c t o r l y 10% i s necessary, I ; S an Y 5 Y exananle, the l n s t r u r e n t i s s e n s l t i v e , i n a IC 4 s observdt lon of t h e Perseus c l t i s t e r , t o t e a t u r e s * i t k 3 0 eV eculva len t h i d t h , In each reso lu t ion elemert. Zuch s e n s l t l v i t y i s s u t f i c l p n t t o Re-is'Jre t t -p resic p l a s r d F s r a n e t e r s O S t e v t c r a t u r e , f e l a t l v e ao\j nci nnc e t o r i r c r , acd l o n i t a t l o r talirnce on ORIGINAL PAGE IS Of POOR QUALITV 0 2 - 1 1 I’C 0 4-2 H 2 1 0 Z 1 U 51 5 T a b l e 3 :.eccssaty sersitikitles dnd r e s o l v l n c ; cabers t c r c l u s t e r r l a s r a ~ : i 6 a n c s t l c s ( f r o ? . Pancall ana Saratin 51 6 s u a t ! ~ l l y - r e s c l v e d scale c f 5 a r c n i n o v e r the extencec! c l u s t e r einlsslon. t i e klrco c l u s t e r l i n e erissicn cdn sinl:arly De de tec tea t n each 5 x 5 a r c r i p s r a t i s l r e s c l u t l o n element i n 10 s 8 dnu t c e cor~clined da ta f r o a t h e wbcle c l u s t e r woul r~ have a sicrlttcince o t d o o u t 15ff. 4 Line erlssion f ror surer rova remnants is genera l ly expected t o ce broadened t’y tt-e excansior v e l o c i t y r r o f l l e , *hicP has teen rrcasiIreG i n scne cases f cr o f t i c a l ftlamclnts, t u t not directly f o r t h e e x p a n d i n q X-ray s p e l l . F o r C a s e , the expansion may be est i r ra ted as ‘ 4 C O k7s-I Fah‘ , and t r i s excansicn is expected t o be ocservatle a s d broadened i r c n l i P e covering a tou t 15 r e s c l u t l c n e lcmer ts , * i t P d n averaqe s i q n i t i c a n c e c f akout 6a i n eacn element . T h e s c n s i t l v l t y i n tPe ( u n r e a l i s t i c ) ccn-excarslon case ane €or t h e case o f an e x r e r c i n c spell source a r c b o t h shonn i n Flclure 7. In the o r t l c a l l y - t h i c k s c d t t e r i n a p l a s r a o f acc re t ion disks arcund Compact objects8 the nafrcw l i n e e a i s s i o n is severe ly reouced from t h c o p t i c a l l b - t P l c c a s e (Felten and kces 1972; G r l f t i t h s 1 9 7 2 ) . Poss e t d l B ( 1 9 7 R ) r.ave ncael lec! t he I l n e emissiar f r o r Her x - 1 8 and the narrow 1lr.e cc rc predicted b y their model is p lo t t cd ir: Figure 8 , iae.8 the r e s i d u a l l i n e core S h P U l d be d e t e c t a c l c i n a 10% ot>servat ion, w i t n s u f f i c i e n t s e n s l t i v t t y to medsiire t r e r e l a t i v e s t r e n G t h s a f tbe resondr.ce, i n fe rconb ina t lo r a ~ d torcldder l i n e s n t Fe X X V , crlvincr a measure c f density in t r e source. I r e s e r e s tdua l l i n e cores a r e an order o f maqnitude bealter thar t r t r e o o t i c a l l y - t r - i r Case, Scmc o t trip 51 7 80 60 40 20 IO i I I I I I I I I I . HER X-l ROSS et al., 19t8 I I I I I I I I I I 6.2 6.4 6.6 6.8 t.0 t. 2 PHOTON ENERGY, keV Y firlure O Cstculatlcn o f c c u r t s from a 10 s okservatlac q f rler x - 1 , u s i n o tr.e r e s i d m l llne i c t c r s i t i e s from t h e mcdel of K O S S e t a l . 1 9 7 8 . 51 8 ticorescerce l r o r l i r e s i n r l r a r b X-ray s c u r c e s , a t C expecter; to b e an order o t aaqnltuac s t r c p q e r than these residual Fe X X V lines ir! Her X - 1 . I t is notebarthy that tFe sensitivity cf an bXAF fccal a lane crystal suectroweter (l.e,, t h e L i F crystal) woule! be about lo-" p h cm--'s-' at the lrcr lines (Schnopger ard Taylor 19811, t w o orders of magnitude hcrse that? the instrument prorased here. The reason for t h i s difference is simply one of area, with the reallzation that sucr srectrcmeters are photon-limited, The etfective area of P X A F at 6.7 kev 1 s betweer! 100 end 200 cma, r h e r e a s the projected area cf t h e crystal Fencls in the Propcsed SCIS is of the order o t 10' cn2, Considering ttiot-resolution crystal soectroscopy e lore , the use of grazing-inclde~ce optics is clearly inefficient. 7Pe same arqunent revalns valid, but to a !esser extert, s t loher e n e r ~ l e s , where t P c A X A F effective area of '1000 cm'= can still be sur~esscd by a c r y s t a l p a n e l with tre aporopriate atomic spacing for Pragg reflection and concentratfon of the sllicon or sulphur lires, for example, SCIS - Prooased Payload The SCIS payload, as rroFosed for J Spaceleb misslcn by SA0 in collaroraticn w i t h tre S a c l a y group (Scbnccper et al, 1978; Schnopper and Taylor 1 9 8 0 1 , is s h o b n in a cut-away drahinq in r'lqure 9, lne larqe crystal Fenels are r c l a t l v e l y cheac t o fabricate, and a total payload c o s t has beer estimated a t around $ 3 million. The l a rqe c e n t r a l area is sho*n here f i l l e d w i t h a monitor proport ional counter f o r measurevent of t k e o v e r a l l s r ec t run o t X-ray sources. A i a s s c i r t l l l a t l o n Droportional counter wculd have t b l c e t h e enerqy r e s c l u t i c n and would b e t t e r coaplement the c r y s t a l spec t rone ter . I d e a l l y , t he c e n t r a l a rea would De f i l l e d Py an e f f i c i e n t q raz i rq incidence c o l l e c t c r , coveting a t l e a s t t h e "1-8 keV ranrle, with a ~ : a r ray Of s o l i d - s t a t e devices in t he foca l plane. Conclusions The frost e f f i c i e n t , a r d by f a r t h e most econorFlcal way o f performina hish- reso lu t icP SFectroscopy of the 6-7 keV i ron l i n e s Is by means of a sphersca l c r y s t a l iweq lnq spectrometer. Sucb an tiistruwent, of Space la t -pa l l e t p ropor t ions , would have a s e n s l t l v i t y two orders cf magnitude b e t t e r than an A X A F focal-plane c r y s t a l spec t r cne te r , and would be s e r s i t i v e in observat ion times of I d a c r l e s s , t o a l l o f the l r o n lines detected t h u s f a r by proport ional COiinterS. CA'.%,i&L PAGE IS OF PCOR QUkLlW 1 521 , REFERENCES Acton, Lob. e t a l . 19RC, S c l a r F h y s . , & 53 , Andresen, R.O., Manzo, G . , F e d c c c k , A . , T a y l o r , B.G., Catura, H . C . , C u l h a n e , J.L., and E e r t h e l s d o r f , R . 1 9 8 1 , PrOC. 1 5 t h ESLRB Symposium, 'X-ray Astronomy', E S P . B a h c a l l , J.N., and S a r a z i n , C.L. 1 9 7 7 , AP.J. [ L e t t e r s ) , 2 1 3 , L99. B a h c a l l , J.IY., and S a f a z i n , C.L. 1 9 7 8 , A p m L 1 . a 2 1 9 , 781. a e r t h e l s d a r f , R.F., M i t c h e l l , R . J . , and Culhane , 3 . 1 . 1976 , Space S C i . I n s t r . , 2 , 3 4 9 . Felten, JOE,, Rees, Y O L 1 . , ard Addms, T.F. 1 9 7 2 , Clstron. and l s t r o o h v s . , 21, 139. Grif€iths, HOE. 19 2 , A s t r c n . end A s t f o c h v s , , a 97. G r i f f l t h s . P.E., P o l u c c i , G . , wak, A . , Murray, S.S., and S c h r d r t z , D.A. 1 9 8 1 , Froc. S O C . P l i o t o - C p t . I n s t t . Enq. ( s P l E 2 , ' S o l i d - s t a t e i r a g e r s f o r astronory', '190, 6 2 . Holt, S.S. 197h, Soace S c l . I n s t r . , 2 , 205 . J a p e s , R.w., 1 9 4 9 , ' T n e 0C;tical P r i n c i p l e s c f t h e D i f f r a c t i o n of X-rays', The C r y s t a l l i n e S t a t e , Vol. 1 1 , G. Bell and S o n s , London. Pushotzlcy, R.F. , S e r l e m i t s o s , P . J . , S m i t h , t3.h., P o l d t , E.A., and Y O l t , E . 5 . 1978, Ap.J.p 225, 21 . P e c k e r a r , M.C., McCnnn, C o h o , a r d Yu, L, 19ki1, Rev , S c i . J n s t r . , In Dress. Pravdo, S.H. 1 9 7 9 , P r o c . CCSPbW S y m o s l u f r on X-ray Astronomy, 522 ORIGlNloL PAQE IS OF POOR QUALIW advances i n space E x r l c r a t i o n ( C O S P A f i I , V o l . 3 8 c169, eds, i . A . B a i t y and L.C. P e t e r s c n . E.G., and f!akke, 3 . c . 1 9 7 7 8 Rev, SCi. fnStr.8 1123. Raymond, J.C., and S r n l t h , P . A . 19778 AP.J . SUFPOt s,. 4 1 9 . R O S S , P . R . , Weaver, I?., i$Pd I-'cCray, R , 1 9 7 9 , Ap.J. , 2 1 9 , 292, Schnapoer, H o e , , a n d Taylor, f.C. 1980, - AiID. C P t . , x , 3306 . Schnopper, H.k.8 C e l v a i l l e , J.P., t i r i f f l t h s , R e , Taylcr, P.O,, KOCh-Mlrafi~Ondr Lo, Eerqeron, J,, I l o v a i s k y , S,, Pontmerle, P R34-10-78. u o o d n a t e , n.E., Lo*inqer, T., acd Schnelder, F r o 19738 4 ~ ~ 1 . O F t . , 1 2 , 2759. LABORATORY STUDIES ON A SPHERICALLY CURVED BRACC SPECTROMETER FOR COSMIC X-RAY SPECTROSCOPY M. Cantin, L. Koch-Miramond, 8. Mougin, R. Rocchia Cent re d'Etudes Nucleaires d e Saclay, France ABSTRACT A spherical array of twenty LiF 200 crystals has been built to test t h e performances of a freestanding, self -focussing spherical crystal cosmic X-ray spectrometer. Measurements presently available show tha t t he s ize of the image for a point source at infinite distance would be 3mm(FWHM) along the focalisation axis and 2.1 mm (FWHM) along the dispersion axis. The mosaic spread on individual crystals is less than 0.1 degree. A slightly systematic deviation from the ideal bending (0.1 degree) is observed at t h e edges of most crystals and this appears to be the major limitation to specirometer performance. 1. INTRODUCTION Precise piasma diagnostic on cosmic ray sources can only be obtained from measurements of line strength ratios with Spherical Crystal Imaging Spectrometer (SCIS e; Bragg spectrometers. A free-standin similar to the one described by H. Schnopper and P. Taylor (1980) has been chosen for the E5A mission X 80 (L. Culhane, this Workshop;. A fre2-standing spectrometer employs a curveo array of crystals to simultaneously collect , focus and diffract the X rays emit ted by a distant source. The concave spherical shape provides the minimum loss of spectral and spatial resolution which may result from source extent, spacecraft pointing and alignment errors (R. Criffith, this Workshop). W e began studies on spherically bent lithium fluoride crystals in 1979 to demonstrate the feasibility of the spectrometer we proposed to NASA in collabo- ration with the Smithsonian and Harvard Astrophysical Observatory (CFA). We describe hereaf ter the results obtained during this feasibility study. We will see tha t these results a r e directly applicable to the Bragg spectrometer of the ESA mission X 80 whose main design goals a r e : - spectral resolution E / A E t 10 at 7 keV - size (full width at half maximum) of the image of a distant point source : 2.5 mm along the dispersion axis and 4.5 mm along the focalisation axis for a radius of curvature of 2.5 m. 3 11. TYPES OF CRYSTALS The following results were obtained with LiF 200 crystals which were proposed in the initial document (ref. ESA.SCL(79)3) for the obseivation of iron lines between 1.7 and 1.9A. These crystals can b e provided by cleavage or c u t and polished. Cleavage along t h e 200 plane does not give very f la t crystals : impurities produce cleavage s teps of a few microns, even tens of microns. Polishing gives surfaces with very good quality which allows optical uses. I t is note-worthy tha t polishing restilts in a bending of t h e crystal due to mechanical surface stress. For thr- tes ts we used square crystals (5 c m x 5 cm), 0.5 mm thick provided by Quartz et Silice. Other types of crystals (220 LiF, PET, TIAp) for fur ther studies will be provided by the same manufacturer.* 111. TESTS AND SELECTION O F FLAT CRYSTALS Planimetry The flatness af each crystal is measured before bending. The required precision on the radius of curvature AR/R A 5-10% imposes a severe selection: to understand that we have to remind tha t a 2.5 m curvature radius produces a sagi t ta of only I25 microns on a 5 c m length. The surface of each crystal is scanned with a planimeter along several lines parallel to the X and Y axis. The two sides of each crystal are explored with an accuracy of I micron. Cleaved crystals with prohibitively 524 ORIGINAL PAGE IS OF POOR QUALlW high cleavage steps were excluded. Polished crystals exhibit a strong uniformity and no selection is required for these. X-Ray tests They a r e devoted to est imate the effect of the large scale mosaicity. The test bench consists of a point source, a crystal holder and a radiographic film. Mosaicity means that, at a given point on the crystal, crystalline la t t ice makes an angle with the upper surface. The different points on the crystal, satisfying the Bragg relatio?, contribute to the diffraction image on the film which consists in a single regular line for a perfect crystal and several portions of bent lines for actual crystals. The vertical spread E of the diffractioq pattern is proportional to the mosaicity W. Calibration gives €/W = 0.7 mm/arc.min. The two lines Km Kw2 of Copper a r e clearly seen for each scaxning position. In most casesthe vertical spread is less than 3 to 4 mm corresponding to a mosaicity less than 0.1'. A few crystals only werr. rejected a f te r this test. IV . METHOD USED TO SPHERICALLY BEND THE LiF CRYSTALS The fiiF crystals a r e 0.5 mm thick and 50 x 50 mm in atea. The nominal radius of curvature is R = 2.50 m. Individual crystal holders 50 x 50 mm and 5 mm thick a r e made in aluminium alloy. They can be indi-iidually aligned by a classical optical method using the rear face. The front face is covered by a spherical depo,it of STYCAST resin obtcined by molding, t he surface being polished against a convex spherical shape to remove any imperfection. Then the crystal is glued on the holder during 72 hours or more pt room temperature under pressure of ~lr 7.10 PascL;s. V. TESTS OF BENT CRYSTALS Me. wical measurements The same instrument as in sect.111 was used to est imate the crystal sphericity. The nrofiles show slight cleavage steps. The set of tested crystals have radius clustered around the nominal vahe +5% with a maximum spread Of abG.Jt 2 8% X-ray tes ts The equipment is shown in fig.1. I t consists of an X-ray generator wit') a copper anticathode.The s ize of the X-ray source is I x lmm2 ; after a tr ip of 6.5 meters in a tube filled with helium, X-rays reach the LiF crystal. The par t of the radiation impinging t h e crystal with Bragg incidence is reflected and focussed, after another 6.5 m*:ters tr ip in helium, on t h e sensor (film, PM, position sensitive proportional counter). For a perfect crystal , reflection should occur cn a vertical line. Test bench in X rays for spherical crystals. Two tests were carried out : (i) for a given position of the crystal t he image is scanned along a vertical axis so as t3 es t imate focussing properties. The image brightness is not gaussian in most cases and may exhibit some spikes or bumps. On the ave iage the width with cleaved crystal is smaller than wh!) polished ones, but the shape is not so smooth. This is probably dine to small cleavage s teps which give rite to different curvature radii. (ii) The crystal is rotated around the curvature center. The detector is located so as to measure r It nominal). From t h e width and shape of the curves one can conclude tha t local angular deviations from a perfectly spherical surface is less than 0 . 1 O . The reflectivity of polished crystals is about twice the reflectivity of cleaved ones. 526 ORIGINAL PAGE IS OF POOR QUALln Fina!Iy the whole panel is rotated around the center of curvature tc test the response in the dispersion plane. Tb *;K.B ShOM t he received intensity versus q&! of ro"dt.u.n for the whole panel. The upper c'rrve cor- csponds to an aperture of 5 x 30 mn. :n front of t he detector, t he lower curve to an aperture of 5 mm diameter. The P;iS 1 diffracted intensity corresponds as expected to 4 times the mean intensity given by individual crystals. The imperfections of the Curvature at the edges of the crystals explain the irregular shape of the curves. From the ratio of diffracted intensities between the upper and lower curves in fig.4 one deduces the size of the image in the dispersion plane : 4.2 mm (FWHM). During these tests we have monitored the focussing properties of the m a y which shewed acceptable variations. . . . . ! . . . ................ . . . . . . . - . . . - FiR.4 Diffracted X-ray intensity distribution along the dispersion direction versus rotation angle of the crystals around a vertical axis going through the theoretical curvature center. VI. CONCLUSIONS The results gathered with a limited size sample of twenty 200 LiF crystals (16 polished and 4 cleaved ones) show that : - the focussing and dispersion curvatures are slightly bet ter with cleaved crystals than with polished ones but reflection coefficient is increased by a factor two by polishing. - a slightly systematic deviation from the ideal bending of less than 0.1' observed at the edges of most of the crystals is the major limitation of the spectrometer performance. i Quartz et Silice : 8, rue d'Anjou 75008 PARIS. This deviation results in : (i) A slight degradation of the energy resolution and focussing properties (non awsian distribution, existence of wings). ii) A non-uniform energy range coverage. The presently available measurements show tha t the size of t h e image for a point source at infinite distance (flight configuration) along the focussing direction would be 3 mm (FWHM) or 6 mm ( F W at 1/8 max.). These figures are better than the estimated ones based on a mosaicity of W = 0.1' (5.4 mm F W H M or 7.8 mm at 1/8 max.). This proves tha t the mosaicity induced degradation has been, on the average of 5 crystals, overestimated. The effect of badly curved individual crystals does not show when considered in a set of several crystals (4 in the present case). The size of the image for a point source at infinite distance along the dispersion direction would be 2.1 mm ( F W H M ) , bet ter than the design goal. These results indirectly demonstrate tha t t he energy resolution of 1000 at 7 keV is obtained together with a spatial resolution of 2.5 min of arc. This will be confirmed won by measurements along the dispersion direction in a parallel X-ray beam. In these conditions point 2 appears to be t he most severe limitation of the system but i t can be overcome if we use staggered or alternative rows of crystals which produce the right overlapping for a nearly uniform energy range coverage. t Future tests will extend measurements to LiF 22C, PET and TIAp. The results obtained so far a r e surprisingly good and should be still bet ter with an improved bending technique (increased curing t ime of the lue, optimization of the bending matrix shape!. References Cantin, M., Koch L., Mougin 8. and Rocchia R., 198 I, CEA Report DPh.EP.Ap./8 I - I I R. European Space Agency Assessment Study, 1980, SCI (8015 Schnopper H.W., Taylor P.O., 1980, App. Opt. 19, 3306 Schnopper H.W., Koch L., Mougin B., Rocchia R., Bethelsdorf R.F. and Culhane J.L. 1981, 15th ESLAB SYmposium X Ray Astronomy, Amsterdam. 8 8 2 ' 2 6 0 8 3 \ Cylindrical crystal imaging spectrometer (CCIS) for cosmic x-ray spectroscopy. Herbert W. Schnoprzr Danish Space Research Institute, Lundtoftevej 7, 9K-2800 Lyngby and Paul 0. Taylor Harvard-Smithsonian, Center for Astrophysics, 60 Garden Street Cambridge, MA. 02138, USA ABSlRACT We have developed a "stigmatic" focusing, Bragg crystal spectrometer that has been used for high spectral resolution x-ray emission line diagnostics on hot laboratory plasmas. Our concept can be applied at the focal plane of an orbiting x-ray telescope where it offers several advantages over conventional spectrometers, i.e., mechanical simplicity, high resolving power and sensitivity, simultaneous measurement of an extended segment of spectrum, and good imaging properties. The instrument features a simple, unambiguous, non-scanning spectrum readout that is not ad- versely affected by either spacecraft pointing error or source extent. We estimate the performance of the instrurcent in the con- text of the Advanced X-Ray Astrophysical Facility (AXAF) mission. 