A study of X-ray observations of Aquila X-1 /

The goal of this thesis is to obtain the most precise radius measurement of the neutron star in Aql X-1. In order to measure the radius, nine new observations have been analyzed along with four previously analyzed observations. Two of the new observations were taken with XMM-Newton. During the times of the XMM-Newton observations Aql X-1 was found not to be in quiescence and so cannot be used for radius measurements. This state of active accretion of Aql X-1 is characterized by basic spectral analysis. Spectral lines from Aql X-1 were investigated using the Reflection Grating Spectrometer (RGS) on XMM-Newton. There was no visual evidence of spectral lines from Aql X-1 observed. The seven new Chandra observations along with four others all caught Aql X-1 in quiescence. These observations were taken over two quiescent periods with an intervening outburst. Joint spectral fitting of the 11 observations revealed significant variability in the high energy (>2keV) emission but that the thermal emission---which emerges from the neutron star atmosphere---was constant, as expected from theory. The high energy component was parameterized with a varying power-law, with the thermal component held constant. The best fit from jointly fitting the observations was Rinfinity= 11.6+0.4-0.1 (D/5kpc)km and LogTeff(K)= 6.49+0.01-0.02 , where D is the distance to Aql X-1. This is the most precise measurement of the neutron star radius for this source, with the dominant uncertainty (not included in the statistical uncertainty) now in the source distance. Finally, a new class of neutron stars, RRATs, have recently been introduced. One of the RRATs includes Aql X-1 in its error circle. A search was performed of the Chandra and XMM-Newton observations for candidate counterparts. A fading X-ray source, CXOU J191121.4+003844, was found to lie within the error circle of the RRAT, which is put forth as a candidate X-ray counterpart