A Study of the Variation of Geometry of Accretion Flows of Compact   Objects through Timing and Spectral Analysis of Their Outbursts
  Temporal and spectral variations of black hole candidates during outbursts have been reported in several publications. It is well known that during an outburst, source becomes softer in first few days, and then returns to hard states after a few weeks or months. In the present paper, we show variation of Comptonization Efficiency (CE), defined to be the ratio of number of power-law photons and number of soft photons injected to into the Compton cloud, as a function of time in several outbursts. Since power-law photons are generated through inverse-Comptonization of the intercepted soft photons, CE is a measure of geometry of Compton cloud. Our investigation indicates that all outbursts start with a large CE and becomes very small after a few days, when the Compton cloud becomes small enough to intercept any significant number of soft photons. CE returns back to a larger value at the end of outburst. We show co-variation of count rates, frequency of quasi-periodic oscillations (QPOs), photon index and CE and establish a general trend of disk geometry variation during outbursts in all systems under consideration.a 
Partha Sarathi Pal, Sandip K. Chakrabarti
astro-ph.HE