528 I. INTRODUCTION Careful interpretation of precise spectral information obtained by the x-ray spectrometers on the Einstein Observatory ( 1 ) has established the role of spectroscopy in the understanding of the underlying physical phenomeca that drive many cosmic x-ray sources. The increased col- lecting area associated with the use of a grazing incidence reflection telescope on the Einstein x-ray observatory has allowed powerful spectroscopic techniques commonly used in other branches of physics to b e applied to many of the stronger x-ray sources. Numerous cosmi,? x-ray emitters, inciuding binary sources, supernova remnants, active galaxies and the intercluster gas have thermal components and many have been found to exhibit characteristic line emission. Fe line or edge structures have been observed, but with very low resolution ( p r o - portional counter), in several of the brightest sources. Emission lines from several highly ionized elements (among them 0, Si, 3 ) have been observed by the solid state ( 1 ) and focal plane crystal spestrcaeters ( 1 ) on the Einstein Observatory. The future need is for more precise ard higher resolution data that can be interpreted within the context of a particular model t o provide information on temperature, density, re- lative abundance, ionization equilibria, mass motion, redshift or part- icle size distribution. Previously, detector sensitivity was the prime consideration in astrophysical observations. For future missions higher resolution (>lo2) will be desirable and, in many instances required to make a choise between various conflicting models, to obtain line shapes for measuring temperature broadening and bulk gas motion, and to discriminate against spurious features from redshifted or satellite lirizz. :-iuch of the quantitative information contained in the spectrum can be obtained only from precise ratios of line intensities. The simul- taneous recording of the He-like triplet and/or lines of H-like o r Li-like series and their satellite lines, will reduce inaccuracies that could result from serial observations of fluctuating cosmic plasmas in the presence of a varying charged particle induced detector background. Parallel recording of all spectral features will aid in line identification and charge state analysis and will also improve the efficiency of finding features with unknown redshifts, absorbtion edges, and continuum levels. We have developed a high wavelength resolution, "stigmatic" focusing, Bragg crystal spectrometer for line shape diagnostics of hot laboratory plasmas. The concept has been studied pre- viously ( 2 - 4 1 , Jut rarely applied in practice. It employs a cylin- drically bent crystal (Fig. 1) to intercept x-rays that diverge with Fig, 1. CCIS concept; a cylindrical crystal surface gives exact point to point focus along the axis. Rays incident on the crystal at constant Bragg angle O1 define a conical surface about the axis with semi angle el. a broad range of Bragg angles8 from an entrance slit located on the axis of the cylinder. At each portion of the crystal those x-rays satisfying the local Bragg condition ( A = 2din8, with d the lattice spacing) are reflected towards the axis where the dispersed spectrum is recorded on a position sensitive detector. This concept is to be contrasted with the more common Johann ( 5 ) design, cylindrical crystal spectrometer in which divergent x- rays are intercepted at almost constant Bragg angle. In the Johann design, a region of spectrum is scanned serially by changing the Bragg angle; hence, the relative position of focal point, crystal and detector must be adjusted repeatedly. When the cylindrically curved imaging spectrometer (CCISj is used at the focal plane of an x-ray telescope it can pro- vide excellent imaging of an extended source. In the plane of the detector spatial information is displayed perpendicular to the cylinder axis while spectral information is displayed along the axis, and both can be recorded with a suitable two-dimensional, spatially resolving, detector. If, in some region of the spectrum, the predominent radiation is in an isolated narrow line, then then a two-dimensional image of an extended source will be recorded when the spectrometer is used without an entrance slit. 530 Finally, the unique focusing geometry of the cylindrical crystal makes it possible to exploit the greatly increased reflectivity of some mosaic crystals without a corresponding loss in resolution. With the inclusion of this benefit the cylindrically focusing spectrometer geometry will achieve a resolution and sensitivity equal to those of any current spectro- meter design. In addition, the CCIS will have a large throughput since several hundred resolution elements are displayed simul- taneously. The most important are: Our instrument has particular advantages for x-ray astrcnomy. 1. An ability to measure simultaneously an extended 2. A spectrum integration mode in which scanning is 3. A simple unambiguous spectrum readout. 4. A minimal loss of energy resolution for extended or 5. A high quality focus perpendicular to the dis- 6. A high resolving power, E / A E lo2. 7. A pc'_r?tial for increased sensitivity without l o s s segment of spectrum. unnecessary. off-axis sources. persion plane. of resoiuiion if the high integrated reflectivity of mosaic crystqls is exploited. c i -. centra t i ng opt i cs . 8. A geometry bhich does not require high resolution 11. CCIS CONCEPT A spectrometer concept in which the crystal is bent to a cylindrical surface that focuses and disperses the x-rays along the cylinder axis has been previously suggested (2,3) and its focusing properties and advantageous features have been examined in some detail (4). The concept and geometry is illustrated in Figs. 1, 2, and 3. Rays from a point on the axis, incident on the cylin- drically bent crystal at constant Bragg angle 8 , define a conical surface about the cylinder axis with semi-angle 8 (Fig. 1). We consider the crystal to be oriented at a central grazing angle e o with respect to the telescope axis about which the x-rays are diverging (Fig. 3). 531 Fig. 2. ORIGINAL PAGE IS OF POOR QUALfTY CCIS at the foca.1 plane of a grazing incidence telescope. Crystal and detector positions for two different wavelength bands are shown. Fig. 3. Definition of spectrometer parameters. Fig. 4. Effect of mosaic crystal structure on focus. Each block reflects over a range of angles Wc. A . Wavelencth dispersion. X-rays of davelength A incident at Bragg angle 8 are reflected in first order when A = 2dsine,where d is the spacing of the diffracting crystal planes. If the crystal surface is a cylinder of revolution of radius p , rays originating on the cylinder axis are refocussed exactly onto thet axis at a distance from the origin given by x = 2 pcote. The wavelength dispersion alor,g the x-axis is A = 2d/[ l + ( x / 2 ~ ) q ’ ’ * and the refocussed x-rays can be recorded by a linear, posit- ion sensitive, detector. 532 Relationships between detector spatial resolution x , angular resolution h e , and energy AE, or wavelength resolut- io? M are obtained by differentiating the expressions above. Thus, dx = (2p/sin26?de and dE/E = dX/X = de cote = (sin28/4p)dx. (2) The crystal rocking curve width ABC=Uc ing resolving power determines the limit- S = (E/AE) = tanO/Wc, ( 3 ) C and the corresponding limiting detector spatial resolution e 1 emen t Several factors could degrade the angulc o r spectral resolution beyond the crystal limited value Wc and they have been analyzed in detail. Exact ray tracings indicate that no significant resolution loss will result from telesco2e aber- ations, pointing drift, or source extent. The maximum telescope angular aperture 2 adetermines the crystal length C required to fuily utilize that aperture (Fig. 3 ) ; Assuming that there is a maximum convenient crystal size, then Eq. ( 5 ) sets an upper limit on p and on the other spec- trometer parameters. The maximum fractional energy range which can be diffracted by one crystal at orientation 8, is 533 (2 a = 7' for AXAF). This simultaneously diffracted energy band is sufficient to encompass the He-like triplet for all multiply charged ions (C: to Fe) and, except for large e o , it also encom- passes the H-like 2s - 2p/3p lines. When 8 , is reset to accom- modate a specific energy band, the distance L = P/sin$ (from slit to crystal, and from crystal to detector) must also be reset as is shown in Figs. 2 and 3. An instrument having L variable over nearly a factor of two and also having five or six interchangeable, cylindrically curved crystals available will accommodate the wavelengLh range from 25 A to 1.5 A , that covers all K lines and absorption edges from 0 to Fe. B. Resolution In this section we show that the spectral resolution of the cylindrical spectrometer is not significantly de- graded by z mosaic crystal. The probability R ( 0 , X ) of reflecting an x-ray of wave- length X incident at an anglee is sharply peaked about the Bragg angle. For a perfect crystal the full width at half maximum of this function is the single crystal diffraction width Wc. If t.he crystal is a mosaic of single cryFt.al domains of appropriate microscopic size, then at each orientation of the crystal an x-ray can pass through many domains with slight- ly different orientations before being either absorbed or re- flected. In this case, the "reflection window" or l'half width" of the probability R(8,X) is the mosaic spread W, which is generally larger than Wc. Fig. 4 is a representation of this concept in the con- text of the cylindrical crystal geometry. A ray reflected at the Bragg angle from a mosaic block oriented parallel to the crystal surface will cross the crystal axis at a distance x given above. Rays of the same wavelength, how- ever, incident on the surface at angles 8 2 6 , can reflect from mosaics tilted at angles 2 6 to the crystal surface, and will cross the axis at the distance 534 which differs from the 6 = 0 position by h,,, - 2p62cote/sin2e. ( 8 ) The distribution of intensity in the image resulting from the mosaic effect is asymmetric with a peak at x = 2 p c o t e and a rapid fall-off towards larger x. Using 6 = W, as a measure of the resolution loss, we find that the mosaic broadening Axm, is equal to the single crystal difft*action limited resolution hC given by Eq. (4) when where S is given by Eq. ( 3 ) . Mosaic spread orthogonal to the dispersion plane results in a large spatial blurring, Azm = 2p Wrn/(sinBcosY), ( 1 0 ) where JI is the half angle subtended by the crystal from a point on its axis (Fig. 1). An orthogonal tilt 6 also re- sults in a shift in Bragg angle of associated spectral res9lution l o s s will equal the diffraction limited value when b e = 62tan . The 1/2 -1/2 id,,, = (ZWccot6) = 2s ( 1 1 ) Thus, the maximum practical value for Wm is limited by Eq. ( 9 ) until tan e = 2’12 and then by Eq. (11) and only if Wm > > Wc will there be significant resolution l o s s resulting from the mosaic spread. mined by the integrated reflectivity The efficiency of the spectrometer is, however, deter- which can be much greater for a mosaic crystal than a single crystal; roughly by the factor Wm/wc = ~ 1 1 2 . 535 Certain crystals can be prepa zd with domain stracture that provides both J narrow diffraction width and a high integratecl reflectivity. Two examples are graphite and lithium hydride which have theoretical single crystal re- solving powers of d 5000 (2-5 keV) and- 0.5 to lx105 ( > 3.5 keV), respectively. Tolerable mosaic spreads are thus 0.8O for graphite at 2.5 keV and 0.4' for lithium hy- dride at 6.5 keV which are values typical of commercially available samples. Integrated reflectivities for ideal mosaic crystals are 18~10'~ for graphite ar.d 35~10'~ for LiH; a factor of 10 c)r more higher than those for typical single crystals that might be chosen to reflect the same energy bands. This benefit of high integrated reflectivity together with high resolving power is not obtainable with scanning Bragg crystal spectrometers since, at each setting of the crystal, these instruments register on the detector the entire energy range determined by the rocking curve width Wm. 111. PERFORhANCE A. Resolution Design parameters for an instrument suitable for use at the focal plane of a large orbiting x-ray telescope are given in Table 1. Fig. 5 shows various spectrometer dimen- sions and Fig. 6 shows the resolving power limits imposed by telescope and spectrometer focusing and alignment aber- rations. It is possible to have a very compact instrument without compromising on desired resolving power (S & lo2. If the spatial resolution of the detector is 0.2 mm x 3 mm it will impose a limiting spectral resolving power of S-4003 and a limiting transverse spatial resolution of 1 arcmin orthogonal to the dispersion plane. To accomodate the full range of Bragg angles (2 a = 7' and a field of view of 0.5', the active area needs to be 20 cm x 10 cm (if P = 20 cm.). 536 TABLE 1 PARAMETERS OF AXAF TELESCOPE* Effective area (wavelength dependent) Pcinting accuracy Aspect determination Focal length, F Focal plane dispersion Focusing cone maximum angle, 2 a Resolution on axis 7, off axis 15 off axis 20 off axis 200-1000 cm2 $ 30 arcsec f 1 arcsec 10 50 darcsec 7 1 arcsec 10 arcsec 30 arcsec 1 arcmin PARAMETERS OF CYLINDRICALLY FOCUSING SPECTROMETER Energy range, E 0.5 to 7.3 keV Bragg angle range, Bo Crystal length, C 100 mm Crystal radius of curvature,p 200 mm Detector pixel size 0.2 mm x 3 mm Equivalent spatial resolution element 1 arcmin Simultaneously recorded energy band ( & / ' E & 20% to 5% 25' to 70' * These data are taken from AXAF Science Working Group Report, NASA TM-78285, May 1980. 537 ORIGINAL PAGE IS OF POOR QUALITY Fig. 5. Spectrometer dimensions corresponding to labels in Fig. 3: length A, bredth B, focal point to crystal distance L, minimum crystal lerigth C, and crystal height H . All scale wiiA the radius cf curvature and have been computed for P = 200 nm. Fig. 6. Spectral resolving power limits imposed by aspezt drift (30 arcsec), telescope blur diameter (2 arisec) and detector limiting spatial resoiution (0.2 mm). The heavy curve combines these effects in quadrature and represents the geometric limit on resolving power for a point cosmic source. (Crystal resol.ring powers are shown in Fig. 7 ) . The dashod curves show degraded resolving power due to source angular extent in the dispersion plane of 10 arcsec, 30 arcsec, 1 arcmin (corresponding to focal plane image widths of 0.5, 1.5 and 3 mm). Telescope blur diameter degrades ap- proximately as the square of the off-axis angle and it is this effect, combined with abberations cf the crystal focusing, which provided the indicated limits on source extent orthogonal to the dispersion plane. 5 38 Shown in Figs. 7 and 8 are resolving power and integrated reflectivities of several crystals (6-11) that can be used to cover the energy range from 0.5 to 7.3 keV. Spectrometer re- solution can be limited by instrumental effects that must be combined with the crystal resolution. This is illustrated in the case of LiF (whose crystal resolving power is greater than 10 by showing in F.g. 7 the instrument resolving iilnit for a point cosmic source (from Fig. 6). The curves for ACP and RAP are similarly modified. When viewing an extended source a field of view of 1' x 1' will degrade the rtsolution to between 150 and 400. 4 B. Sensitivity A spectral feature that contributes signal to a given detector pixel will be superimposed on ar, x-ray continuum (from bremsstrahlung, recombination, etc.) and a non x-ray background (from radiation belts, cosmic rays, etc). After an observing time t, the number of continuum photons re- corded in a pixel of energy width AEp from a point source contributing a flux FC(E) is where f is the fraction of the source imaged on the entrance slit and Aeff is the effective area Aeff = (A/2a)Fcn whern A/2 ais the everage telescope collecting ---ea per unit Bragg angle, and rl is the proportional counter sensitivity. If nb is non x-ray background count rate per detector pixel then the cornbilled background is NB = NC + nbt. The response to a line of strength (FL AEL) will be where it is assumed that the line covers p pixels. If he line is broad ( A E L > instrument resolution AE) then p = AEL/AEpi if the line i3 narrow p = AE,'AEp. I,-.* integrated line strength can be expressed in terms of the continuum, FC and an equiva- lent line width, 6EL; 3JY ORIGINAL PAGE IS Of POOR QUAL17 L i .. i. I .L. .. Fig. 7. Measured single crystal resolving powers for bendable crystals useful in the energy range 0.5 to 7 keV operating at Bragg angles 25O < e o < 70°. RAP TAP ; ADP (distorted crystal); Graphite ; LiF . In many cases the resolving power will be limited by instrumental effects (Fig. 6 ) as illustrated by the solid curves for LiF (whose crystal resolving power is greater than 10 1 ADP and RAP. 4 Fi3. 8. Measured integrated reflectivitips for crystal use- ful in the energy range d . 3 to 7 keV used at Eragg angles 25 < 8 < 70' . R A P , TAP, ADP ; PET ; LiF ; Graphite ; LiH, WS2 (values for LiH and WS2 are approximately one third the theoretical mosaic values). Fig. 9. Expected effective detection areas, A,ff =ARcn/afo;- the proposed cylindrically focusing spectrometer at the focal plane of the AXAF. Effective area falls off at low energy due to proportional counter window transmission. For comparison equivalent effective areas for t h e HEAO-2 foca l plane crystal spectro- meter (FPCIS) are shown (dashed curves). 540 If the line is detected in p channels, the minimum detectable number of counts signal N L can be written according to a 40 It is assumed that the uncertainty in the background measure- merlt is made negligible by sampling many background channels on either side of the line. Therefore for one channel criterion, P N ~ , ~ / ~ ( ~ N ~ m) = 4, 9s where u ( P N ~ , ~ ) = P 112 (NL,m +N p;z* The minimum detectable line flux is -2 -1 F ~ s ~ = P ~ L , ~ /ifAefft) photon cm s , and the minimum detectable equivalent width is ( 1 7 ) The last quantity ( a measure of signal to noise) is mini- mized by chosing p = 2 which is the smallest number of pixels consistent with proper sampling of the line width. using the parameters listed in Table 2. We have set f = 1 , p = 2, and used a AEp < in Fig. 9. They are used to calculate the minimum detect- able line fluxes for a 1000 sec observation that are plot- ted in Fig. 10. Equivalent widths in keV can be obtained from the minimum fluxes by dividing by the ccrresponding continuum fluxes. The sensitivities are valid for line width AE ( 0.5 CUI" s-l keV-l) the line sensitivity is signal limited and is valid for all line widths if the pixels are binned such that p = 2 effectively. For the Si line at-2000 eV the gain in sensitivity using a mosaic graphite crystal versus a PET crystal is illustrated. Sensitivities for three sample energies are calculated A E = E/S. Effective areas are shown < 2AEp, but at low continuum flux levels I 54 1 TABLE 2 PARAMETERS USED I N CALCULATING SPECTROMETER SENSITIVITIES FeXVII S i X I ' . S i X I V FeXXV Enemy E (keV) 1.0 2.0 2.0 6 . 7 Disperser TAP PET Graphite L i F Bragg Angle (deg) 28 45 6 7 . 