| One of the most intriguing challenges in high-energy astrophysics has been understanding the dynamics of accretion flow onto compact objects. X-ray binaries exhibit many different patterns of accretion flow, manifested as "X-ray states". Various states show dramatically different spectral energy distributions and temporal behavior. In particular, two canonical states are seen in most X-ray binaries---a "soft" state characterized by strong thermal emission and low variability in the light curve, and a "hard" state characterized by nonthermal power law emission and high variability. Besides these canonical states, a variety of intermediate and extreme states are also seen often. A complete understanding of these X-ray states is essential not only to understand the physical mechanisms associated with the different types of accretion flows, but also in enabling one to test physics in the regime of strong-field general relativity.; While it is generally known that the mass accretion rate onto the compact object influences a transition from one state to another, recent observations have shown that there are additional factors that determine the state of a source. In this dissertation, I have used X-ray spectral and temporal information from neutron star and black hole systems to explore the factors that might be contributing towards a state transition. The X-ray data were obtained using the Rossi X-ray Timing Explorer (RXTE) satellite, triggered by our optical/near-IR monitoring of X-ray binaries using the CTIO 1.3m telescope operated by SMARTS consortium.; I have also used data from our long-term (∼ 8 years) optical/near-IR monitoring of the neutron star system Aql X-1 to show that the outer accretion disk heats up like a simple blackbody during the course of evolution of an outburst. Optical data, along with soft-X-ray data from the All Sky Monitor aboard RXTE, demonstrated that besides the canonical "fast-rise and exponential-decay" (FRED) lightcurve morphology seen during outbursts, Aql X-1 frequently went through "low intensity state" outbursts characterized by a prolonged, somewhat variable light curve and significantly lower X-ray-to-optical flux ratio compared to the FREDs. From the optical/near-IR observations, the use colormagnitude diagram as a tool for detecting non-thermal radiation from X-ray binaries is also shown. |
