SIMBAD references

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 this paper, we show variation of Comptonization efficiency (CE), defined to be the ratio of the number of power-law photons to the number of soft photons injected into a 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 the Compton cloud. Our investigation indicates that all outbursts start with a large CE, which 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 the outburst. We show co-variation of count rates, frequency of quasi-periodic oscillations, photon index and CE, and establish a general trend of disc geometry variation during outbursts in all systems under consideration.