5 27 .5 Instrument E f f e c t i v e Area A e f f (cm2) 1.0 1.2 12 0.5 Crys ta l Resolut ion Instrument Resolut ion A€ (eV 1 2.6 3 1 6 (eV> 2 . 6 3* 1 ~ 0 . 5 % (eLf ' 1 1 1 3 hEP P i x e l Energy bi idth P i x e l Length ml 1 0.4 0.6 0.4 Pixe 1 Width- mm 1U 10 10 10 1.2 1.8 1 M -4 -1 Background per p i x e l nt (10 s i 2 . r Simultaneous p i x e l s I 200 500 330 5 00 Extended Source Frac t ion f 1 P ixe l s per L ine F i d t h P 2 Radius of Curvature P mm 2oc Detector Length 2c m 200 7 * Energy reso lu t i on for a mounted and v ib ra ted PET c r y s t a l ; Best. PLT c r y s t a l s show reso lu t i cns of 0.7 eV . 7 H The E i n s t e i n (HEAO-2) p i x e l area i s about 1 cm2 and back- grounds are scsled from referecce 1. 542 ORIGINAL PAM IS OF POOR QUALITY w - n I 10-3 4 I I 1 1 1 1 1 1 1 1 I I I I l l l l I I I 1 1 1 1 10-2 lo-I I 10 CONTINUUM FLUX. %(E) (Cm'* 8-' W-', Fig. 10 Minimum detectable line fluxes at 4 a level for the proposed cylindrically focusing spectrometer at focal plane of AX4F ( l o 3 sec observation) and for the HEAO-2 f w a l plane crystal spectrometer (10 sec observation). These sensitivities can be compared directly since the AXAF telescope has about ten times the co1lec:ion area of Einstein. 4 C. Comparison with Einstein Focal Plane Spectrometer. Figs. 7 and 9 also show curves of resolving power and effective area for the Einstein foc;tl plane spectrometer ( 1 2 , 1 3 1 . The Einstein instrument suffers from severe focusing aber- rations ( 1 2 ) . It also has an order of magnitude wcrse re- sclution loss associated with drifting or extended sources and a more complicated spectrum readout. The crystal must be rocked and scanned during spectrum integration. CCIS, how- ever, can achieve more uniform crystal limited resolving powers while still maintaining a large effective area per spectral resolution element. In Fig. 10 minimum detectable line fluxes for IO4 sec Einstein observations are compared with our 543 predicted performance for a lo3 sec observation with CCIS at the focus of the AXAF telescope. At 1 to 2 keV the A X A F tele- scope will have about 10 times the collection area of the Einstein telescope and, therefore? the curves may be taken a3 a comparison of the sensitivities of the two spectrometers. Thus, CCiS achieves comparable or better sensitivity per smaller resolution element while recording simultaneously several hundred channels of resolved spectrum thereby giv- ing it a distinct throughput advantage. - IV. THE CONCEPT IN PRACTICE. The components of a high resolution cylindrical focusing Bragg crystal spectrometer have been developed an2 fabricated. The instrument is currently operating at Alcator, the MIT tokamak, where it is used to obtain high quality, high resolu- tion spectra emitted from the hot (approximately 2 keV) pulsed plasma. The instrumeat has a large ( 1 6 . 9 x 9.6 cm) PET crystal, (2d = 8.742 A ) bent to a radius of curvature of 60 cm, that is used at a Bragg angle of 3'1' to diffract MOL radiation near 2500 eV (4A) with a dispersion of-2 mA/mm. The energy range detected by the 250 mm long position sensitive proport- ional :ounter is 350 eV. The resolution of the system is limit- ed by the present detector to a value of E / A E = 400. Data ac- quisition and analysis is controlled by a minicomputer that provides a real time plasma diagnostic. Test spectra covering a 300 eV range from a Mo target x-ray tube have been recorded on film and they show the expected line widths for MOL lines (Fig. 11). Spectra have been recorded for several different plasma conditions at Alcator A. The main spectral features in the re- gion around 2.5 keV are MOL x-ray emission and K emission from Helium-like C1 and S. A typical spectrum is shown in Fig. 12. The strong doublet at 4.4A is the resonance line 2p + 1s from Helium-like C1 and the broader feature is a superposition of emission linea from Mo 24+ to 30+. 544 Fig. 1 1 . Photographic piate and densitometer trace of a spectrm of a Mo target X-ray tube obtained with a cylindrically focusing spectrograph. CHANNEL NUMBER ?ig. 12. Data from Alcator 545 Photographic spectra ( 1 4 ) have also been obtained with a smsl- ler ( p = 5 cm) spectrometer using mica to disperse x-rays emit- ted in the 10 - 1 1 A range from glass microballoons imploded by a two-beam Nd: Glass laser system at the University of Ro- chester, Laboratory for Laser Energetics. The results are shown in Figs. 13 and 14. Of particular importance is the trace or- thogonal to the dispersion direction shown in Fig. 14. It illu- strates the excellent spatial resolution by displaying the brightening of the target poles caused by the two incoming laser beams perpendicular to the cylinder axis. Fig. 1 3 . Fig. 14 J73q Spectra obtained from curved and flat mica spectrographs. A comparison of collecting areas. A trace orthogonal to the dispersion direction of a silicon line from a target of diameter 7 0 ~ . These laboratory results show clearly the many advantages of the CCIS system aiid demonstrate its potential f o r cosmic x-ray spectroscopy. - V. CONCLUSIONS The rich scientific yield from the spectrometer instruments carried by Einstein have emphasized the need to evaluate carefully the spectroscopic reqcirements of future missions. Throughput, spatial and spectral resolution, and wavelength range are variables in any combivtion of telescope fncal plane spectrometer concept. For a given dispersing element 546 the question of throughput (i.e. sensitivity) is largely de- termined by the collecting area of the concentrating telescope. fbesoh~tions, both spectral and spatial, are determined by the re- flecting properties of the crystal and by the geometrical pro- perties of the combined optical system. The upper limit on energy is usually set by the reflection efficiency of the telescope. The dispersive spectroscopic systems on Einstein (transmission grating and Bragg spectrometer) require a high angular resolution telescope. As a result of the geometry of the optical system these instruments provide only modest spectral resolution and are almost totally ineffective when viewing extended sources. It is difficult to couple these instrwents to any of the lot9 resolution ( 1 arc min.) high throughput LAMAR s3stems without a severe degradation of performance. The telescope/CCIS optical system is flexible with respect to both telescope resolution and source extent. In common with other good design.:, CCIS will achieve crystal limited spectral resolution when it views the image of an on axis point source formed by a telescope having a high resolution. The distinguishing feature of CCIS is that it will achieve sufficient resolution to resolve the structure of the He-like eaission structure when it views the image of an extended source formed by a telescope having only modest resolution and, in addition, it will provide simultaneously the intensity distribution across the source. 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Schnopper, and P.O. Taylor, Rev. Sci. Instrum, 50, 1609 (1979) SPECTROSCOPY I N THE 10 keY TO IO Me" RANGE JAMES L. MATTESON C e n t e r f o r A s t r o p h y s i c s a n d Spnct, S c i e n c e s U n i v e r s i t y of C a l i l o r n i a , S a n Diego ABSTRACT Spectral l i n e s i n t h e 10 keV t o 10 MeV r a n g e c a r r y i n f o r m a t i o n of fundamc:ntal i m p o r t a n c e o n many of t h e objects d i s c u s s e d a t t h i s w r k s h o p . S i n c e t h e l i n e s are d i r e c t l y re- l a t e d t o s p e c i f i c p h y s i c a l processes t h i s i n f o r m a t i o n is model i n d e p e n d e n t a n d g i v e s t h e p h y s i c a l c o n d i t i o n s i n t h e ob jec ts . A t t h e s e n s i t i v i t i e s a c h i e v e d t o da te , Fu t o lo-' ph/cmL- sec f o r s t e a d y s o u r c c s a n d % s o u r c e s , l i n c s have h e e n detected f rorn t h e ga lac t ic c e n t e r , gamma-ray b u r s t s a n d t r a f i s i e n t s , X - r a y p u l s a t o r s , t h e Crab p u l s a r a n d s o l a r f l a r e s . F u t u r e i n s t r u m e n t s w i t h a f a c t o r of % 100 s e n s i t i v i t y improvemen t w i l l allow d e t a i l e d s p e c t r o - scopic s t u d y of thcsc. classes of o b j e c t s a s well ah s u p e r n o v a r e m n a n t s , a c t i v e g a l a s i e s a n d the in t e r s t e l l a r medium. T h i s s e n s i t i v i t.y i n p r o v e m c n t c a n be obta ined t h r o u g h t h e u s e of' detector technology already p r o v e n i n b a l l o o n and s a t e l l i t e i n s t r u m e n t s . t o 1 p h / c m 2 - s e c for t r a n s i e n t 1. INTRODUCTION O b s e r v a t i o n s of spec t r a l l i n e s i n t h e h i g h .energy ranp ;e , 10 keV t o 10 M e V , a r c direct ly related t o t h e u n d e r s t - a n d i n g of man!: of t h e c lasses oE ob jec t s discusses a t t h i s w o r k s h o p . N e u t r o n stars, black h o l e s , s u p e r n o v a r e m n a n t s , t h e i n t c r - s t e l l a r n i e c i i u m , t h e g a l a c t i c n u c l e u s a n d a c t i v e g a l a c t i c n u c l e i are known o r p l - e d i c t e d t o be s o u r c e s o f sy2c t ra l l i n e s w h i c h c a n bc s t u d i e d w i t h h i g h - e n e r g y s p e c t r o s c o p y . L i n e f o r m i n g processes w i t h p h o t o n e n e r g i e s a b o v e 10 kcV are a n a t u r a l c o n s e q u e n c e of t h e > 10'1: t e m p e r a t u r e s a n d > 10" g a u s s f i e l d s w h i c h o c c u r i n o r n e a r these ob jec t s . T h e l i n c s are d u e t o e l e c t r o n - p o s i t r o n a n n i h i l a t i o n , c y c l o t r o n p r o c c s s e s , r ad ioac t ivc d e c a y , a n d n u c l e a r d e e x c i t a t i o n f o l l o w i n g i n e l a s t i c c o l l i s o n o r n e u t r o n c a p t g r e . S i n c e t h e l i n e s are d i r e c t l y r e l a t e d t o s p e c i f i c p h y s i c a l p r o c c s s e s , t h e y ca r ry niodel- i n d e p e n d e n t i n f o r m a t j o n o n t h c p h y s i c a l c o n d i t i o n s i n t h e ob jec ts : t u m p e r a t u r c , d e n s i t y , b u l k m o t i o n , a b u n d a n c e of i s o t o p i c s p e c i e s , t h e s ta t (? of mat te r , thc e n e r g y s p e c t r u m o f p a r t i c l e s and the inass of a c e n t r a l object . t i o n can be u s e d t o g u i d e t h e o r c t i c a l m o d e l i n g a n d p r o v i d e c r i t i ca l tests of n iodc ls i n order t h a t t h c n a t u r e of the objects may be d e t e r m i n e d . T h i s i n fo rmi l - 550 i)R\GINAL PAGE IS OF POOR QUALlW The major d i 1 f i c u l t . y i n t h e d e v e l o p m e n t of t h e f i e l d of h i g h v n c r g y s p c ( t r o s c o p y b a s b e e n t h c a t t a j n m c n t of s u f f i c i c n t s e n s i t i v i t y . T h c ca r ly p r e d i c t i o n s by h lo i* r i son ( 1 9 5 8 ) i n d i c a t e d l i n e f l u x e s g rea te r t h a n However t h e s t r o n g e s t s t e a d y s o u r c e s h a v e now been ohscrved t o h a v e f l u x e s of % 10“ p h / c m L - s a c a n d so t h e i r d e t e c t i o n h a s o n l y been p o s s i b l e w i t h t h e i m p r o v e d i n s t r u m c n t s ava i l ab le i n t h e l a s t f e w y e a r s . L i n e s h a v e now b e e n detcctec! i n a t l eas t 4 1 d i f f e r e n t ob jec t s r e p r e s e n t i n g 6 c l a s ses of a s t r o p h y s i c a l phenomena . T h e s e i n c l u d e 9 i m p u l s i v e so l a r f lares (Chupp 1981) a n d 27 gamma-ray b u r s t s (Nazets e t a l . 1981) where line f e a t u r e s are c h a r a c t e r i s t i c r a t h e r t h a n e x c e p t i o n a l . p h / c m * - s e c from s t e a d y sources. -- I n t h i s p a p e r I r e v i e w t h e l i n e f o r m i n g p r o c e s s e s , t h e p h y s i c a l i n f o r m a t i o n c a r r i e d by t h e l i n c s a n d t h e i r e n e r g y d i s t r i b u t i o n a n d c x p e c t e d w i d t h s . T h e n t h e o b s e r v e d a n d p re - d i c t e d fluscs arc! u s e d t o d e r i v e t h e i n s t r u r n c n t a l s e n s i t i v i t i e s recll : irctl f o r e f f e c t i v e f u t u r e o b s e r v a t i o n s . F i n a l l y , i n s t r u - men t c o n c e p t s w h i c h c a n a c h i e v e t h e s e s e n s i t i v i t ies are p r e - s e n t e d . 1 1 . PHYSICAL PROCESSES OF 10 keV TO 10 MeV LINE FORMATION I n s y s t e m s w i t h e f f e c t i v e t e m p c r a t u r c s greater t h a n r~ 1 0 R K Lind/or rnngnctjc. fjelds g r e a t e r t h a n 10” g a u s s l i n e emission abo\ro 10 kcV w i l l o c c u r b y s e v e r a l processes. l’hcse h a v e bccn rev iewcci by Hamaty (1$I78), Rnrnaty a n d L i n g e n f e l t e r (1979) ant1 R a m a t y .-- et. - - a l . (1981). a ) C y c l o t r o n P r o c e s s e s C y c l o t r o n 3 i nc>s r c s u l t f r o m t r a n s i t i o n s k t w e e n t h e Land:rii l e v e l s of c l l e c t r o n s i n s t r o n g magnetic f i e l d s . T h e 250 keV ! i n e i n t h e b i n a r y X-ray p u l s a t o r Her X - 1 is i n t e r - p r e t e d as d u e t o e i t h e r c y c l o t r o n a b s o r p t i o n or e m i s s i o n a n d i n d i c a t e s a f i c l d o f %5 x 1 0 l 2 g a u s s ( T r u m p e r -- et a l . 1978). T h e s h i f t of t h e l i n e c n c r g y w i t h p h a s e of t h e 1.24 see p u l s a t i o ~ i ( G r u b c r ct. a. l 9 S O ) is n p p a r e n r l y d u e t o beamed emission f r o m d i f fFi;t.nt f i e l d r e g i o n s b e i n g r e s p o n s i b l e f o r t h e pulscid emissior.. C y c l o t r o n a b s o r p t i o n h a s been o b s e r v e d a t % 20 kcV i n the t r a n s i e n t b i n a r y S-ray p u l s a t o r 4U0115+63, r e q u i r i n g a f i e l d of 2 x 10” g a u s s (Whcaton et al. 1980). T h e % 50 kcV a b s o r p t i o n f e a t u r e s obse rved i n gamma-ray b u r s t s h a v e bevii i n t e r p r e t c d as due t o c y c l o t r o n p r o c e s s e s i n d i c a t i n g t h a t h i g h l y m a g n c t i z c d nc i i t i*on stars a r c the site of t h e b u r s t s ( h l a z c t s ct .- al. _- 19S1 ) . S i n c c the c y c l o t r o n 1i.tw w i d t h is a s c n s i t i v t . f u n c t i o n o f the m a g n e t i c f i e l d g c o i n e t r y (Hussard l 9 S O ) , future? high s c n s i t i v i l y s p c c t r o s c o p i c o b s c r v a t i o n s will dctci.mir:c t h c magnetic f i e l d s t r u c t u r c i n thc e m i s s i o n regions of a11 tt :cDse 0bjc.ct.s and its tcniporal t x v o l u t i o n i n t h e case of -- ORIGINAL PAGE IS OF POOR QUALITV 55 1 I gamma-rpy b u r s t s . A s e n s i t i v e s e a r c h f o r c y l o t r o n l i n e s i n X-ray b u r s t e r s a n d g a l a c t i c b u l g e s o u r c e s c o u l d e s t a b l i s h w h e t h e r these objects c o n t a i n h j g h l y m n g n c t i z e u n e u t r o n stars a n d d e t e r m i n e t h e i r r e l a t i o n s h i p t o X-ray p u l s a t o r s a n d gamma-ray b u r s t s (Lamb 1981). b ) E l e c L r c n - P o s i t r o n A n n i h i l a t i o n P o s i t r o n s are p r o d u c e d by pa i r p r o d u c t i o n , i n e l a s t i c co l l i s ions a n d r a d i o a c t i v e d e c a y of n u c l e o s y n t h e s i s products. Since t h e 511.003 keV a n n i h i l a t i o n q u a n t a h a v e an e x a c t l y known e n e r g y i n t h e rest f r a i n e , s p e c t r o s c o p i c o b s e r v a t j o n s c a n pi*ovidc a g r e a t d e a l of i n f o r m - t i o r , . L i n e b r o a d e n i n g wou ld i n d i c a t e t h e t e m p e r a t u r e a n d / o r K c p l c r i a n or r a d i a l m o t i o n of t h e a n n i h i l a t i o n s i t e . When p o s i t r o n lifetitlie i n f o r m a t . i o n e x i s t s t h e l i n e w id th c o u l d be u s e d t o d e t e r m i n e t h c d e n s i t y of t h e p o s i t w o n s l o w i n g down r e g i o n a s u e l l a s limit t h e mass of a c e n t r a l ob jec t . A l i n e c e n t r o i d s h i i t wou ld i n d i c a t e a b r a v i t a t i o n a l r e d s h i f t o r a Dopplcr red- or b l u c s h i f t . F i g u r e 1 shows t h e g a l a c t i c c e n t e r 511 kcV l i n c m c a s u r e d b y PEAO-3 ( R i e g l c r g . 1 9 S 1 ) . T h e 2.5 keV l i m i t o n t h e l i n e w i d t h a n d o b s e r v a t i o n of v a r i a b i l i t y i n 6 months r c q u i r c , ‘I’ < 7 x lo4 K and a c e n t r a l m a s s < 2 x 10’Mo. r e m a r k a b l e fcatdre of t h e g a l a c t i c c e n t _ r a n n i h i l a t i o n rad ia- t i o n is t h a t . i ts l u m i n o s i t y , 5 x 35,’ erg/sec, is a factor. of % 200 g r e . x t e r t h a n t h a t o f t h e ? , r i g h t e s t 1-4 kcV sourcc’ i n the GCX conip lox ( \Sa t son et - -- al. 1981). F u r t 5 e r t h c o r e t i c a l and o b s e r v a t i o n a l s t u d y of the a n n i h i l a t j o n r a i A , . i o n wi 11 be c r u c i a l t o the u n d e r s t a n d i n g of ‘he g a l a c t i c n u c l e u s . A l h e r e d s h i f t e d a n n i h i l a t i o n l i n e s s e e n i n t h e spectra of gamma-ray b u r s t s a t 400 kcV can obta in u p t o 10 p e r c e n t of t h e b u r s t . t’ne:.gy a n d c a n v a r y o n shorter t i m e s c a l e s t h a n t h e t o t a l I u r i i i n o s i t y (hirtzets al. 1981) Ramnty c t a l . (1980) h a v c i n t c . r p r c t c d the> 400 kcV l i n e i n t h e 5 March 1..~79 burs t a s d u e a n n i h i l a t i o n of p o s i t r o n s p r o d u c e d by p a i l . p r o d u c t i o n i n t h e rnd i a t ion d u i n i n a t e d , h i g h t e m p e r a t u r e a t n i o s p h c r c of a n c u t r o n s tar w h i c h is h e a t e d t o T % 109K and m a g n e t i c a l l y c0nf inc .d . Conf i n c m c n t is n e c c s s a r y t o p r o d u c e g r a v i t a t i o n 3 I ly recishi f ted r a t h e r than Doppler b l u e s h i f t e d a n n i h i l a t i o n rad] a- t i o n . P a i r productjon s h o u l d o c c u r when a r a d i l l t i o n f i e l d ’ s c h a r a c t e r i s t i c t e m p e r a t u r e e x c e e d s % 109K. S i n c c t h i s is o b s e r v e d t o o c c u r i n a c c r e t i o n f u e l e d e m i s s i o n n e a r b l a c k h o l e s s u c h as Cyg X - 1 a n d those t h o u g h t t o e x i s t a t t h e c e n t e r s o i act,ve g a l n s i e s , these objrcts s h o u l d be s o u r c c s of 511 kcV p h o t o n s (Lightn lnn 1981). O b s e r v a t i o n s of t h i s r a d i a t i o n wou ld s i g n i f i o : a n t l y a i d modcls of thcse objects a n d i m p r o v c the u n t l c r s t a i i d i n g of t h e i r r e l a t i o n s h i p t o the g a l a c t i c n u c l e u s . ! 