Geometry variation of accretion disks of Compact Objects
  The Temporal and Spectral variations of black hole candidates during outbursts have been reported in several publications. It is well known that during an outburst, the source becomes soft in the first few days, and then returns to the hard state after a few weeks or months. In the present paper, we show the variation of Comptonization Efficiency (CE), obtained from the ratio of the black body photon number to the power-law photon number, as a function of time in several outbursts. Since the power-law photons are generated through inverse-Comptonization of the intercepted soft photons, the CE is a measure of the geometry of the Compton cloud. Our investigation indicated that all the outbursts starts with a large CE and becomes very small after a few days, when the Compton cloud becomes very small to intercept any significant number of soft photons. CE returns back to a larger value at the end of the outburst. 
Partha Sarathi Pal and Sandip K. Chakrabarti
astro-ph.HE