A study of the long-term behavior of galactic X-ray sources with RXTE

Abstract

I have carried out several investigations of the long-term behavior of galactic X-ray sources using the 4.5-year database of the All Sky Monitor (ASM), and pointed observations made with the Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer (RXTE). I have conducted a systematic search through the ASM data of all the 300 X-ray sources monitored for evidence of periodic behavior. Follow-up investigations are pursued on the discoveries I made in (1) the state transition and orbital modulation of the black hole candidate Cyg X-1; (2) a 4.4-d period in a previously poorly-known X-ray source X 1908+075; and (3) a 98-d period in another previously poorly known X-ray source XTE J1716-389. In Cyg X-1, I have detected its orbital period in the low-hard state but not in the high-soft state. I show that absorption of X-rays by a stellar wind from the companion star can reproduce the observed modulations in the hard state. To explain the low orbital modulation in the soft-state data, a reduction of the wind density during the soft state would be required. In addition, I have discovered an evolution of the correlation between the 1.5-12 keV X-ray count rate of Cyg X-1 and its spectral hardness during the 1996 spectral state transition. I present a quantitative study of this evolution using both the ASM and the PCA data. Implications of our findings are discussed. I have discovered a 4.4-d period in the ASM light curves of the X-ray source XTE J1716-389. I present the results of the investigations on this 98-d periodic modulation using both the ASM and the PCA data. The possible cause of this period and the nature of this system are discussed.