552 ORlGlMAL PAGE IS OF POOR QUALITY c ) Radioactive Decay Gamma- ray ines are predicted to be producec by the raaio- active decay of the products of explosive-nucleosynthesis events in supernovae and novae (Clayton 1980, Woosley and Axelrod 1980). Although none of these lines has been dis- covered, the predicted fluxes are within the sensitivity of future instruments based on today's techniques. Since the gamma-rays' flux directly indicates the abundance of the rele- vant isotope, the abundance of synthesized material may be measured in a model independent manner. In contrast, the inter- pretation of atomic lines in the few keV range to determine abun- dances in an expand-ng supernova remnant is a model dependent procedure made partizulary difficult since thermodynamic equilibrium docs nci occur. Earlier at this workshop Shull (1981) dcscribcd these difficulties and the resulting large unccrt::intics in elernmted abundances. Gamma-ray 1 ines csn provide additonal impoTtant informa- tion. Used as tracers they can idcntify the positioiis of the galactic supcrnovae which have occurred in the past 300 years This does not appear to be possible in any other wavelcngth band. l'he Doppler bro:idening of the lines give tile expansion velocity of the core oi the supernova remnant, not the velocit) of the shock-heated outer envelope which is measured optically. Ratios of gamma-ray lines give the density of the cnvc lope which overlays the synthesized material (Clayton 1974). Long Jiv.:d isotopes indicate the total rate and sites of Kalactic nucleosynthesis over the last *u 10' years. These observations would provide detailed .tests of the highly developed models of supernova and nova explosions and make the study of young galactic supcrncva available to other wavelength ranges. d) Nuclear Decxcitation Gammn-ray line production by nuclear deexcitation is expected whenever the particle energies exceed the threshold for nuclear reactions, typically a fc * MeV. The expected nucl~ar ganma-ray emissivities for tet.iperal:ures in the 10d-10'2€i range havt- been calculated by HiEdon and Lingen- felter (1977). Ramaty -- 2t al. (1979) and Ramaty -- et al. (1981). Since several huqdred MeV/nucleon must be released ,n accretion onto a neutron star, it is espccted that nuclear reactions should be a si1:ni ficant process near theac objects. Neutrons would be produced by many cf t h e I-c-actions and inter- act themselves by '-.:lastic scattering and neutron capture. Thc result nf a11 t;le.,e reactions would be excited nuclei vhich wol;ld produce characteristjc nuclear gamma-rays. Thcse indicate til:. composition of ; . -e medium as well 3s the enerqy spectrutr of the fast particles. Line widths indicate the I 553 Onl::;lrr :.. i As; . :< OF PC2R Q’JALt2 ’ s t a t e of matter s i n c e l a r g c DoppI.?r broadening occur? J . I lines from a n ambictit gas w h i l c small broadc? l l ing occurs i n d o s t and s o l i d s . P a r t i c l e bc;iiuing r e su l t s i n a D o p p l e r s h i t t of t h e l i n e e n e r g y due t o t h e recoil of t h e excited Iiirclci. g r a v i t a t i o n a l r e d s h i f t ’nd ica tes t h e M/R v a l u e of the c e n t r a l object. S i n c e the n c w t r o n t h e r m a l i z a t i o n t i m e is d e n s i t y dcqwndcAnt , tht. t imc. :lt>lay d ncutron c:~ptui-c? g;inima-rays i n d i - c a t w tht . a r n b i c n t d c ~ n s i t p . A 554 ORlGlNAL PAaE 1s OF POOR QUALlrV 555 Also s h o w n i n Figurc: 5 a re t h o i n s t r u m e n t s e n s i t i v i t i e s a c h i c v c d t o (late iind t h o s v r e q u i r e d for f u t u r e i n s t r u m e n t s . The lfE:AO-- 3 i n s t rurnctn t ( hl:ihoncxy ct a 1 . 1980 j used 4 l a rge c o a x i a l G ( b d e t c ~ c - t o r s i n an i n s t r u m e n t o p t i m i z e d a t od 1 MeV. Below "L GO kuV t h c i n s t r u m e n t was c u t o f f duc to d e t e c t o r dead laycbt-s. The s c . n s i t i v i t y v a r i e d from 'L t o lo-' ph/cm'-: .c~c, dcpt3nJii:g o n e n e r g y . S i n c e HEAO-3 was a s c a n n i n g syaccc!-a f t , '-<, 30 d:::.-s wrc? I-cqui rcd t o observt. a source d u r - i ng which ''< 100 h o u r s of e f f e c t i v a t i m e o n a p o i n t sourcc occ-urrctl. l'he s e n s i t i \ ? i t y that p r c s e t i t b a l l c o n ins t i -umcxnts w h i c h arch o p t i m i z c d : iL 1- 1 X1.V c a n o b t a i n i n a s i n g l e b a l l o o n f l i g h t ( n o t shown i n F i ~ u r c l 5 ) is a f;ictor of ' L 2 wrsc t h a n t h a t of l l l~ l \O-S. Thcb s c l i - s i t i v i t y of prc-scr;t ba l loon i n s t 1-u- m t h n t s w h i c h ;in' o p t i m i z t ~ d at r ' . GO kc>!', also s h o w n i n F i g u r c ~ 5. is 2 s IO-* phjcm'-scc: a t 60 1;cV. B a l l o o n i n s t r u r w n t s p1: inned for thcx t i c t s t . 5 ycai -s \ v i 11 hnvcx n ictor of % 4 impro\vemcnt ovcr t h i s stwsit ; \ . i t ? - . 2. 0 b s t : r v a t i o n s are conducted f w i i i spacc and i n m i s s i o n s w i t h y e ; i r s (11 d u r ~ t i o n . T h i s wi 1 1 n s s u r c t h a t ;i 1ai-g~. number a s wcll a s discovcxr 1 i nc exission f r o i n u n s u s p c c t c d sources. t objects ;ire s t u d i e d i n o r d c r t o 1cDnrn tlicir c1;tss p r o p e l - t i c s 556 OAIG~NAL PAGE IS OF POOR QUALIW The following cxamplcs indicated illustrate these requirements. The gamma-ray burst in which Tcegarden and Cline (l9SOj dctected redshifted line emission from '"Fe at 3.50 signjficznce had a total energy flux of 3 x lo'' erg/ sec. Since bursts of this intensity occur only Q once/year (Jennings and White 1980) b6th greater sensitivity and years of observation arc required to obtain high quality spectro- scopic data on a larfze nunibcr of garmaa-ray bursts. A t 100 times better sensitivity > 205 detections of the 847 keV Fe line would be expected in 10 t o 20 bursts per year and detailed analysis could be performed on the class of gmn-ray burst sources. Similar considerations a p p l y to steady sourc's. For example, the pulse-phase rcsolvcd study of the Her X - 1 cyclotron line requires 10 times the present sensitivity and t h e opportunity to observe the entire 35-day cycle of Her X-1. brighter than a few UFU requires an additional factor of Q 10 sensitivity improvement. A similar study of the % 100 galactjc objects V. CONCEPTS FOR FUTURE SPECTROSCOPY INSTRUMENTS Thc scnsit ivity requirements dcvcloped ab0L.e can be fulfilled by a combination of thrce instruments which are based on tcchnology u s e d in tuday' . s balloon and satelli tc instruinen:s. E i I C h instruInCrit is optirnizcd fcr 2 specific energy range and f icld of v i e w . A l l the dctc.ctors ir-ould be germanium sinc-c it is capable ol resolvinz ncarly all the expc .ctcd linc>s and is available t o d a y in large volume gcometries, 2. 150 cm' . T h c o b j c h c t s for which the instru- m e n t s arc: suited, their physical Imi-;imcters and sensitivities :? 1 gjv(bn in Tahl(. 1 . The. scBnsitivit i p s c l f the instruments Instrument (1) is for the s tudy of (quasi-) steady sourc"s of lines in the 10 keV to % 000 keV rar-ge. Optimum sensitivity would occur at 2, 60 keV. A field of view of a few degrees would be proviaed by a passive collimatJr in order to reduce background and source confusion. ilanar detectors would providz 4 0 times the collecting area of present instruments, an energy resolution of 1 keV and minimum background. a l s o indicatzd in Figure 5. Instrument ( 2 ) : s for the study of transient sources. I t would i = ~ l * . : ~ * ~ - the 10 keV t o 10 MeV energy band and abservc tht- t-r.'. ire sky. above a few hunL'ed keV, the volume would be 6C times that of tne Tecgarden and Cline (1980) detector. Unshielded c3- axial detectors would be used in order to have good sensi-tiv- ity abovc a few hundred keV. Since detector volutnc determines sensitrvity 55 7 I n s t r u m e n t (3) is for t h e s t u d y of (quas i - ) s t e a d y s o u r c e s of l i n e s i n t h e 100 keV t o 10 M e V r a n g e . Optimum s e n s i t i v i t y would o c c u r a t a few MeV. The detector volume is 25 times t h a t of HEAO-3 a n d v e r y l o n g o b s e r v i n g times, % 200 d a y s , w o u l d be required t o r e a c h t h e des i red s e n s i t i v i t y . B e c a u s e of t h i s , a wide f i e l d of v iew, % 40°, w i t h a coded a p e r t u r e would be required t o o b s e r v e a n acceptable number of objects d u r i n g a several y e a r m i s s i o n . H e a v i l y s h i e l d e d coaxial detectors w o u l d be u s e d i n order t o h a v e minimum b a c k g r o u n d a n d good s e n s i t i v i t y a t t h e h i g h e s t e n e r g i e s . A l l t h e i n s t r u m e n t s w o u l d r e q u i r e c o o l i n g of t h e detectors t o % 90K. On HEAO-3 t h i s was done w i t h a s o l i d c r y o g e n r e f r i g - erator w i t h l ifetime of o n l y Q 6 m o n t h s . M e c h a n i c a l r e f r i g - erators h a v e also b e e n u s e d f o r ge rman ium de tec tor c o o l i n g i n space (Nakano a n d Inihoff 1978), b u t h a v e h a d prol:,lems w i t h r e d u c e d e f f i c i e n c ; ir, e x t e n d e d l i f e a l s o c a u s e d m i c r o p h o n i c n o i s e i n t h e detector s i g n a l . The problems w i t h mechafiical refrigerators c a n be s o l v e d w i t h a mw..iiBst d e v e l o p m e n t program a n d i n the 1990's they t r i l l be t h e preferred m e t h o d of a c h i e v i n g t b n r c q u i r e d detector temperatures . R a d i a t i o n d a m g e was 8 problem w i t h t h e p - t y p e IIEAO-3 d e t e c t o r s (Xahoney ~t _ _ a l . - lOY1). I iowever , t h e n - t y p e d e t e c t o r s w h i c h are y ' 'w c o m m e r c i a l l y a v a i l a b l e a r e u n a f f e c t e d by r a d i a t i o n damage i n a l o w e a r t h o r b i t . T h e s e detectors a l s o have a n c g l i - g i b l c dead l a y e r , so t h e y o y n r a t e f rom < 10 keV t o > 10 MeV. V I . CONCLUSION F r : + n ~ - ~ ~ o b s e r v a t i Q n s of spc- t r a l l i n e s i n t h e 10 keV t o 10 hlcV rallde h i l l be of f u n d a m e n t a l i m p o r t a n c e i n o u r u n d c r - s t a n d i n g of a wide v a r i e t y of objects a n d phenomena which h a v e b e e n d i s c u s s e d a t t h i s w o r k s h o p . T h e s e i n c l u d e n e u t r o n s t a r s &nd b lack hi les a n d p r o c e s s e s n e a r t h e m , e x p l o s i v e n u c l e o s y n - t h e s i s a n d s u p e r n o v a r e m n a n t s , so l a r f l a r e s , t h e i n t e r s t e l l z r medi 'I, t h e g a l a c t i c c e n t e r and a c t i v e ga lac t ic n u c l e i . S p e c t r a l l i n e s have a l r e a d y b e e n detected from many of t h c s e by t h c mod- est i n s t r u m e n t s t h a t h a v e b e e n u s e d t o d a t e . T h e s e h a v e a c h i e v e d a s e n s i t i v i t y of lo-' t o lo-' p h / c m 2 - s e c f o r s t e a d y s o u r c e s a n d lo-' t o 1 ph/cm2-scc for t r a n s i e n t s o u r c e s . T h c i n s t r u m e n t c c n c e p t s p r e s e n t e d here w o u l a .?chieve a fac tor of 100 s e n s i t i v - i t y i m p r a v e m e n t , a l l o w i n g d e t a i l s t u d y of many ob jec ts . S i n c e t h e 1ir.es are d i r e c t l y r e l a t e d t o specific p h y s i c a l p r o c e s s e s , model i n d e p e n d e n t i n f o r m a t i o n o n t h e p h y s i c a l c o n d i t i o n s i n the ol1,ject.s r e s u l t s a n d t h e c lass properties a n d n a t u r e c f t h e ob jec t s c a n be d c t c r m i n e d . 558 ORIGINAL PAGE IS OF POOR QUALITY The instrument concepts are based on dctectcr technology that has already been developed for balloon and sztellite instru- ments. lnstrunents using these concepts would easily be accom- modated by a Shuttle-launched Explorer mission in the 1990's. Since thc scientific return from such a mission would be both large and relevant to a wide variety of problrms in high energy astrophysics it should be given t h e highest priority in the planning of future space missions. 559 ORIGINAL PAGE IS OF POOR QUALITY REFERENCES B u s s a r d , R.W. 1980, Ap. J . , 237, 970. Chupp, E . 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Tru inper , J . , P i c t s c h , W., R e p p i n , C . , V:zes, W., Stauber t , R . , .. and K e n d z i o r r n , E . 1978, Ap. J . ( l e t t e r s ) , 219, L105. Watson, M . G . , ' .Vi l l ix?galc , R . , G r i n d l a y , J.E., a:.d H e r t z , P . 1.981, p r e > 1 . i n t , C e n t e r f o r A s t r o p h y s i c s , t o be p u b : i s h e d i n Ap J . , 1 November 1981. Wheaton , W . A . , D o t y , J . P . , P r i i A n , F . A . , Cooke , B .A .> Dob- son, C . A . , ,d ldman, A . , H e c h t , M . , Hoffman, J . A . , !Iowe S . : < . , S c h e c p m a k e r , A . , T s i a n g , E.Y., Lewin , W . l - . G . , Matteson, J.L., G- u b e r , D . E . , S a i t y , W . A . , R o t h s c t i l d , H.E., K n i g h t , d.K., N o l a n , P . , a n - . P e t e r s o n , L . E , 1979, N a t u r e , - 2 8 2 , 2 4 0 . Woosley, S . E . , A x e l r o d , T . S . , and Weavcr, T.A. 1980, ;re- p r i n t 1 T R L - 6 5 1 1 8 , t o be p u b l i s h e d i n Xi lear P a r t i c l e P h j s i c s . 561 TABLE 1 IIIGH-LKCRCY SPECTROSCOPY I NSTI'\V[IMTS FOR T l l C 1990's ~ ~ ~JCCT/PIiEHD:.!Cl:O:.1 DETECTOR SENSITIVITY 1. X-RAY PULSATOR PULSAR SUPERNOYA ACTIVE GALACTIC iiucLEus BLACK HOLE GALACTIC CEt:TER 2000 LM2 X 1.5 Chl lHlCK PLANAR CE ARRAY Fcw FOV UP TO 30 DAY ~X.ERVATION 10 T O 600 KEV 2003 cn' GE COAX ALRAY 3, Sl 'P tRNOVA 1O,C)c10 ut3 GE cokx ARRAY l H r E R S T E L L A R 1;CDIUM 1.40~ rov 3Xlo-G PH/CM2-SEC 3 60 KEV, c 3x1W5 PH/CM -SEC 2 511 KEV, SPECTRA OF 1 UFU SOURCE, PtlASE RESOLVED SPECTRA FOR > 10 UFU SOURCES. DISCOVER YOUNG GALACTIC SUPERNOVA V I A 7 -RAY L INES < 600 KEV. D ~ S C O V E R 511 Y.EV ~ . Y I S S I O I I F ROM ACT I VE GALACT 1 C NUCLE 1 LINCS FRCM G!\Lt.CTIC AND SEARCK FOR UhSUSpFCTED 1.1' POSIT I O N DLTI:K;?i NAT I O N SOURCES OF G,v.m- RAY S T N S l T I ' ; i TO P O I N T AND E X I R A t A L A C T ! C SUPERNOVAE. LINES D I F F U S E SOJRCES MEASURE AND NAP Tti; INTERSTELLAR 200 - D A Y 0% E RVA T 1 ON GAHtNA-RAY LINES. 100 KEV TO 10 K t V DISCOVER UNSUS~TCTEI) SOCRCES 01: CAWl4-RAY LIKES I 562 0.016 0.014 0.012 = 0 t s c 0 0.010 $ 0.031) Q, s c 0.035 3 0 3 0.004 ‘A c. q 0,032 0.000 e -0 .om ORlGlNAL PAOE IS OF POOR QVALITY ENERGY (keV) 490 500 510 520 530 -0.034 4 09 41 0 420 430 440 CHARNEL NUMBER Figure 1. Tile spcctruin o f t h e g a l a c t i c center r eg ion m c a s u r c d by 1113l0-3 i n Scpt/Oct lLI’i9 (Hic::lcNr -- e t a l . 19Sl). ‘l’he 5 1 1 3 k c V eIcr : t~c~n-posi t : i~on a n i . h i l ; i t . i o n l i n c ? f l u x , (1 . . 8 2 . 2 ) x 10- ph/c.m’ - w c , was f o u n d io bc a f a c t o r of 3 lower wl~cn HSAO-:3 r c w l ~ s ~ r v c ~ d t h c g a l a c t i c cc’ntcr 6 months Inter . The observed litw w i d t h is due. t o thr 2 . 7 keV I’KIiJ.1 eiicrgy resoliltion of thc dctcict o r s . Tlicsc d;it:i c . o n s t r : i i n t h e g,ralnc.t,ic center I i n c wid th 10 less thLit1 2.5 l < e V ( l o ) and 3.4 I i c V (20). 563 ORIGINAL PAGE IS OF POOR QUALITY 16' X 2 16' IO' F: guul'e 2. Thv prcdicl c d spcctrr im of gamma-ray l i nes a n d bwr t i ss t rah lung fi-om t h c J i r c c t i o n of t h c g a l a c t i c center (Harnatp -- ot a ] . 1979, Izigui-c 30). t h e c sc i t a t ion of n u c ' l ~ i i n g r a i n s a n d w i d e l i n c s a r e due t o th( . c i x c i t a t i o n of nyslei i n gas - 1 4 T h e shor t lifetime of t h e r l . 4 : ~ S I ~ V i c ~ \ i or c , 5 x s e c , results in a broad line. a t t 1 ) i s cn('r4:y i n b o t h cases. 2 1 0 - ' ' p l i / c r n ~ - s r ~ , - r n d , LL f a c t o r of q.20 below t h e b c s t s e n s i t i v i t y azh icv t .d t o d ; i t c , but w i t h i n t h o cbnpab i l i t i e s of t!ie f u t u r e i n s t r u i n c t n t:; discussed i n t h i s papc.r. Narrow ljnes a r e due t o The f l u x e s a re t y p i c a l l y 564 ORIGINAL PAGE IS OF POOR QUALlW t i t 4 n a W > w In 5 P w t W W IC - n ENERGY DISTRIBUTION OF HIGH-ENERGY LINES LOG ERERGY (KEW 1 2 3 PROCESS I I C - CYCLOTRON TRANSITION R - RADIOACTIVE DECAY-- -- ! % -i-- OBJECT THE GLLAXY - C? GALACT 1 c CENTER t\’EUTRCH S-IAR G A W A - R A Y BURS (5 SEC) -- I GAx::<:-fiAY T R A k S I E N T (2i) M1:i) A X-PAY P U L S A T O ~ - - PULSAR SUR FLARE -- - ;--c-- N-E-€ SUN s U 3 t R N OV A THE GALAXY A I I 1 INTERSTELLAR K E D ~ U M --- ACCRETION DISK NUCLEUS ---- i - BLACK HOLE I I ! -- I I A ACT I VE GALAXY I I 1 2 3 LOG ENERGY (KEW Figure 3 . T h e energy d i s t r i b u t i o n of l’ncs produced by t h e procesws d i s c u s s e d i n t h e t e x t . and c n c r g i e s of l i n e s observcd and p r e d i c t e d in a v a r i e t y of astronomical o b j x t s . Also shown are t h e p r o c e s s e s 2 0 ! F j g u r c I . The w i d t . h s of l i n e s p r o d u c c d by t h c processc's tlisc:issc.rl i n t h e t c *x t . The p l o t t c d w i d t h of a d c t c * c t c d , b u t n o t ~ * ~ s o l v c ? t l 1 1 1 1 ~ ~ i s a n upper l i m i t to t h c l i n e ' s t r u e w i d t h . Thc cnei-gh- r c s o l u t i c B n of C c and K;iI d c t e c t o i - s a r e a l s o p l o t t e d . I t is o b v i o u s t h a t Gc is rcquircd for d c t a i 1t.d atut l ics which measur(' the w i d t h s and profiles of I i n c s . T h c proct'sscs and SOUI-ccs o f thc l i i i r - 3 31-c i n d j c a t c d by thc not : \ t i o : i X,Y, w h e r e X = process and Y = source, !is g i v e n b e l o w . S o u r c e --P l - 0 c: C? s s c C y c l o t l * o n GC G a l a c t i c center A E l c t c t ron-posi Lron GHR Gmina-ray burs t R Rad i uac- t i v c d c r a y obs(-rved from n b a l l o o n E I n e l n s t i c c s c i t n t i o n SY Sula1. 11;11*c of n: , . l c . i ISM J n t crstc-21;ii' m c d i u a N N C ~ I i'on <.itPtUrc. SNR Supc3riiov:t remnant , i . c. : inn ih i l a t i m GRT 20-min ganmxi-ray t r a n s i e n t e s p l o s i VP nuc I c o s y n t h c l s i s X I V X- r i a ) pii 1hrlt or CP C1*ilt) p t ~ l s i ~ r -~--------. - . . . . - . . 566 ORIGINAL PAGE IS oFpooRQuAuM INTENS11 IES OF HIGt1-LNCRGY LINES k ICCTED, 0 DEIECTED C,\ PREDICTED ANU RESOLVED I I -7 G 1 i_._. 2 3 4 LOG ENERGY (KEW Figure 5 . The i n t c n s i t i c s of high-energy l i n e s . The most i n t e n s e l ines of e a c h class of objcct are p l o t t e d . Also p l o t t e d are t h c s c n s i t i v i t i s of s c t v c r a l i n s t r u m e n t s . (a ) KEAO-3: 400 c m coaxial G c d e t e c t o r array, 30-days of s c a n i i i ng o b w r v a t i o n . !b) Present b a l l o o n instrument: 50 c m G e detector array, 6 h o u r poi n t c d olsei'v;~ t i on. ( c ) Future q p a c c i m t r u n i c n t : I n s t r u m c n t ( 1 ) from t h e t e x t , 2000 c m p l a n a r CP array , 30-d:iy p o i n t e d o b s e r v a t i o n . ( d ) Future spqcc. i n s t r u m c u t :. 1nstr:imtwt ( 3 ) from the t e x t , 10,000 cni coaxial Ce array, 200-day o b s c r v a t i o n . s 2 '2 - SPECTROSCOPY FROM 2 t o 200 keV David J. Relfand, Gary A. Chanan, and Re Novick Columbia Astrophysics Laboratory, Columbia Universi ty Crawford J. MacCallum Sandia Laboratories aad Marvin Leventhal B e l l Telephone Laborator ies ABSTRACT The as t rophys ica l processes responsible f o r l i n e and continuum emission i n the s p e c t r a l range 2 keV t o 200 keV are examined from the viewpoint of designing a spectrometer which would operate i n t h i s regime. Phenomena consi- dered include f luorescent l i n e r ad ia t ion i n X-ray b ina r i e s , magnetically s h i f t e d i ron l i n e s and cyclotron emission from neutron star su r faces , l i n e emission from cosmically abundant elements i n thermal plasmas, and nuclear deexci ta t ion l i n e s i n f r e s h nucfeosynthet ical ly produced matter. An i n s t ru - ment cons i s t ing of a -100 c d a r ray of planar germanium de tec to r s surrounded by a large sodium-iodide ant icoincidence s h i e l d is descr ibed and d e t a i l e d considerat ion of projected background rates and s e n s i t i v i t i e s are presented. A sample observing program f o r a two-day Shuttle-based mission is included as an example of the wide range of s c i e n t i f i c quest ions which could be addressed by such an instrument. 