SEQUENCING THE VARIABILITY CLASSES OF GRS 1915+105

The galactic black hole GRS 1915+105 exhibits at least 13 types of X-ray variability classes. Transitions from one class to another take place in a matter of hours. Within each class, the spectral state transitions take place in a matter of few seconds, hinting at the fact that it is the free-falling, and not the Keplerian disk that is involved. In the present paper, we establish that one of the reasons of the class transitions is the variation of the Comptonizing efficiency (CE), i.e., the ratio of the power-law photons and the black body photons in the average spectrum of each class. Around a mean CE, there is a scatter in time scales of a few to a few tens of seconds which can be achieved by variation of the cooling effects of the outflow through Comptonization. Thus, we pinpoint two major causes of the variability of the enigmatic black hole candidate GRS 1915+105.