X-Ray Observations of Dwarf Novae

The details of a survey of the X-ray properties of ~65 dwarf novae are presented: four objects are discussed in detail and the null results for the remaining stars are summarized. Pulsed soft X-rays (0.1 - 0.5 keV) with a period of 9s and a pulsed fraction that varies between 0 and 100% were detected from the dwarf nova SS Cygni at the peak of an optical outburst. This detection confirms for the first time the supposed high-energy origin of the optical pulsations seen from dwarf novae. The pulse shape is remarkably sinusoidal for such a large amplitude oscillation. The pulsation is not coherent, in contrast to previous claims for the related optical oscillations. Instead, the phase of the pulsation apparently executes a random walk with a Q of ~ 25, whereas the period is quite stable: P ~ -1 x 10 -5 s s -1. There is no evidence for any other periodic behavior on timescales between 160 ms and 3 h. A blackbody fit to the observed spectrum yields T ~ 3.5 x 105 K and a flux at the Earth of ~ 4.5 x 10-11 erg cm-2 s-1 in the 1/4 keV band. This in turn implies a total luminosity (integrated over all frequencies) at the source of ~ 1.8 x 1033 x (d/200 pc)2 erg s-1. A soft X-ray pulsation has also been detected from the dwarf nova U Geminorum on the declining portion of an optical outburst. The pulsation is quasi-periodic with periods ranging from 20 to 30s and present throughout all binary phases, including eclipse, with an average amplitude of ~15%. Although superficially the quasi-coherent pulsation from U Gem seems qualitatively different from the more coherent soft X-ray pulsation of SS Cygni, we find that a change in only the strength of the random walk in phase can account for the difference between the two stars. We have developed a simple, powerful technique for analyzing the unstable pulsations; our analysis shows that the phase noise in U Gem is from 10 to 30 times greater than in SS Cyg. A blackbody spectral fit to the U Gem data gives the best-fit parameters kT ~ 25 eV and NH ≲ 5 x 1018 cm-2, which imply a flux at the Earth of 3 x 10-10 erg cm-2 s-1 in the energy interval 0.13 - 0.5 keV. Previous models for the optical oscillations fail to account for the properties of the X-ray pulsations. It is possible that an as yet unspecified instability in the boundary layer between the white dwarf and the accretion disk is the origin of the pulsation. A low-intensity soft X-ray source has been discovered in a region of the sky containing the dwarf nova AY Lyrae. The upper limit to the temperature of the source is T ≲ 5 x 106K . X-ray emission below 2 keV has also been detected for the first time from a source whose X-ray variability and position favor identification with EX Hydrae. The soft X-ray intensity was observed to vary by a factor of three in less than one hour. A temperature in excess of 1 x 107 K was measured for the dwarf nova, which was in an optically quiescent state during the X-ray observation. A thermal bremsstrahlung spectrum with kT = 4.5 keV and a column density of 1.7 (+1.4-1.0) x 1021 cm-2 gives the best fit to the data, and implies a total X-ray luminosity (between 0.15 and 10 keV) of ~2.4 x 1032 x (d/100 pc)2 erg s-1. Models for the production of X-rays by accretion onto a white dwarf are discussed in light of this new evidence. The HEAO-1 X-ray satellite was used to make these observations. HEAO-1 fortuitously scanned ~20 dwarf novae while they were undergoing optical outbursts, yet soft X-ray emission was detected only from SS Cyg and U Gem. The 3o limits to the flux from the nondetected objects, several of which were observed by the spacecraft during more than one outburst, range from 1 to 10% of U Gem's mean flux. We present these limits and discuss various hypotheses for the failure to detect soft X-rays from the majority of erupting dwarf novae. A preliminary survey of the soft X-ray emission from ~65 dwarf novae during quiescence has also been made. Thus far, soft X-rays have been detected only at positions coincident with those of AY Lyr and EX Hya. We report the average 3o upper limits to the flux from the nondetected dwarf novae. These limits are ~ 7 x 10-12 erg cm-2 s-1 in the energy interval 0.18 - 0.43 keV, and ~ 3 x 10-11 erg cm-2 s-1 in the interval 0.74 - 2 keV. We compare the quiescent state spectra of dwarf novae with their outburst spectra and deduce that variations in the mass accretion rate can account for the variety of X-ray behavior we observe.