568 I. INTRODUCTION From the serendip i tous discovery of the f i r s t celestial X-ray source less than 20 years ago, X-ray astronomy has ca tapul ted t o the fo re f ron t i n many of the most e x c i t i n g areas of cur ren t a s t rophys ica l research: black holes , supernovae and t h e i r remnants, quasars and o ther ga lac t i c nuc le i , t he i n t e r s t e l l a r medium, and cosmology. The de tec t ion and i d e n t i f i c a t i o n of numerous classes of X-ray epitters, along wtth the recogni t ion and in t e rp re t a - t i o n of the wide va r i e ty of temporal v a r i a b i l i t y they exh ib i t , cons t i t u t ed t h e d r iv ing force behind the e a r l y growth of the f i e ld . More recent ly , t he launch of the Eins te in Observatory extended s e n s i t i v i t e s by a f a c t o r of -lo3 and provided t h e capab i l i t y f o r imaging X-ray emi t t e r s with spatial r e so lu t ion comparable t o that achievable i n the rad io and o p t i c a l regimes. Major mis- s ions planned f o r t he fu tu re include a l a r g e r s o f t X-ray te lescope f a c i l i t y and a satell i te dedicated pr imari ly t o d e t a i l e d t iming s t u d i e s of br ight g a l a c t i c sources. neutron stars, The development of cosmic X-ray spectroscopy has been slower than t h a t of s p a t i r l and teepora l s tud ie s , al though recent advances ind ica t e t h a t t h i s f i e l d holds great p o t e n t i a l for providing important new i n s i g h t s i n t o many areas of as t rophys ica l research. Lines from s i l i c o n , s u l f u r , neon, oxygen, and o the r cosmically abundant elements have been de tec ted from t h e m 1 plasmas i n a var i e ty of s e t t i n g s with the epectrometers on Einstein. Iron l i n e emls- s ion has been discovered i n the spec t r a of galaxy c l u s t e r s , confirming the hypothesis t ha t these X-rays arise from p a r t i a l l y processed material. A t h igher energlee, cyclotron f ea tu res have been seen i n t%e spec t r a of Reveral I 569 I i i ! g a l a c t i c X-ray sources thought to contain magnetized neutron stars including Her X-1 and other b inar ies , the Crab pulsar, pad y-ray bursters . A wide var ie ty of rad ia t ion processes are involved i n producing the high energy photons we detec t from celestial ob jec t s and X-ray spectroscopy ia an e s s e n t i a l t oo l in i den t i fy ing these techanisme and charac te r iz ing the physical environments in which they operate. Bot t h e m 1 and nontherael plasmas pro- duce the in tense continuum and l i n e emission we observe from X-ray binary stars, supernova remnants, and c l u s t e r s of galaxies. Synchrotron r ad ia t ion and inverse Compton emlssion are thought to be responsible for the energy output of ac t ive g a l a c t i c nuclei (e.g., Seyfer t s , BL Lac objec ts , aad qua- sars), while cyclotron rad ia t ion is dominant i n the reglon of in tense magnetic f i e l d s near the sur face of a neutron star. nuclei from novae and supernovae as well as exc i ted nuclei produced v i a the in t e rac t ion of cosmic rays with the i n t e r s t e l l a r medium are expected t o emit c h a r a c t e r i s t i c X- and gamma-ray l ines . F ina l ly the rad ia t ion from a l l of these processes is arodified by the material through which it passes, both in the near source environment and along our l i n e of s igh t t o the source. Rxplosively produced rad ioac t ive The instrumental l imi t a t ions encountered thus f a r I n X-ray spectroscopy can be summarized as the need t o make unsa t i s fac tory t r adeof f s among three hndamental parameters: the reso lu t ion of the instrument, its s e n s i t i v i t y t o l i n e and continuum e d s s i o n , and the t o t a l range of energies over which it can operate. Tradi t iona l proport ional counters (with which the i r o n emission f ea tu res in c l u s t e r s have been detected) have good s e n s i t i v i t y and reasonable s p e c t r a l range, but inherent ly poor resolut ion. On the o ther hand, the high reso lu t ion obtainable vi t h c r y s t a l opecttometers requi res severe s a c r i f i c e s f n s e n s i t i v i t y anti s p e c t r a l range, while s c i n t i l l a t i o n de tec tors (such as the one used t o discover the Her X-1 cyclotron l i n e ) are l imi ted i n r e so lu t ion and, thus, i n t h e i r s e n s i t i v i t y t o narrow l ines . F ina l ly , any spectrometer located a t t h e focus of a grazing incidence te lescope is l imi ted t o the spectral regime below-8 keV. keV . For inst ruments flown t o da te , t he cutoff has been -4 We have designed a wide-band germanium X-ray spectrometer capable of obtaining high r e so lu t ion spec t r a over two decades of energy i n the region of t he spectrum most crucial to our understanding of the o r i g i n of both the l i n e and continuum emission from celestial X-ray sources. A t the lower end of t h e instrument 's s p e c t r a l range, i t o f f e r s an improvement i n energy r e so lu t ion of a f a c t o r of 3 over t r a d i t i o n a l propor t iona l counter experiments, while i n t h e hard X-ray region (50 keV to 200 keV) i t provides a gain i n s e n s i t i v i t y and reso lu t ion of more than an order of magnitude over e x i s t i n g instruments. is t h i s unique combination of s e n s i t i v i t y , reso lu t ion , and s p e c t r a l range which would allow one t o carry out , i n t he br ie f period ava i l ab le on a S h u t t l e mission, an extensive, var iegated observa t ioqa l program addressing many impor- t an t quest ions of current as t rophys ica l i n t e r e s t . Below, we discuss s e v e r a l of t he sources of X-ray l i n e emission which would be detec tab le with t h i s instrument (511) including a summary of previous work and estimates of t h e observing time required t o meet var ious objec t ives . instrument design and develop d e t a i l e d estimtes of the backgrounds expected and the s e n s i t i v i t i e s achievable. In the f i n a l s ec t ion , we comment b r i e f l y on the place such experiments might occupy i n the space science program of the fu tu re . It In 5111, we descr ibe the 571 11. SPECTROSCOPY FROM 2-200 KKV - a) t i n e s from Thermal Plasmas Emission l i nes from highly ionized spec ies of i ron i n the energy range from 6 t o 7 keV have been discovered i n the spec t r a of a v ide va r l e ty of c e l e s t l a l X-ray emi t t e r s including supernova remnants (Charles et al. 19781, c l u s t e r s of galaxies (Mushoteky et al. 19781, cataclysmic va r i ab le stars (Swank et el. 19781, and binary X-ray sources (Pravdo 1978). The latter sources a r e pa r t i cu la r ly i n t e r e s t i n g i n t h a t t h e i r continua cannot be charac- t e r i z e d simply as t h e m 1 emission from an o p t i c a l l y t h i n hot plasma; a super- posed source of f luoresc ing material is required t o account f o r t he l i n e emission. Po ten t i a l loca t ions f o r t h i s material include t h e atmosphere of t h e companion star, a s h e l l a t the Alfven sur face of the neutron s ta r , and an accre t ion d isk (Pravdo 1978). Evidence f o r a dependence of l i n e energy and i n t e n s i t y on pulse and o r b i t a l phases has been reported for seve ra l sources including Her X - l and Cyg X-3 (Becker e t al. 1977; Becker -- et al . 1978). I n addi t ion , lron-abundance determinations f o r the binary sources ind ica t e an enhancement of metal-rich material i n these systems over t h a t found i n the thermal sources (supernova remnants and c l u s t e r s of galax ies ) . This may, however, be more ind ica t ive of our crude knowledge of the e x c i t a t i o n and emission mechanisms involved than of an overabundance of hfgh-Z mhtter. A wealth of Information can be obtained from the determination of prec i se The exact l i n e energies and the meaclurememt of high-resolution l i n e p ro f i l e s . spec ies responsible f o r the emission can be determined, with the r e l a t i v e contr ibut ione of each y ie ld ing informetion on the ion iza t ion balance of t he e l s i t t ing region. In addi t ion , more r e l i a b l e abundance determinations become 57 2 poss ib le , and kinematic condi t ions i n the source can be explored. Such information is c ruc ia l , f o r example, i n choosing among the various models proposed for the source of the f luoresc ing plasma i n X-ray b inar ies . Most of the e x i s t i n g i ron-l ine measurements have been made with propor- t i c n a l counters which have t y p i c a l energy reso lu t ions a t 6 keV of -1 keV. Thus, i n mny cases, the i n t r i n s i c l i n e widths are unknown and, even i n t h e few sources where resolved f ea tu res are seen, the d e t a i l e d s t r u c t u r e of t h e l i n e cannot be determined. The r e so lu t ion of the proposed spectrometer is -370 e V a t 6 keV; t h i s represents near ly a f ac to r of 3 improvement over most proport ional counter experiments and w i l l allow one to pursue many of the important quest ions out l ined above. The s e n s i t i v i t y of the instrument is such that i n less than three hours' abserving time, we could obta in high s ignal- to- no ise (210 u i r o n l i n e s ) , high-resolution spec t r a fo r a l l of t he e igh t bmary X-ray sources known t o exh ib i t i ron f ea tu res , e a s i l y resolving emlssion due t o neu t r a l i ron (Ka a t 6.4 keV) from t ha t of various highly ionized spec ies (e.g., Fe XXV a t 6.7 keV and Fe XXVI a t 6.93 keV). I n 12 hours, we could de t ec t these same l i n e s (at the 5 u l e v e l ) from a dozen thermal source^ inclu- ding four supernova remnants, s i x c l u s t e r s of galax ies , and two cataclysmic va r i ab le stars. Observations over one complete 4.8 hour binary o r b i t of Cyg X-3 would y i e ld high reso lu t ion i ron-l ine p r o f i l e s (at the 20 u l e v e l ) i n each of 30 o r b i t a l phase in te rva ls . A six-hour observation of any one of the ' br igh te r X-ray b ina r i e s could lead t o the f i r s t de t ec t ion of the nucleosynthe- t i c a l l y important n icke l l i n e a t 7.5 keV. Similar durat ion point ings would de tec t the s t rong s u l f u r , s i l i c o n , and neon l i n e s observed i n seve ra l g a l a c t i c SNR with the Einste'.r. So i id S t a t e Spectrometer (SSS), a l b e i t with near ly a f ac to r of 2 worse reso lu t ion . How- 573 ever , the wider f i e l d of view of t he Germanium experiment -sill y i e l d t o t s 1 spec t r a of the l a rge r remnants (e.g., Vela, Puppis A, and the Cygnus Loop) f o r t he f i r s t time. I n addi t ion , t h e simultaneous determination of t he l i n e s t r e a g t h s f o r i ron and the s i l i c o n group elements w i l l be invaluable i n s o r t - ing out some of the abundance anomalies found i n the SSS data. Such broad s p e c t r a l coverage of both the l i n e and continuum rad ia t ion is required f o r determinig the run of temperatures and the r e s u l t i n g ion iza t ion d i s t r i b u t i o n i n these objects . - b) Line Emission from Magnetic Neutron S t a r s The exc i t i ng p o s s i b i l i t y of s tudying the in tense magnetic f i e l d s t o 1013 gauss) a t the poles of a neutron star v i a X-ray spectroscopy was f i r s t pointed out by Gnedin and :'.unvaev (1974) and Basko and Sunyaev (1975). Elec- t rons i n the ionized plasma acci*eting onto the sur face of X-ray pu l sa r s w i l l undergo cyclotron motion i n t he magnetic f i e ld . Trans i t ions between d i s c r e t e Landau l e v e l s can occur, giving rise t o a series of X-ray l i n e s a t t he funda- mental cyclotron frequency and i ts harmonics. Observations of t!iese l i n e s c o n s t i t u t e a d i r e c t measurement of the magnetic f i e l d i n t e n s i t y , and a de ter - mination of the l i n e p r o f i l e s could y i e l d important new information about the s p a t i a l d i s t r i b u i i o n of the f i e l d and the acc re t ing matter. Recently, t h e observat ion of such a l i n e from Her X-1 a t 55 keV was announced by Trumper - et - al. (1977) and Coe et a l . (1977). The l i n e emission is reportedly pulsed, with a f l u x of of A12 keV. The implied magnetic f i e l d i n the line-forming region is 4.6 x 10' 2 gauss. photons (cm2 SI-' and an iv.strumentally determined width The e x i s t i n g observat ions a r e i n s u f f i c i e n t even t o determine whether t he l i n e is seen i n emission a t 55 keV or i n absorpt ion around 45 keV. High 57 4 s p e c 2 r a l r e s o l u t i o n d a t a are t h e only way to choose between t h e s e t w o possi- b i l i t i e s and t o provide the input requi red i n modeling kinematic and plasma c o n d i t i o n s i n and around t h e source. I f t h e repor ted l i n e from Her X-1 is i n t r i n s i c a l l y narrow (-1 keV), it w i l l appear as a 40 a d e t e c r i o n i n a 12-hour , observa t ion with t h e proposed instrument ( a 10 keV wide l i n e w i l l y i e l d a 30 a r e s u l t ) . The e n t i r e 40-70 keV continuum w i l l be measured a t a r e s o l u t i o n of 1 keV and both t h e second ( repor ted a t 110 keV) and t h i r d c y c l o t r o n harmonics should be d e t e c t a b l e . The amount of l i n e broadening due t o Doppler (1978) and s e l f - a b s o r p t i m e f f e c t s (Mesearos 1978) he lps t o determine t h e angle between t h e magneeic f i e l d and t h e observer ' s l i n e of s i g h t . The proposed instrument w i l l be a b l e t o s p e c i f y t h i s angle unambiguously, and its v a r i a t i o n as a f u n c t i o n of p u l s e phase could provide an important c l u e t o t h e geometry of t h e e m i t t i n g region and t h e r e l a t i v e o r i e n t a t i o n of t h e star 's magnetic and r o t a t i o n &xes. The r e l a t i v i s t i c f i n e s t r u c t u r e (Daugherty 1978) of t h e proposed l i n e emiss.'on provides another estimate of the viewing angle. More important ly , k x e v e r , t h e s p l i t t i n g of t h e f i n e s t r u c t u r e l e v e l s (w, , o , wz , 1 , w3 ,*, e t c . ) i s r e l a t e d t o t h e f i e l d s t r e n g t h ; thus , a measurement of t h i s s p l i t t i n g provides a d e t ' t - minat ion of t h e f i e l d s t r e n g t h which is independent of t h e l i n e energy. i n t u r n al lows one t o c a l c u l a t e t h e g r a v i t a t i o n a l r e d s h i f t at t h e e m i t t i n g reg ion , l o c a t i n g i t p r e c i s e l y above t h e s u r f a c e of t h e neutron star (whose mass is known from observa t ions of its binary companion). Such a r e s u l t is w e l l w i t h i n t h e c a p a b i l i t i e s of t h e proposed spectrometer and would y i e l d t h e f i r s t f i r m upper l i m i t on t h e r a d i u s of a neutron star (an extremely important input for c a l c u l a t i o n s of t h e equat ion of s ta te f o r matter at u l t ra -h igh d e n s i t i e s ) as well as prov?ding a weal th of information on t h e magnetospheric T t d a s t r u c t u r e a t the base of t he acc re t ion column (Mesearos 1978). The r a d i a t i v e t r a n s f e r ca l cu la t ions of Weaver (1978) and o the r s i n d i c a t e t h a t a la rge f r a c t i o n of t h e cyclotron-l ine r ad ia t ion should be s c a t t e r e d t o lover energies. i n which the l i n e photons escape through holes i n the c i rcumste l la r s h e l l above the a g n a t i c poles of the s tar and those i n which the r ad ia t ion passes d i r e c t l y through the she l l . Thus, the l i n e can a l s o be used as a probe of p l a s m condi t ions a t the Alfven rad ius of the star, y i e ld ing da ta re levant t o the problems ef the s o f t component of the X-ray emission from Her X-1 and the ove ra l l accre t ion process. Detailed l i n e and continuum shapes can choose between models Observations of o ther pu lsa t ing X-ray b ina r i e s such as Cen X-3, Vela X-1, and the t r a n s i e n t source 4U0115+63 ( i n which evidence f o r a 25 keV l i n e has been reported) as w e l l as of i s o l a t e d neutron stars such as the Crab pulssh ( i n which a t r ans i en t 77 keV emission f e a t u r e was seen [Strickman, Kurtess, and Johnson 19811 could extend the usefulness of t h i s s e n s i t i v e probe t o o the r objects . A f e a t u r e ten times weaker than the reported Her X-1 l i n e would be e a s i l y detected i n Vela X-1 i n ha l f a day'& t i m e . These observat ions would, of course, a l l be simultaneous wlth those examining p l a s m condi t ions i n these systems f a r t h e r from the neutron star v i a high-resolution spectroscopy of the fron lines as discussed above. Recently, i t has been suggested t h a t atomic l i n e s of i ron , s t rongly s h i f t e d by the in tense magnetic f i e l d , might a l s o be detec tab le from neutron s ta r sur face regions. ca lcu la ted by Ruder e t el. (1981) f o r a va r i e ty of sur face temperatures, abundances, and f i e l d s t rengths . f i rs t d i r e c t observation of neutron s ta r sur face material, and would add Energies and s t r eng ths of the Lyman l i n e s have been The de tec t ion of these l i n e s would y i e l d the 57 6 s u b s t a n t i a l l y t o our knowledge of plasma condi t ions i n the region where the accreting material impacts the s ta r and the primary r ad ia t ion is produced. Wunner e t a1 ( t h i s volume), have ca lcu la ted the requirements of a spectrometer necessary t o observe such l i nes : 10 keV 6 E and l i n e s e n s i t i v i t y of 2 ph em-* s‘l . A l l of these s p e c i f i c a t i o n s are exceeded by t h i s proposed instrument. 20 keV; E/AB - 10 t o 100 ; Explosive Nucleosynthesis in Supernovae The modern e r a of t h e o r e t i c a l nucleosynthesis may be dated from 1957 with the publ ica t ion of the classic paper Burbidge et al. (1957) (B2PH). t h e r e i n is t he basis f o r t he cu r ren t ly held be l ie f t h a t t he elemental abun- dances as we know them derive from stellar evolut ionary processes r a t h e r than being a primordial arrangement of matter. These workers gave, f o r t he f i r s t t i m e , a charac te r iza t ion of the s p e c i f i c physical processes (r-, s-process , etc.) which lead t o the formation rlrf d i f f e r e n t classes of isotopes. In a sense, most of the research i n t h i s f i e l d f o r the p a s t 25 years represents a search f o r the as t rophys ica l sites of these processes, o r i g i n a l l y de l inea ted by B’PH. t h e s i s (Baadc e t al. 1956) t o explain t h e i r l i g h t curves, aupernovae were he- 1ieve.l t o play an important r o l e ir. nucleosynthesis. During the la t ter p a r t of the 1960s and e a r l y 19708, la rge , f a s t computers became ava i l ab le f o r general use. This made it possible t o follow phenomenologically the complex nuclear reac t ion matrix t h a t takes place when a highly evolved star explodes and hydrodynamically expands. The ca l cu la t ions of Arnet t , Cameron, Clayton, Colgate, Fowler, Schramm, Tinsley, Truran, Woosley, and o the r s (Schramm and Arnett 1973) led t o the remarkable r e s u l t t ha t many of the elements can be produce6 explosively i n the co r rec t cosmic abundance. Set down From the teginning, with the advocation of the cal i fornium hypo- This conclusion l e n t 577 grea t credence t o the theo r i e s of explosive nucleosynthesis. An important ob jec t ive of a germanium spectrometer would be t o provide an observa t iona l bas i s f o r these theor ies . Many of the s t a b l e spec ies are f i r s t produced as unstable rad ioac t ive elements which then decay, o f t en with the emission of c h a r a c t e r i s t i c X-ray and gamma-ray l i n e s (Clayton 1973). The unequivocal de t ec t ion of even a s i n g l e l i n e from explosively produced rad ioac t ive matter would represent a fundamental breakthrough i n t h i s area of research. Predic t ions of l i n e f luxes t o be expected from radioac t ive spec ie s pro- duced I n the various s h e l l s of a presupernova s ta r have been pursued most a c t i v e l y by Clayton, Colgate, and Pishman (19691, Clayton (1973, 1974, 19751, and Ramaty and Lingenfe l te r (1977). Several l i n e s of p a r t i c u l a r iaterest f a l l within the s p e c t r a l range of our proposed icstrument: t h ree l i n e s from t h e "Co -+ 5 / P e decay a t 14 keV, 122 keV, and 136 keV, the 59 keV l i n e from the 60Fe -* 6oCo + 6 o N i chain, and a doublet a t 68 keV and 78 keV from the r eac t ion 44Tl + 44Sc + 44Ca. years, 4.3 x lo5 years, and 68 years , respect ively. These decay schemes have average l i f e t i m e s tav of 1.1 (See Table 1 f o r a sum- mary of p o t e n t i a l sources and required observing times.) A t a t i m e t follow- ing the supernova explosic- , t he f l u x F observable i n l i n e s such as these is given by where N is the number of nuc le i produced i n the explosion, q ts the number of photons produced per d i s in t eg ra t ion , and D is the d is tance t o the source. Using some t y p i c a l values f o r these q u a n t i t i e s , equat ion (1) becomes: 578 To observe a gama-rap l ine from the short-lived process "Co .+ 57Pe, a very young (t < 10 years) supernova reanant is required. ga lac t ic supernova rates range from one per f ive years t o one per f i f t y years ( T a r m n 1977; Clark and Stephenson 1977; Manchester and Taylor 1 Y i 3 ) , wfth s a recent r e su l t s (Jones 1975; Taylor 1978) tending t o favor the higher rate. Thus, i t is not a t a l l unlikely that a remnant less than 5 to 10 years old e x i s t s soaewhere t n the Galaxy; such an object would prode a 122 keV l i n e f lux of 4 x 10- photons (cn? s1-l at 20 kpc. pointed along the ga l ac t i c plane, we could search over 75% of the e n t i r e volume of the Galaxy i n which supernova remnants are known t o e x i s t (R < 12 kpc; l e ! C 150 pc) \ > t h i s l eve l of s e n s i t i v i t y i n less than two days. The only excladed regions would be within a few kiloparsecs of the ea r th within which we are qui te ce r t a in no such recent event has occurred. Current es t imates of With our 1O.S f i e l d of view While the prospect of detect ing such a young remaat is an exc i t ing one Indeed, the i n i t i a l program fo r such a spectrometer would probably concentrate on the other t w o d i s in tegra t ion chains noted above, s ince t h e i r longer half- l i ves allcw us t o search fo r these l i nes in known objects. The 44Ca doublet wil l be detectable with our instrument i n a three-hour in tegra t ion i f i t is present i n the Cas A remnant a t the expected level. These searches fo r nu- c lear l l nes from the explosively produced matter can, of course, be car r ied out simultaneously with the de ta i led study of the atomic spectra of both the ejected and swept-up yiterial v ia the 6-7 keic' t ron and nickel l ines . Although less energet ic by several orders of magrrizude, ga lac t i c novae a re a l so thought t o be the s i te of s ign i f ichnt nucleoepnthesis, and t h e i r 579 f requent occurrence 0 2 5 per year i n t h e Galaxy) makes them an impcrtant p o t e n t i a l t a r g e t . Aseuging a y i e l d of u n s t a b l e nuc le i of 10 times t h a t for supernovae ( L i a g e n f e l t e r and Ramaty 19781, we could e a s i l y d e t e c t both t h e "Co and bbTl l i n e s from a nova 3 kpc away for t h e first year or two af te r i t s outburs t . In a d d i t i o n , t h e p o t e n t i a l l y quick response time of t h e S h u t t l e sugges ts t h e e x c i t i n g p o s s i b i l i t y of observing one of t h e most abundant of t h e i r o n peak i s o t o p e s ("Ni, 163 keV) d e s p i t e its s h o r t l i f e t i m e of -10 days (a launch wi th in 3-4 months of t h e o u t b u r s t of a nearby 1-2 kpcl nova would be requi red) . In conclusion, then, our spectrotneter o f f e r s t h e f i r s t oppor tuni ty to put t h e so-far b r i l l i a n t l y s u c c e s s f u l theory of explosive nucleosynthes is t o a d i r e c t test a t t h e sources of t h e c r e a t e d mater ia l . - d ) Continuum Spect ra No instrument has every bee- flown which covers t h e e n t i r e energy range from s o f t X-rays to s o f t gaaum-rays. Consequently, a i l of t h e c u r r e n t l y a v a i l a b l e X-ray source s p e c t r a are composites of d a t a taken wi th d i f f e r e n t instruments and, more important ly for t h e s e v a r i a b l e sources , a t d i f f e r e n t times. Deta i led continuum s p e c t r a are a va luable d iscr iminant among var ious emission and r a d i a t i v e t r a n s f e r models, and with the cur ren t o b s e r v a t i o n a l s i t u a t i o n , progress i n understanding t h e s e sources has been slow. An obvious by-product of t h e s p e c t r a l l i n e observa t ions proposed above is t h e determinat ion of source continuum s p e c t r a over t h e spec t rometer ' s range of 2 t o 2 0 0 keV. The high r e s o l u t i o n a v a i l a b l e over the e n t i r e band w i l l enable u s t o pinpoint s p e c t r a l breaks and absorp t ion edges, and or i l l provide d a t a which w i l l be most u s e f u l i n s tudying d e t a i l e d models f o r aource emission componenta. Assuming a Crab-like spectrum and a I keV * s o l u t i o n element , we can d e t e c t i n t h r e e hours a l l sources b r i g h t e r than 15 Uhum counts out t o 50 580 keV and all sources brighter than 300 Uhuru counts (-23 sources) out to 200 beV. - e) Solar Physics -- High resolution solar spectra have never been obtained in the energy range from 50 to 200 keV and standard, free-flying spacecraft constraints oc:en preclude solar observations entirely. Thus, the closest object in the universe of interest to high energy astrophysicists remains an enigma. The relationship of radio noise storms and bursts to the energetic particles evident in X-ray flares is not well understood, and detailed continuum shapes are required to aake progress in the theories of particle acceleration and storage mechanisms. Lines from the hot thermal plasm of the corona will be easily observable. line which may have originated in the quiet SV.I, and although theoretical estimates of their strengths are quite low, lines from the deexcitation of nuclei produced in surface nuclear reactions and in the spallation of the thermal nuclei of the photosphere by the high energy flare particles Grlght well be detected. Detailed line shapes can yield kinematic and poeitional information about the emitting particles. By following the time development of the spectrum through a solar flare, densities and temperatures in the emitting volume can be obtained (see, for example, Parkineon 1975). Although solar astronomy would probably iIOt be the primary objective of the initial flight of the proposed instrument, its spectral range and resolution meke it an obvious candidate for use in studying the Sun. We have included a solar observation of a few hours in our sample observing plan to obtain the first high reeolution spectrum of the quiet Sun available in this energy regime. If an active region is present at launch time, we could specify a more extended Jacobsen (1977) has reported the detection of a 73 keV 58 1 1 i I pointing in an attempt to obtain a flare spectrum which would contribute substantially to our knowledge of the energetic phenomena taking place on our closest star. - f) Sample Observing Program In Table 2 we present a sample c ' .erving program that could be carried out with this instrument in two days of pointed observations on a Shuttle mission. 40% of our observing time and have based our integration time requirements on the sensitivity calculations found in 5III . f . - possible in this short time, and there are manifest advantages in being able to combine search program (e.g., for cyclotron and nuclear deexcitation lines) with observations which we are confident will yield positive, important results (the high-resolution spectroscopy of the 6 keV iron lines and the detailed continuum spectra). We have assumed that on-source data prill be collected during only A wide range of targets is 111. EXPERIMENT DESCRIPlION - a) Overall Design The Wide-Band Germanium X-Ray Spectrometer is illustrated in Figure 1. The detector package, consisting of 19 planar germanium crystals and their associated lowlroise preamplifiers in a vacuum cryostat, ic surrounded by a massive active Polyscin (NaI) ehield/collimator and Its associated complement of phototubes. FWBl field of view, and includes two movablo shutters used in acquiring back- ground data, along with an aluminum filter to allow solar observations. solid Argon refrigerator sits at the base of the unit, providing sufficient The two-section collimator provides each detector with a 1?5 A cool ing capaci ty t o maintain the de t ec to r c ryos t a t at T ,$ 90 K f o r a per iod of up t o f i v e weeks. This experiment w i l l provide the f i r s t single-instrument coverage of t h i s e n t i r e energy range, a f a c t o r of 3 improvement over t he reso lu t ion of proport ional counters i n the 6-7 keV iron-l ine region, and more than an order of magnitude increment i n both r e so lu t ion and l i n e s e n s i t i v i t y i n t h e 30 t o 200 keV region. components, along with quest ions of s e n s i t i v i t y and background rates, are d is - The design d e t a i l s f o r each of t he system's cussed be low. - b) Detector Package The de tec to r itself c o n s i s t s of a hexagonal close-packed a r r ay of 19 planar h igh-pur i ty germanium c r y s t a l s of 1 inch diameter ( 5 c d ) . ind iv idua l i n t r i n s i c de t ec to r s are r ead i ly ava i l ab le commercially i n a v a r i e t y These of thicknesses; we have chosen 10 mu as the optimum value for energy range and r e so lu t ion (but see 9III .gbelow). The quantum e f f i c i ency of t he de t ec to r s is g rea t e r than 80% from 3 to 90 keV and remains g r e a t e r than 25% between 1.5 and 200 keV; the r e so lu t ion is better than 400 e V a t the low end of t h i s energy range and degrades only slowly t o a value of -650 e V (0.4%) a t t h e upper end. The de tec to r s are mounted on a cold p l a t e t o minimize thermal generat ion of e lec t ron/hole p a i r s and t o increase the mobil i ty of the p a i r s c rea ted by the ion iz ing X-rays from the source. Each c r y s t a l is equipped with a low-noise PET preamplif ier which r e s ides in s ide the de t ec to r c ryos ta t . The preamplif ier output s igna l s are sen t t o t h e e l e c t r o n i c s box mounted beneath the s h i e l d assembly where they are amplif ied, shaped, and processed i n a pulse-height analyzer. Four multiplexed 12-bit analog-to-digital converters (one f o r each f i v e de t ec to r s ) assign each event t o one of the 4096 5O-eV wide energy channels. The da ta are s e r i a l l y 583 t r ans fe r r ed t o the Shu t t l e telemetry sys tem through a buffer ; an t ico inc idence t r i g g e r s recorded by the phototubes and o ther secondary sc ience and housekeep- ing da ta are subcommutated with the primary da ta stream by a microprocessor. - c) Shield/Collimator System In the low-energy X-ray regime (< 20 keV), l a rge ant icoincidence e c i n t i l - l a t o r sh i e lds have not t r a d i t i o n a l l y been employed, s ince the instrument background is typ ica l ly dominated by d i f f u s e X-ray emission. A t high ener- g i e s , however, t he spectrum of the d i f f u s e background s teepens, and t h e charged particle background becomes the sens i t i v i ty - l imi t ing fac tor . In addi t ion , passive co l l imators become increas ingly d i f f i c u l t t o bu i ld as the energy increases above 40 keV. In the proposed spectrometer, t he s h i e l d allows f o r good co l l imat ion over the e n t i r e s p e c t r a l range, d i f f u s e back- ground-limited observat ions out t o near ly SO keV, and remarkably low counts ( 8 keV1-l) t o t a l background rates throughout the hard X-ray por t ion of the spectrum. The sh ie ld /co l l imator assembly cons i s t s of four pieces of sodium iodide housed i n nermetically sealed containers with g l a s s windows f o r t h e i r asso- c i a t ed phototube arrays. The sh ie ld design assures tha t t he path t o any de tec tor from the outs ide world is a t least 4 inches. The sodium iodide pieces surrounding the de t ec to r box on the bottom and s i d e s are each viewed through symmetrical arrays of s i x 5-inch ruggedized phototubes. The 18-inch high col l imator s ec t ion on top of the de t ec to r is a honeycomb of nineteen 1-inch holes (one above each de tec to r ) and requi res a dozen phototubes f o r e f f e c t i v e l i g h t co l lec t ion . Similar d r i l l e d co l l imators have been flown successfu l ly with so l id - s t a t e de t ec to r s i n seve ra l experiments (e.g., on OSO-7 and HEAO-1). The forward col l imator s ec t ion with i t s own set of 3-inch photo- 584 I tubes r e s ides 18 inches above the top of t he instrument t o provide a reduct ion of a f a c t o r of 2 over the unpracollimated f i e l d of view to 1?5 FWHN (5.4 x IO-' ster). ' I i With an e f f e c t i v e geometric area of -4000 cn?, the instrument should j 1 i experience inc ident background rates of -4 x le events 8-l. coincidence t r i g g e r mechanism w i l l provide a shor t (1-2 ps) gate f o r normal A b i l s v e l a n t i - ! ! ! background events and a wider (-50 ps), adjus tab le ga te f o r those events t h a t deposi t very l a rge amounts of energy i n the sh i e ld ("stars"). from the normal events w i l l be -5%; we estimate t h a t t he re m y be lo3 "stars" per second, leading t o an add i t iona l 5% dead t ime . Since the rate of "stars" is d i f f i c u l t t o pred ic t accura te ly , the exact ve to width of t he "star" ga te can be uplinked once the background o r b i t is determined. The dead t ine - d) Background Shu t t e r s Mounted atop the main co l l imator a r e two motor-driven in t e r lock ing shut- ters, each with its own phototube. They can be commanded closed ind iv idua l ly or together t o cover e i t h e r 9, 10, or a l l 19 of the de t ec to r s f o r the purposes of obtaining simultaneous background da ta while t he source is i n the f i e l d of view. Such s imultanei ty may be required t o e x t r a c t , for example, the weak nuclear deexci ta t ion l i n e s from the s t rong , time-varying background spec- t r u m . For most observations, however, we w i l l be able t o obta in s u f f i c i e n t background data during port ions of the o r b f t when the source of i n t e r e s t is not access ib le (e.g., Earth occu l t a t ion ) and thus will u t i l i z e the f u l l e f f ec - t i v e a r ea of the de t ec to r s f o r source observation. - e ) Background Calculat ions The ca lcu la ted instrument background spectrum is shown i n Figure 2. following background components h a w been included i n our conetderatione: The 585 (1) The diffuse X-ray background (both inside the field of view and incident on the remainder of the instrument) Cosmic gama-ray-induced spallation in the germanium crystals Trapped gamma-rap-induced spallation in the germanium crystals Charged particle-induced radioactivity I n the Ne1 shield, and Neutron activation in the germanhim. (2) (3 ) (4) ( 5 ) We have taken the diffuse background spectrum dPtl/dE from Schwartz (1978); for our 95 ca? detector with its 1?5 field of view, this implies 0.50 E-lO4 ( 1 < E < 21 keV) dFbl 2E -= ( 9.90 E-2.38 (21 < E keV) This contribution dominates the instrurnent background up to -40 keV and then is rapidly swamped by the nan-X-ray background, contributing lees than 10% above 95 keV. detector, the contribution to the background from the isotropic diffuse flux incident on the instrument is negligible; calculations indicate it can be neglected completely below -400 keV. Due to the thick "a1 shield which completely surrounds the We have performed detailed Monte Carlo simulations using the computer codes at Sandia Laboratories1 to estimate the contribution to the background from long-lived (covared to the veto time gate width) radioactivity induced in the germanium crystals from both cosmic rays and trapped protons. The cosmic-ray proton flux was taken from Figure 17 of Webber's review (19671, ' We wish to acknowledge the InvaluaUe assistance of Jim Morel and John Halbleib of Sandia Laboratories for performing these important simulations. 586 using the values f o r sunspot maximum. e l ec t rons were t r e a t e d using Proton Model AP8 Max (Sawyer and Vette 1976) and solar meximum models AE4 and AE6 (Teague, Chan, and Vette 1976). The trapped proton f l u x w a s ca lcu la ted f o r a time Corresponding t o one o r b i t a f t e r t h e last South At l an t i c Anomaly passage on the t en th day of t he mission. The s imulat ions were performed f o r sh i e ld geometries c lose ly approximating our present design and with minimum sh ie ld thicknesses of 8 an and 12 cm. There were no s t a t i s t i c a l l y s i g n i f i c a n t d i f f e rences between the two runs, although the assumption of neglect ing the noncollimated d i f f u s e r ad ia t ion begins t o break down f o r th inner sh ie lds . The South At l an t i c Anomaly protons ad I n p r inc ip l e , one can ca l cu la t e the background cont r ibu t ion due t o charged particle-induced rad ioac t ive s p a l l a t i o n products i n the NaI s h i e l d i t s e l f with the same d e t a i l e d procedures used above f o r t he germanium. How- ever , s ince the vast major i ty of these nonvetoed events leave the instrument without i n t e r a c t i n g i n the de t ec to r , a vas t number of p a r t i c l e and photon h i s t o r i e s must be followed before s t a t i s t i c a l l y meaningful r e s u l t s are achieved, and the computing cos t s become prohib i t ive . Thus, we have modeled t h i s cont r ibu t ion t o the background by ca l cu la t ing the p a r t i c l e and photon flux out of the sur face of a block of NaI with the appropr ia te amount of induced, nonvetoed r ad ioac t iv i ty . This has then been t r e a t e d as an i s o t r o p i c background bathing the Ge c r y s t a l s and can be converted t o a background count- ing rate by simply mult iplying by the i s o t r o p i c e f f i c i ency of each de tec to r and summing over a l l detectors . This cont r ibu t ion turns out t o be roughly comparable to t h a t from the s p a l l a t i o n i n the germanium over most of t he energy range of interest (see Fig. 2). In addi t ion t o the continuous background produced by charged p a r t i c l e s and d i f f u s e X-rays, severa l sharp l i nes t y p i c a l l y appear i n the background spectrum of germanium de tec to r s due t o t r a n s i t i o n s i n the crystals induced by inc ident neutrons. While a massive a c t i v e NaI s h i e l d is e f f e c t i v e i n r e j e c t - ing charged-par t ic le and uncollimated X-ray events , i t is not p a r t i c u l a r l y use fu l i n reducing the f l u x of these neutrons. Ear th albedo and spacecraf t - generated neutrons ranging i n energy from a f r a c t i o n of a keV t o >lo0 keV cont r ibu te i n two ways t o the production of a metastable exc i ted s ta te of ‘3Ge: Two of the s t rong background l i n e s produced by t h e decay of t h i s metastable state occur a t inauspicious loca t ions i n the spectrum: a t 54 keV (near t he Her X-1 cyclotron l i n e ) and a t 67 keV (near the 68 keV nuclear deexci ta t ion l i n e of 4 4 T i ) . The high gain s t a b i l i t y and a t t e n t i o n t o obta in ing background da ta inherent i n the instrument design w i l l al low the accura te sub t r ac t ion of such background fea tures . (1) neutron capture i n ’2Ge, and (2 ) i n e l a s t i c s c a t t e r i n g of f ‘3Ge . - f ) Instrument S e n s i t i v i t y The minimum l i n e f l u x FL detec tab le at an M-sigma l eve l i n o r b i t t i m e t is given by where AE (keV) is the instrumental reso lu t ion or the i n t r i n s i c l i n e width (whichever is g r e a t e r ) , A ( c m 2 ) i s the geometric area of t he de t ec to r with e f t t c i e n c y E E: is the f r a c t i o n of o r b i t time i n which good data are col- l ec t ed , and E is the background counting rate, cons is t ing of - both t h e non-X- ray counting r a t e plus the d i f f u s e X-ray background, dFb/dE, and the source A, t 588 I continuum f lux , dFc/dE, i n the v i c i n i t y of t h e l i n e ( i n counts [cm2 s i - keV1 ' l a Thus, the time t o de t ec t a f ea tu re of known s t r eng th is given by ( 1 -t M* E F t L 2- dFb BE dE F ~ o E ~ + AE 2- dFC dE 1 FL The geometric area of the de t ec to r is 95 c d and the reso lu t ion i n the i r o n l i n e regton of the spectrum is -400 eV. We have assumed t h a t u se fu l da t a w i l l be obtained during 40% of our observing t i m e (€.e., E = 0.40). Note t h a t f o r many of t he s t rong X-ray binary t a r g e t s , the source continuum is the dominant cont r ibu t ion t o the "background" counting rate. In es t imat ing the observing t t i m e s requried f o r the observat ions of br ight binary sources discussed i n SII.a), - we have used the 6 keV continuum flt ixes reported i n the re ferences from which the l i n e f luxes were taken and have ex t rapola ted these values t o the s u l f u r , argon, and n icke l energies using an E-l power l a w spectrum (a good approximation of these objec ts over t h i s narrow s p e c t r a l range [Pravdo 19781). Using these parameters, t he 3-3 s e n s i t i v i t y obtainable i n lo4 8 of o r b i t time is approximately dF FL(roin) dE 9 = 6 x (K) dFb + ('f'2 photons ( c d s)-l implying l i n e s e n s i t i v i t i e s of N < 4 x photons ( c d s1-l and N photons (cn2 s)-l at 6 and 60 keV, respect ively. & Other Detector Configurations The de tec tor system described above was chosen t o optimize s e n s i t i v i t y and reso lu t ion while extending the usefu l s p e c t r a l range down t o 2 keV. In view of the extensive spectroscopic results from the Eins te in SSS i n t he 2-4 589 keV range, it is usefu l t o consider o ther poss ib le de tec tor configurat ions which deemphasiee t h i s s o f t X-ray regime. s u b s t i t u t e t h i cke r germanium c r y s t a l s f o r those i n the base l ine design t o provide increased s e n s i t i v i t y at the higher energies . planar c r y s t a l s r e t a i n 10% of t h e i r e f f i c i ency up t o 550 keV. element a r r ay would provide -10 cm2 of e f f e c t i v e area i n the region of the 511 keV pos i t ion ann ih i l a t ion l i n e which has been observed from the directLon of the g a l a c t i c center . The narrow f i e l d of view and high s p e c t r a l r e so lu t ion of t h i s experiment would provide new information on t h e source of t h i s l ine . The p r i ce of such a change is a worsenlng of the r e so lu t ion i n t h e 6-7 keV i r o n l i n e region by near ly a f a c t o r of two ( t o -lo%, i.e., comparable t o t h a t achievable with gas s c i n t i l l a t i o n proport ional counters - see Rai ley e t al. and Culhane, t h i s volume). Al te rna t ive ly , t he complexity of t he de t ec to r and sh ie ld assemblies could be reduced considerably by chdosing an a r r ay of seven 15-cm2 c r y s t a l s i n place of the nineteen 5-cd a r r a y i n the cur ren t design. Again, however, t he r e so lu t ion is degraded from 360 eV t o 560 e V a t 6 keV. Detai led s c i e n t i f i c /enginee t ing tradeof f s t u d i e s are cu r ren t ly under way t o determine the optimum configurat ion f o r a f l i g h t design. One such p o s s i b i l i t y would be t o For example, 1.6 c m Thus, a 1.9- IV. CONCLUSIONS A number of outs tanding as t rophys ica l problems require c r i t i ca l inputs from moderate-to-high r e so lu t ion X-ray spectroscopy i n the 2-200 beV band and beyond. A l a rge number of the atomic, nuclear , and cyclotron t r a n s i t i o n s t h a t occur i n the regime o f f e r unique probes of t he physical condi t ions which 590 \ obta in i n sources ranging from neutron star b ina r i e s t o c l u s t e r s of galaxies. i We have designed a planar germanium spectroabeter which would operate i n t h i s r i c h s p e c t r a l domain t o address such quest ions as the s t r u c t u r e of neutron I s ta r magnetospheres and accre t ion d isks and the o r i g i n of the chemical ele- ments. With the el iminat ion of the germanium spectrometer on the Gamma Ray Observatory, no cur ren t ly planned NASA mission w i l l be ab le t o meet the pa ten t : need f o r such a spectrometer. The sample observing program displayed i n Table 2 i l l u s t r a t e s t h a t a subs tan t ive s c i e n t i f i c program can be ca r r i ed out by such an experiment i n the br ie f observing time avai lab le on a week-long Shu t t l e f l i g h t . As the cos t of a free-f lying mission escalates past $100 M and the funds ava i l ab le f o r new starts dec l ines prec ip i tous ly , it is perhaps appropriate t o reexamine the premises on which the space science program of the past decade w a s b u i l t . Ten experiments of the class described here and elsewhere i n t h i s volume can a l l be b u i l t f o r half the cos t of one f ree- f lyer i n less than half the t i m e . The continued exis tence of an American program i n high energy astrophysics may w e l l t u rn on which of these options is chosen. This work w a s supported by t h e National Aeronautics and Space Administra- t i o n under Contract NAS 5-26041. This is Columbia Astrophysics Laboratory Contribution No, 216. ORIGINAL PAGE IS OF POOR QUALlM TARLE 1 Nuclear Deexcitation Lines from the Sites of Explosive Nucleosynthesis Life- Nuclei Phot on8 Dis- Time Nuclei (keV) (yr) per Event Decay Source (kpc) (hr) 5 7 ~ o + 5 7 ~ e 14,122 1.1 7x1OS2 0.88 5 yr SNR 20 0.34 Energy time Produced Per tance Required* 136 1.1 7x1OS2 0.11 4.8 57Co+57Fe 14,122 1.1 2 x 1 0 ~ ~ 0.88 1 yr Nova 3 0.61 136 1.1 2x1049 0.11 14 60Fe.*60C, 59 4.3 5 ~ 1 0 ~ ~ 1.0 Sof : X-Ray 0.2 38 xi05 loop +60Ni 4 4 T i . * 4 4 S ~ 68, 78 68 6 ~ 1 0 ~ 1 .o Cas A 2.8 2.4 + 4 4 ( 3 56~1+56 ~0 163 0.025 9~10~' 0.85 1 mo Nova 3 0.01 3 mo Nova 5.9 +56Fe *To detect a 3 d detection based on line strength calculations discussed above . 592 TABLE 2 Sample Observing Program for Two Days' Observing Time - O b j e c t i v e s Fe Ni Other Con- Cyclotron Nuclear Time Line Line Lines tinuum Lines Lines Source (hr) (16 a) (4 0 ) ( 3 a) Spectra ( > l o a) (3 a) X-,saY Binaries cyg x-3* 5 J J J J J 0 . 0 ... 0 . . d ... 0 . . J J J ... Cen X-3 0.1 J Vela X-1 6 J V Her X-1 12 J J 0 . . J J 0 . . GX 301-2 0.1 J ... 0 . . J ... ... 1626-67 0.1 J 0 . . ... J ... ... Cir i-1 0 .3 J 0 . . 0 . . J ... 0 . . C a s A 20 J d(6 a) J ... J CX 287.8-0.5 1 J 0 . . ... J ... ... Recent nava 0.2 ... ... ... J ... J "lusters of GaLasiss Perseus 0.15 J 0 . . 0 . 0 J ... 0 . . Virgo 0.25 J 0 . . ... J ... ... coma 0. a J 0 . . 0 . . J ... ... 0251+41 1.0 J 0 . . ... J ... 0 . . 3 0 . . 0 . . J J ... J *?he swl I Supemova ,?emants ... - Total 50 %e complete orbit . 593 REFERENCES Armstrong, T. W., Chandler, K. C., and Barish, J. 1973, J. Ceophys Res., 70, 2715. Baade, W. et al. 1956, P.A.S.P., 68, 296. Basko, M. M., and Sunyaev. R. A. 1974, Aetron.Ap., 42, 311. Becker, R. H., Boldt, E. A., Holt, S. S., Pravdo, S. H., Rothschi ld , R. E., -. Serlemltsos, P. J., Smith, B. W., and Swank, J. H. 1977, A p e J . , 214, 879. Becker, R. H., Rothsci ld , R. E., Boldt, E. A., Holt, S. S., Pravdo, S. H., Serlemitsos , P. J., and Swank, J. H. 1978, p r e p r i n t . Burbidge, E. M., Burb!dge, G. R., Fowler, W. A., and Hoyle, F. 1957, Rev. Mod. Phys., 29, 597. Charles , P. A., Mason, K., Cash, W., Lampton, M., and Bowyer, S. 1978, Bull. A.A.S., 10, 420. Clark, D. H., and Stephenson, F. R. 1977, M.N.R.A.S., 179, 878. Clayton, D. C. 1973, In Explosive Nucleosynthesis, eds. D. N. Schramm and W. D. Arnett , (Texas: U. Texas Press ) , and r e f e r e n c e s t h e r e i n , p. 264. Clayton, D. D. 1973, i n Explosive Nucleosyntheais, eds. D. N. Schramm and W. n. Arnett , (Texas: U. Texas Press ) , and r e f e r e n c e s t h e r e i n , p. 265. Clayton, D. D. 1974, Ap. J., 188, 155. Clayton, I). D. 1975, Ap. J., 198, 151. Clayton, D. D., Colgate, S., and Fishman, C. J. 1969, Ap. J . , 155, 75. Coe M. J. et el. 1977, Nature, 268, 509. Daugherty, J. K. 1978, "Gamma Ray Spectroscopy in Astrophysics," NASA Technical Memorandum 79619, p. 364. Dyer, C. S. 1977, "Assessment of Background Radiation Effects in Low-Energy Game Ray Astronomy," 1977 Gamma-Ray Symposium, Frascati, Section 7. Gnedin, Y. N., and Sunyaev, R. A. 1974, Astron. Ap., 36, 379. Hess, W. N., Canfield, E. H., and Lingenfelter, R. E. 1961, J. Geophys. Res., 66, b65. Hess, W. N., private communication. Jacobsen, A. S. 1977, "High Resolution Gamma Ray Spectroscopy," invited paper presented at Spring Meeting of A P S . Jacobsen, A. S. et. al. 1975, N.I.M., 127, 115. Jones, E. M. 1975, Ap. J., 201, 377. Kanbach, G., Reppin, C., and Schonfelder, V. 1974, J. Geophys. Res, 79, 5159. Lingenfelter, R. F., and Ramaty, R. 1978, Physics Today, 31, 40. Manchester, R. N., and Taylor, J. H. 1977, Pulsars (San Francisco: Freeman), 160. Meszarc , P. 1978, Astron. Ap., 63, 19. Meszaros, P. 1978, in "Gaomoa-Ray Spectroscopy in Astrophysics," NASA Technical Memorandum 79619, p. 425. Mushoteky, R. F., Serlemitsos, P. J., SMith, B. W., Boldt, E. A., and Holt, S. S. 1978, preprint. Parkinson, J. H. 1975, Solar Phys., 42, 183. Pravdo, S. H. 1978, presented at the 2lst COSPAR Plenary Meeting, Innsbruck, Aus t ria . 595 P r e s e l e r , A. M., S i n e t t , G. M., and White, R. S. 1974, J. Ceophys. Res . , 79, 17. Ramaty R., and L i n g e n f e l t e r , R. E. 1977, Ap. J. (Letters), 213, L5. Ruder, H., Wunnet, G., Harold, H., and Trumpet, J. 1981, Phys. Rev. Lett., 46, 1700. Sawyer, D. M., and Vette, J. I. 1976, NSSDC/WDC-ARLS 76-06, National Space Science Data Center, NASA GSFC, Greenbel t , Maryland. Schramm, D. N., and A m e t t , W. D. eds. , 1973, Explosive Nucleosynthesis (Texas: Univers i ty of Texas Press ) , and r e f e r e n c e s t h e r e i n . Schwartt , D. A. 1978, presented a t 21st COSPAR Plenary Meeting, Innsbruck, Austr ia . Swank, J. H., Boldt, E. A., H o l t , S. S., Pravdo, S. H., Rothschi ld , R. E., and Ser lemi tsos , P. J. 1978, Bull. A. A. S., 23, S81. Taaunan, G. A. 1977, Proceedings of Eighth Texas Symposium on R e l a t i v i s t i c Astrophysics, Ann. N. Y. Acad. SCI., 302, 61. Taylor , J. H. 1978, p r i v a t e c o m u n i c a t i o n . Teague, M. J., m a n , K. W., and Bette, J. I. 1976, NSSDCJWDC-ARLS 76-04, Nat ional Space Science Data Center, NASA GSFC, Greenbel t , Maryland . Trumpet, J. et al . , 1978, "Gamma-Ray Spectroscopy i n Astrophysics," NASA Technical Memorandum 79619, p. 331. Weaver, R. P. 1978, "Gamma-Ray Spectroscopy i n Astrophysics," NASA Technical Memorandum 79619, p. 418. Webber, W. R. 1967, Handbuch d e r Physik, 46, (Springer-Verlag). White, R. S. 1973, Rev. Geophys. and Space P h p ~ . , II, 595. 596 FIGURE CAPTIONS FIG. 1 - Schematic diagram of Wide-Band Germanium X-Ray Spectrometer. FIG. 2 - Background counting rates for the Wide Band Germanium X-Ray Spectro- meter as a function of energy Including contributions from nonvetoed charged-particle events in the detectors (Gel and shield (MI), and the dif f use X-ray backgrourrd . 598 ORIGINAL PAGE IS OF POOR QUALm lo-' - z 2 0 \ u c 0 U 2 lo-a 4 c3 c 2 3 0 0 a I f lo-! 3 Y c3 2 i ?IO-' d 4 c lo-' ,/ V ' '. \ 'COSMIC RAYS I I 1 1 I I IO IO0 IO00 \ ENERGY (rev) FIGURE 2 HARO X-RAY ASTROPHYSICS 600 ORIGINAL PAGE IS OF POOR QUALITY design vI.1 they were years .go ( money is ! A and mays of acintilktion awntor8 are no bqer at a dbwhnnt.gs wlth mpud to gu countera It is possible today to fiy thoumnds of square centfmeten of SciClnMktot with no technical d- lmcemuy. !n order to ortcuhte poWble sen~.vlttes of future a r m the efhciwh 8 of O W m@lWO-inCh mnt~ ( the HEAO-t A-2 m- f i l ko HEm 1 .nd 3 mm p h e scintilbtw t the H € B 1 A-4 W LED1 1 are compueu in Figure 1. Above 15 heV the scintillator is more efficient In order to translate this Into ~~~% 3- an2 and 1 0 0 , ~ seawls livetime have auumad, urd Figure 2 $9 the 30 n w m m d.1.d.#~ flux tor IJOW deteor types, p(U8 the spectrum d a orn wu (or 1.a WYI E- SOU~C.. ckuly. arhophyska abom 20 koV CUI be bettor 8wv.d by a sointilktor. In a similar cornperkon gormurium dehdord 8emitWy k not much diWerent from that for scintillators, excopt at high onergios where the srndtlvity wwld ramah flat and not rise with tho krs of ewiwmy. slme luge area detectors have the wMltMty to measure one W U murce6, what .bout tho h v d x-rry sky? la them mything there to see? Figure 3 shows the representation d the preliminuy Hndiqs of tba H€AO-1 4060 keV SKYMAP (Levine, .I d. 1- Over 50 source8 are presented to a limiting sendtivity of attout 3 x IO'* phdom/cm2 8 hv. or a u t IO wo tor an E-' spectrum. Within this umpre are acttve gahxiw, clu- d gakxhs, a possibk quasar. x-ray binaries, ndlo puI8us. globular durten, bulge sources, urd evm a nova Mema, a wealth of sources era avaikMe to study mtrophyacal probkms I shall now address mymu to some of these questiorw. PHyslcs OF ACTIVE GALAXIES AND THE MFFUSE BACXGROUNO Many aspects of terbng the masme Mack hole model of the power source In active gelaxiw am related to the total luminosity (Lightman 1381L Maasumments of this luminouty must be mado in the hard x-ray range, where most of the power resides in the case of achve galaxio& The lower energy 2-20 or W-fi keV spectra cannot be extrapolnted to highor energy artntranfy , since a break In the hard x-ray spectrum at something lib the characterlstlc energy must occur. In the pn8f breaks at 40 ISV wero predicted, ~MOCI om the break in the diffuse background spectrum Fgure 4 shows the mean HEAO-1 21-165 keV spactrum of 10 act~ve gabties -- the map* of whwh are Seyfert 1 galaxma -- and no 40 keV break is seen (Rothschild. d d. 198ll Fitbng the data confirms what IS apparent - that IS, no break IS seen up to 100 keV. Ereaka uwnd 100 keV have been invoked in the HEM)-1 analysis of Cem A (Baity. el a/- 19813 and NGC 4151 (8.ity. d J. 198lb) Other published rerulb have chmed breaks in W e r t s as hlgh as 3 MeV (Perotb. d .1. 1979A Tho break energy must be measured. not hypothesized Otherwise, only upper limits to system parameters Wrll k Obtamed. Another important hard x-ray estrophysml questlon relates to the relatwnship betweon actwe 9ahxi.r. including quarrtq and the diffuse x-ray background. In the lower energy 2-40 keN regime M 000 ckrr of dtscrete dbstant obtects contributes more than half of the observed backQround in the knn of u n & M point sources. Quasars are 30-408 (Oiacconi. d e/. 19lsI. Seyferts are about 20% (Mushot2ky. ut d. lseo), and dusters are kms than 10% (McKee, d e/. 1981). Hence. comprmn ot spectral sh4pes of c l r u ~ ~ of extngalactlc object8 to that of the dtffura bsckground yl&s IIWO information about It8 natura Above 40 k.V t h i q ~ s change drastkally. The diffuse background, shown in Fqure 5 and meuured by the HEAO-1 instruments from 3-400 kev (Marshall. d a/ 19€Q bnatteran, .I rl 1979) and by oth.rs above thts onorgy (Tromblm. e( .I 19TI; FIC~W. d a/ 1977). starts to drop rawly about 40 keV, Classes of obj- w(th soft Sp.Ctr8, like clu8ters of gala%IOS, tmcome unimportant at these hrgh energi?a The h d o n of t b x-ray background due to the r.nutntng chses of objects mutt increase rapidly Wnn, energy ff thdr 8poctr.l 8hap.r remm mcbngd. Our results monttoned above indlclle that the .ctm galaxy power law qtoctml indar holdr out to at leut 100 kaV, .nd the duhed line in Fqure 5 ehowo their contrikrtron. Adm grmie$ malm up about b v , ~d WM be rerponsim for IOOW by 250 kev. ACWO ga- may .ISO be nrp0nrle10 for tho dop. cfmqp in tho dtfhrm x - r y b$ckpround -rum at 300 kaV to 3 M.V. I ) Vn actw grlnk, u. rorpondbk for the di fhm background at 280 I k V and beyond, whal h u hapg.md to tho q u u u wntributfon coon at lower onorgbs? Tho qua= spectrum thon must break before 260 keV, and this h u yet to k at oRIC;ii\;AL PAGE IS OF POOR QUALITY obsomd. It the qucwr spectra do indeed break before the active galaxy spectra do, this mbght be .vM.nco tor ovolutknuy d h d s in quam spectra 601 PHwlCs OF BUCK HOLES Bkck hoks wore not mentioned in the theoretical papers at thir workhop, but a great amount of interest still d s t s in tho audy of t b m bizarre and fundamental O b W r This is another uoa in which hud x-ray memtremonts can yWd intormation on the pre6sing quesbns in astrophydcf~ Qg X-1 is dmost uniquo unong g a W k x-ray murtbs due to its 50 keV emisbion. The spectrum of a thermal proam 0 dorcrlbod by Sunmev and Tituchuh (1960) or by OuIlbert and Fabian (1981) would bo cutoff at an onorgy determined by the mulmum brnprature. Our HEM-1 6poctrurn of Cyg X-1. spanning the broad onergy range of tho A2 and A4 instruments, 10 koV-8 MeV, are shown in Figure 6, The modo1 shown Is the Sunymv-litarehuk type whkh fits quit0 mll up to a few hundred keV, and indicates a temperature near 30 koV for a singk therm.1 or onos near 15 and 40 k V for a dud hmperature model. Andy Fabian has indicated that his non-equilibrium modd dascnbes this data well also with a tempwature near 100 keV As you can see. there is an excess to tho model at high onergios. Figum 7 displays this excess mora clearly, and a broad (640 kaV FWYM) 511 keV lima has been added to the previously tmentionod model. Note. however, that most any additmnal typo of modd would ftt the excess with the uncertainties involved, and the OR0 mission should be a b to inveswab this in more detsll in the future. We can further study the emission process near black holes by investigating the temporal rtpettr of the scattering of photons by the hot plasma Since the higher energy photon6 may bet the result of more scatterings in the plasma than the lower energy photons. OM) would expect a time W a y between the emerge- of the low and high energy photons after the lnlection of some soft photons into the plasma. If such a time delay wore confirmed, not only would we be more conhdent of our understanding of black hob accretion disks, bJt wo might be abk to measute the size of the emitting rogion. HCAO-1 results (Nolan. .I d. 1981) indicate a possible 10 ms lag betwean and 60 keV photons. Instruments aboard the Ark1 VI spncocraft have observed e 7 ms lag at lower energies (Fabian, private communicatmn). Future instruments must bo able to accomplish this study over as broad an energy range as possible, in order to estapllsh the energy dependence of this time delay, and to determine the physics of the emission region Ultrafast temporal variability -- on the order of milliseconds or less -- may contain ntal information on both the accretion process erd the angular momentum of the black hole ttself. General consideration6 of black holes show that the ortd time for the innermost stable orb# is much shorter k r Kerr metrics as opposed to the Schwarrschild metric (Novikov and Thorne 1973). If the energy spectrum of the ultrafast variability is Hatter than that of the overall emision above 20 keV. hard x-rays would veld a dearer .signal of the bursts and mereby be more able to study the fast temporal behanor. Studylng the temporaVapectrsl variability w i l l ftrst-cf-611, indicate if the millisecond bursts arise from the same process as the dower variable emiulon. Secondly. it will determine if a time delay occurs between the bursts as a function of energy? finally, it may answer whether or not tho time structure, or of lack of it, perwsts at higher energw Such i n v e m l o n s WJI guide us in krmulrtion of theories of black hole accretion and may link the miili6econd bursting to the $lower. more persistent. variability. i No totally consistent picture of radm pulmr x-ray emiston ha6 yet emergod. Pulrrs are spinning nmutron Mar$, but that is about all we are 6ure of Premtly only a handful of pulmrs are mon in x-rays, and only tho Crab pulsar ii m t n to pulse. Yo1 in the future this wcll change Wh the urn of mom mnsittve in6tnlmnt6. Pulse phase spoetn~py. or more propnty, phasr: rerohfed ipectromnpy, he8 reveald a new 602 ORlOlNAL P A M Is OF POOR QUALITY component in the Crab pulnr 8p.drum. Stuting a h the lowor onorgy work of Pravdo urd 8.rlomlt#8 (1Wl) wlth OSO-8 data, KnigM (1801) ha8 shown trom UEAO-7 hard x-ray data that tho int.rpul# r8gion of tho cnb light cum (ahown in Figure 8) is signlfkantly dWrent from the pmk rogiona Th. two peaks' rp.ctn uo m r l y W n t M and uo soen over a wid0 r q p of . ~ r g i e r (Figure B). The p..lr emrwion h u beon d.sorlbod in terms of the rynchmtron proc888 invoMng htgh energy oWron8 in tho pulwr mrgmtk h.IQ and polarkation nnuumnent8 contin, thia The interpub spectrum. shown in Figure 14 i8 dgn- dHhnnt It is flatter and appear8 to h a k around 200 keV, pombbly indlcrtlve of the emission process enorgy @.rhrpr tho cydotron 81).1oy~ Attempt8 to this have incmed a 150 kov t tmw k.mmtnhiung process, compton scattering of low onorgy photon8 by 26 keV p k s m of momson depth 5, end scattered rulkth trom thkk atmoaphen in a 8trong magnotic bid. Con8Meratiow such u emimion mion dzz temperature. md f W dreq th pore probkms rrlth dl that. modola Undwbtodd)y, however. a new m p o m n t h u beon mmtd -- perhaps otlglWng ham qubte doaa to the neutron star itmlf. Further study of thm apoctra in tho hud x-ray range is eawntw to a Mer understadlng of radio pulsar% Naturally, tha most excitirlp m a p o d of the hard x-ray astrophydcs of w t r o n stur w t l y is tho prospect of memuring their magnetic riokj strengtt by nbssmng the cyclotron E#turor it; thdr rprctn Thooretml work a6 pres0nt.d at this workdtop is ulvemng on predicbng the size and shape of th.m hduna as a functlon of Hold strength. Her X-1. &own in Fqure 11, is the mo8t prominent uumplo of mi8 phenomenon. md oth.rr indud. 4UO115+63 (Wheaton. .I a!, 1979) and numerous gun-ray bum (M.t.tr, d d. 1981). Tho H€AO-1 rordtr (Qruber et 01. 1980) of model htting the Her X-1 data r e m 1 the cyclotron feature to be brord (28 b V . zt in emisston) Md occuring around 45 kov. (The mnsrgy ot the controid depend6 upon whether it 18 m omkdon [W keV] or abmrptlon [39 b V ] J The hrture's crntrod wMe8 with pllem phaW. mt kut durtng tho pubes portion d the light curve. as I6 shown in Figuro 12 A 15% or -on is seem, depending upon emi&on @r absorption. We are unable to follow the W u r e through tho 'bfbpulm" ph- region due to uwdoqwt6 detector a m but the vukbldy is not enough to aun tfw bmadth of tho feature. Thir changing cantrod anergy t t probably a rwult of v m n g the specrhe g.0- of the Her X-1 system from dllhrent q k s , md thut tm geometry is open for study. E m tor those x-my pulsars without an obnous cyclotron Mum, intomutton on thou mgnotk h.ld strength 16 av.llrMe in tho form of tho hard x-rcv spectrum (btdt, .I .I 1976L As 6hown in Ftgure 13. the spectrum conforms to a poww k w with index rround one until sbout 20 koV, whero it breaks to a st..p.r spectral brm. Thts is caused by the complex interplay of cyclotron and sca'toring pr0co-s in the rccrstion plasma in the strong magnotic hdd. Several x-ray p u b r s display thir behawor and m am con-t that th& may albw undwrtanding tho procemms involved and may lead to further measurmentr of neutron star msgnotk held strongths md distributions Her X-1 is just the prlmo oxample d a class of r-rry binaries. The Hard X-ray and Low Energy Ohmma-by Exp.riment r m t d UEAO-~ ha6 discovered long torm vrriaklcty P W ~ A rtmikr to mat #.n in tior X-1 (Oorwcki, et at. 1Wl). In two other odipstng Mnuy pubam LMC X-4 (Lang, ef el. 1981) and W C X-1 M-r, d u. i m ~ f h i 8 now 1-8 me ckss ot p~sing, dipsing binarb6 with altlond longor tom "On/Off' Whvtty to m-. Tho LMC X-4 IQht C U ~ W a d th. SMC X-1 light cum am ahom kr Figurn 14 .(16 1A Mem, tho x-ray community can w i n the detuhd study of this class of obiwta (and hopdully X l E will) in 0rd.r to dotermtm which chusd.ri&ca arm unqw to ind-1 sources and whFh can k -tad to Mo dam in gonenl In odor to acoonrplbh thu detrrkd study of accrohon onto magnitizd neutron rtur. lug. d.toctor u w am mamuy. F W U ~ 13 8howS the tp.drum of ~ . r x-i md mt at suc x-1. rno )rtkr toum is rbout b n hmos wr.laar than th. former, a d conwquontly. ton timer the 6arUltnlty 46 noceuuy to rtudy It at th. unn 1-1 u U E M E ~ rtudkd h r X-1. Porttaps me SMC X-i spectrum hat a cycbtron featurn but m win not know without tho higher sendtMty. A poulbh (Iourc. of aolhnrtbn d the x-ray pulw bwm lt tho hok in the n\.oneto8phere nom the neutron atar magnetlc pokr through whid, the accrotkn plasma rerch.8 the x-ray emtmbn region. It this hole t 603 is responsible for the overall x-ray pulse shape, the observed pulse8 should broaden by about 10% a8 o m goe8 to higher energy due to the change in the Compton cr088 section wbth energy. If this effoct is not sea& the collimabn must be due to the source region. htsize. further informatlon on the pulse formatmn In x-ray pulsrrr is ~ H . b k at hard x-ray energies. PHYSCS OF CLUSTERS OF GALAXIES Clusters of galaxies are believed to contain an intraduster magnak field. This ba6ic cluster purfnder can be measured by 0bW.ving the weak hard x-ray component in cluster x-ray spectra (Lea and Holman 1Sm due to the microwave photons inverse Compton scattering off of the tntracluster electrons that am spidling wound field lirns and generating radio emission. This x-ray component will reflect the rhap d the iloctron power law spectrum, and Its mrgniturle will be directly reletsd to the field strength. Thus, wen upper l i m b to the high energy flux wlll mt lower lirnlts to the magnetic held. Hard x-ray spectral component8 have beon detocted by the Hard X-Ray and Low Energy Gamma-Ray Experiment aboard HEAO-? hom the Per~eu8 (Pnmlni. d a/. 1981) and Virgo clu6ters and upper limits from Abell 2142 (Lea, et a/. 1981) were also 6.tennlnsd (see Figure 16). Ths Perseus hard tail is due, however, to thu active galaxy NQC lm at it8 center, and thus the limits to the inverse Compton component are even lower. In the caoe of the Virgo duster it Is quepionable whether or not-ye galaxy M 87 at the cluster cor? re8ponsible. In any event we have B >5 x 10- QIW W ~ S ) , and B 35 I $0 'Gauss (Abell 21421. These results indicate that the magmttc fieus in duster radio suurces must be &e to the squiputit;on a u e a G a m [wloo). p > 10 I would like to empha8ite the importantance of further remrch in hard x-ray aEtrophyJCr. Them invaabgatms could utilize largo scintillator arrays, large cooled getrmanium srrays. coded aperturm Bragg concentrators. etc. Msswrements above 30 keV extending to hundred8 of keV must be mode 10 attack some of the major Identmed astrophysical questlons before us. C&rrelated analyses over a6 broad an energy range u posrrbk 18 vwy important for dittinguishing b e m n competing models tor a given source and for the m r . ) ImowledQe of the phenomonology of cosrnffi x-ray sources. 604 ORGINAL PAGE iS OF POOR QUALln Bdty. el J. 1981& Ap J, 244. 429, Wly. el a/. 1981L. in preparation. Bola el al. 1916, A&Ofkrktr0phy8., 60, 161. Ftchtel, et J. 1977, Ap J, 217. La Glacconi, el J. 1979, Ap J. 234. L1. Qoreckr, rl J. 1981, Ap J, rubmittd. Qruber, el d. lSe0, Ap J., 240, L l n . Qrukr, d d. 1981, in pmpamtb, Guilberf P. W. .nd Fabian. A 1981. praprtnt Knight F K 1981. PhD db&ath. Uc80. Lang. el d. 1901. Ap J, 246, L2l. Lea, S. M. Urr Hdman. C3. 0. 1978. Ap J. 222, a L w d J. ! 2 1 , Ap J, 248,388. Levine. rl J. rats. BMS, 11. 428. 11ghim4n, A 1961, Worbhop on X-Ray Astronomy and Spectroscopy. GSFC. hhrshall. d J. 1seO. Ap. J, 296, 4. Matteron, .( rl. 1979, BAAS. 11. 663. Mazaq d af. 1981, Nature, 280.37a WKw, d d. 1- Ap. J, 242, 843. Murhotaky, d d. 13W. Ap. J, 23s. 377. Nolm, r( rl. 1961. Ap. J, 246, 494. Nonkw, 1. 0. and Thorne, IC S. 1973. Black Holes. (Gordon and Breach%tw York), 343. hrott:. et d. 19zs. N8tUre. 282, 484. Pravdo. S. and Sorbmitam P. 1981. Astron. Asttophya. submitted. Rimmi, .I at. 1981 Ap. J, 213, L13. Rothcchild, d .1. 1981. in pr.p.ration Sunyaev. R. A and Titarchuk, L. G. 1900. Aaron Astrophys. 86,131. lrombkr. et al. 1977, Ap. J, 212. 925. Wheaton, d al. 1979, Nature, 282. 240. 605 ORlGlNAL PkSE IS OF POOR QliALlN PHOTOELECTRIC DETECT ION E F F ICIEMCY (Including K-Escape Effects) I I I I I 1 m . g I I m m - 1r.v . . I ,I.. - c - I 1 1 I I 1 1 1 1 I I 1 I 1 1 1 1 1 I I I . . . . & IO 100 1000 01. - 1 ENERGY (keV) 606 ORIGINAL PAGE IS OF POOR QUALITY / / ro0,ooo s 1/8 inch B fil 1.5 - 3 x cts/cm2 s kaV 60 7 h x t c Y 608 SEYFERT CONTRIBUTION TO DIFFUSE BACKGROUND ORIGINAL PAGE IS OF peen QUALITY HEAO-I DIFFUSE BACKGROUND SPECTRUM \ GSFC/CIT (MARSHALL, a.u., 1980) 4 UCSD/MIT LED'S \ t UCSD /M'T MED'S (PRELIMINARY RESULTS) 3-165 keV (KINZER, et. - 01 1981) C ALCU L AT E 0 SEYFERT CONTRIBUTION (MARSHALL, e. oJ., 1980, +THIS REPORT) 'I '1 HIGH ENERGY DIFFUSE BACKGROUND \ FICHTEL, et. - - al. , (1978) 1 TROMBKA, - - et . ol . , (1977) .-AL 10' ENERGY 61 0 + t \ + + + k 61 1 I I I l l ’ 1 3 ENERGY ( k e V ) plou, 7! Th.NPA0.f rg.otrwn o)* x-1 .t Mgn ORIGINAL PAGE rs OF POOR QlJALln --‘t CRAB PQLSAR fOWEH LAW IKDEX Auuoge 1.83 2 .03 X2 f 18.8 fot 7 dol 0.0 0.5 I .r3 1.5 2 .o REI.,\\ PtlASE ORIGINAL PAGE IS CRAB PULSAR PI: FIRST PEAK OF POOR QUALITY MA0 - I UCSO / MIT 10'' 1 ' 1 1 1 1 1 1 1 1 1 ) 1 1 1 - - - a - POWER LAW, r : 1.81 2 .02 - - X2 = 73.6 for 40 dot - - w \ PHASE-AVERAGED POWER LAW - F lo-% 2 1.74 f .02 - .) - - - > a3 s 1 0) a 0 L IC - - - ' 10-3 - - 0-4 + 050-6 4 - 60 keV 3.17,16.78 + LEDI t LED^ 15 -200 kev 9.30.78 LEO1 50- 700 LOV 3-24-78 NEW t NEB4 lOO-27OOhV 3-24.78 1 IO - 6 6 IO0 IO' IO2 ENERGY fkeV) 61 3 3 IO3 104 61 4 ~ri.G,bJ.i. PAGE 1s CRAB PULSAR f : BETWEEN PEAKS OF POOR QUALITY HEAO-I UCSO /MIT F x 3 I LL 9 t + POWER LAW, ? 1.63 f .03 \ '\ THERMAL, h f t I51 f I7 LoV X2 : 73.5 tor 44 dot X2 : 73.9 far 43 dof \ PHASE-AVERAGED POWER CAW r r.74 * .oz os0 - 8 LCOl + LE02 15 -200 LOV 9.30-78 LED1 FO-tOOh@V 3-24-70 4 - 60 ~ o V 347,l8*78 IO' IO2 IO' 61 5 lo-2 IO-^ A IO+ IO Energy ( k e V ) FQum 11: fh. rprctrum of Mer X-1 from HEAO-1. The model rhowr both tho continuum m d tho m0d.ll.d line in omtuion io3 61 6 60 50 40 30 20 IO W Q) s I I I HER X - l 0.8 0.9 0 0. I Is.238 P H A S E 0 61 7 0 0 0 0 I IO E (KEV) Fbgure 13 The rpectrum ot Her X-1 a d SMC X-1 horn H E m 1 . Th. did l i n ~ the b08t-m modd to th. HU X-1 drh md tho data polnta we tor HEM)-1 obsomtionr of SMC X-1. 100 61 8 d- Q I 0 Q: W I - O2iGiiJkL FAZE IS OF POOR QUALlW c;f I X 0 r J -- QN033S / SlNn03 1 * a0 a0 - CD cc cc lr) rc) cc N d- d- 620 IO IO6 I - OR:GINAL PAGE IS OF POOR Quam IO' + A 2 LED A 2 MED, HE0 + A - 4 H R I NUCLEUS JULY 1979 I 1 4 \ e f \ F \ e \ I I I IO' I IO IO E (KeV) 621 LIST OF PARTICIPANTS Lor in W . Acton Code 602.6 N ASA/GSFC Greenbelt, MI 20771 8. Aschenbach Max Planck I n s t Garchi ng 8046 Munich, WEST GERMANY J.H. Beal l Code 4152.2 Gamma Ray Astrophysics Naval Research Lab Washington, DC 20375 E l ihu Boldt Code 661 NASA/GSFC Greenbelt, MD 20771 Stuart Bowyer Astronomy Department Univ. o f Ca l i fo rn ia Berkeley, CA Hale Bradt M.I.T. Cambridge, MA 02139 37-581 Alan N. Bunner Perki n Elmer Corp MS 979 100 Wooster Heights Road Danbury, CT 06810 Claude R. Canizares Ins:* t u t e of Astronomy Univ. cf Cambridge Madi ngly Poad Cambridge CB3 OHA ENGLAND Webster Cash LASP Univ. o f Colorado Boulder, CG 80309 R.C. Catura Bldg. 255, k p t . 5212 Lockheed Research Lab 3251 Hanover St reet Palo Alto, CA 94304 George W. C1 ark MIT ROOI 37-611 Cambridge, MA (12146 Len Cowie HIT cambridge, MA 02139 J. Len Culhane Mu1 1 ard Space Sci . Lab. Holmbury St . Mary Dorking, Surrey UNITED KINGDOM Linda Dressel Code 661 NASA/GSFC Greenbelt, MD 20771 Andy Fabian I n s t i t u t e f o r Astronomy Madi ngly Road Cambridge UNITEC KINGDOM G i l b e r t G. F r i t z Code 4129 Naval Research Lab Washington, DC 2C375 Gordon P. Garmire 504 Davey Lab Pennsylvania State Univ. Univers i ty Park, PA 16802 R i ccardo G i acconi D i rec to r Space Telescope Science Ins t . Homewood Campus Baltimore, MD 21218 Paul Gorenstei n Harvard/Smi thsonian Center for Astrophysics 60 Garden Street Cambridge, MA 02138 Richard E. G r i f f i t h s Harvard/Smi thsonian Center f o r Astrophysics, B-440 60 Garden Street Cambridge, MA 02138 Jonathan G r i ndl ay Harvard/Smi thsoni an 60 Garden Street Cambridge, MA 02138 Herbert Gursky NRL Code 4100 Washington, DC 20375 Andrew Hami 1 ton Leander McCormick Obs. P.O. Box 3818 Univ. S ta t ion Char1 o t tesv i 11 e, VA 22903 David J. Helfand Columbia Astrophysics Lab 538 W 120th Street New York, NY 10027 Steve H o l t Code 661 NASA /GSF C Greenbelt, MD 20771 Esther Ming Hu Code 661 NASA/GSFC Greenbelt, MD 20771 Hugh Hudson UC SD La Jo l la , CA 92093 CASS, C-011 James Imamura Dept. A s t r o n w Univers i ty o f V i r g i n i a P.O. Box 3818 Univers i ty S ta t ion Char lo t tesv i l le , Va 22903 Lou Kaluzienski Code SC NASA Headquarters Washington, DC 20546 Fred Knight Code 4153.4 Naval Research Laboratory Washington, DC 20375 Lydie Koch CEN, 6.P. No. 2 91190 G i f Sur Yvette Sac1 ay FRANCE W i l l i a m Ku 538 West 120th Street Columbia Astrophysics Lab New York, NY 10025 Nikolaos 0. Ky la f i s I n s t i t u t e f o r Advance Study School o f Natural Sciences Princeton, NJ 08540 Don 0. Lamb Center fo r Astrophysics 60 Garden Street Cambridge, MA 02138 D-?22 Fred K. Lamb Dept. Physics Univ. of I l l i n o i s Urbana, I L 61801 Steven Langer Dept. o f Astronolily Univ. o f I l l i n o i s Urbana, I C 61801 Alan M. Levine Cambridge, MA 02139 N I T , 37-601 Margaret Lewis Univ. o f Leicester X-ray Astronomy Group Leicester LR1 7RH UNITE0 KINGDOM Alan P. Lightman Center fo r Astrophysics 60 Garden Street Camhridge, MA 02138 J e f f L i nsky Univ. o f Colorado LASP Boulder, CO 80309 Knox S. Long Columbia Astrophysics Lab 538 W 120th Street New York, NY 10027 Frank Marshal 1 Code 661 NASA/GSFC Greenbelt, NO 20771 Frankl in 0. Martin Code SC NASA Headquarters Washington, DC 20546 James L. Matteson Univ. Cal i fornia, San Diego La Jo l la , CA 92593 CASS, Code C-011 Dan McCamsnon Dept. o f Physics 6207 Chamber1 i n Ha l 1 1150 University Avenue Madison, W I 53706 J e f f McCl intock M I T Cambridge, MA 02139 Bldg. 37-521 Richard McCray JILA Univ. o f Colorado Boulder, CO 80309 John F. Meekins Code 4125 Naval Research Cab Washington, DC 20375 Peter Meszaros Code 665 NASA/GSFC Greenbelt, MI 20771 George Newton Code SC NASA Headquarters Washington, D.C. 20546 Patr ick No1 an CASS, C-011 UC SD La Jo l la , Ca 92093 Robert Novi ck Columbia Astrophysics Lab 538 West 120th S t r e e t New York, NY 10027 Jonathan Drmes Code 661 NASA/GSFC Greenbelt, MD 20771 Robert Petre 77 Massachusetts Ave. Cambridge, MA 02139 M.I.T., h 37-576 Raymond Proctor Univ. Cal i fornia, San Diego La Jo l la , CA 92093 CASS, C-011 Saul Rappaport HIT Cambridge, MA 02139 G.R. Ricker 77 Mass Ave. Cambridge, MA 02139 37-551 CSR M . I . T . , Rm 37-527 Guenter R. Riegler J e t Propulsion Lab Pasadena, CA 91109 M.S. 169-327 Richard Rothschild UCSD La Jo l la , CA 92093 CASS, C-011 J u l i a Robinson-Saba Code 661 NASA /GSF C Greenbelt, MD 20771 623 Herbert W. Schnopper Danish Space Res. Ins t . Lundtoftevej 7 DK-2800 Lyngby DENMARK A1 l y n Tennant Code 661 N ASA/GSFC Greenbelt, MD Dan Schwartt SA0 60 Garden Street Cambridge, M A 02138 Peter J. Serlemi tsos Code 661 NASA/GSFC Greenbelt, MD 20771 Gerald Share Code 4152 Naval Research Lab Washington, DC 20375 J . Michael Shull Univ. o f Colorado J ILA Boul der, CO 80309 Robert Smi ther Argonne National Lab 9700 S. Cass Argonne, I L 60439 Jean Swank Code 661 NASAKSFC Greenbelt, MD 20771 Andy Szymkowi ak Code 661 NASA/GSFC Greenbelt, MD 20771 Yasuo Tanaka I n s t i t u t e o f Space and 6-1, Komaba 4-Chome Megu ro- ku Tokyo 153 JAPAN Astronautical Sc i ence Harvey Tananbaum Center f o r Astrophysics 60 Garden Street Cambridge, MA 02138 207 7 1 M e l v i l l e P. U mer Dept. Physics 1 Astron. Northwestern Univers i ty Evanston, IL 60201 Hugh M. Van Horn Dept. Physics 1 Ast ronoq Univ. o f Rochester Rochester, NY 14627 Giuseppe S. Vaiana Center for Astrophysics 60 Garden Street Cambridge, MA 02138 I r a Wasserman Center for Radiophysics 8 Space Research Cornel 1 Univers i ty Ithaca, NY 14853 Michael Watson Univ. o f Leicester Le icester LRl 7RH UNITED KINGDOM Nick White Code 661 NASA/GSFC Greenbel t , R i chard Wh Code 661 NASA/GSFC MD 20771 t e Greenbelt, MD 20771 Bruce Woodga t e Code 681 NASAKSFC Greenbelt. MD 20771 Guenter Wunner I n s t i t u t f. Theoreti sche Glueckstr 6, 0-8520 E r l angen WEST GERMANY Physik der Unlversi t a e t PRECEDlFiG PAGE B M K NOT flUAE0 625 INDEX OF CONTRIBUTORS Brown, W .A. 271 Schropper, H.W. Schwartz, 0. Serl mi tsos , P. J . Shull, J.M. Smither, R.K. Stern, R.A. 527 227 441 107 477 289 Cantin, M. Cash, W.C. Catura, R.C. Chanan, G.A. Cowie, L.L. Cul hane, 3 .L. 523 403 271 56 7 119 183 Tanaka, Y. Tananbaun, H. Taylor, P.O. Trumper, 3. 219 iii ,3 527 471 Fabian, A.C. 133 Garmire, G.P. Gorenstein, P. G r i f f i t hs , R.E. Grindlay, J.E. iii ,3,289 22 7 49 7 349 Ulmer, M.P. 325 Vaiana, G.S. Vartanian, M.H. Vescelus, F. 371 301 289 Hailey, C.J. Helfand, D.J. Herold, H. Holt, S.S. 30 1 56 7 47 1 iii ,3 Wunner, G. 471 Joki, E.G. 271 523 30 1 Koch-Mi tamond, L. Ku, W.H.4. Lamb, 0.0. Lamb, F.K. Leventhal , M. Liewer, K. Lightman, A.P. Linsky , 3 .L. 37 77 567 289 143 13 MacCallum, C.J. Matteson, J .L. McCannon, D. McCray, R. Mestaros, P. Mougin, B. Murray, S.S. 567 549 379 iii ,3 461 52 3 349 Nousek, J.A. Novick, R. 289 567 Petre, R. Proctor, R.J. 31 9 335 iii ,3 289 523 599 47 1 Rappaport , S. Ricgler, G.R. Roccnia, R. Rothschi 1 d , R .E. Ruder, ti.

Extraterrestrial application of X-ray diffraction

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Extraterrestrial application of X-ray diffraction

Abstract

Extracted data

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