Permissible transitions of the variability classes of GRS 1915+105
  The Galactic microquasar GRS 1915+105 exhibits at least sixteen types of variability classes. Transitions from one class to another could take place in a matter of hours. In some of the classes, the spectral state transitions (burst-off to burst-on and vice versa) were found to take place in a matter of few to few tens of seconds. In the literature, there is no attempt to understand in which order these classes were exhibited. Since the observation was not continuous, the appearances of these classes seem to be in random order. Our goal is to find a natural sequence of these classes and compare with the existing observations. We also wish to present a physical interpretation of the sequence so obtained using two component advective flow model of black hole accretion. In the present paper, we compute the ratios of the power-law photons and the black body photons in the spectrum of each class and call these ratios as the `Comptonizing efficiency'(CE). We sequence the classes from the low to the high value of CE. We clearly find that each variability class could be characterized by a unique average Comptonizing efficiency. The sequence of the classes based on this parameter seem to be corroborated by a handful of the observed transitions caught by Rossi X-ray timing explorer and the Indian payload Indian X-ray Astronomy Experiment and we believe that future observation of the object would show that the transitions can only take place between consecutive classes in this sequence. Since the power-law photons are produced by inverse Comptonization of the intercepted soft-photons from the Keplerian disk, a change in CE actually corresponds to a change in geometry of the compton cloud. Thus we claim that the size of the Compton cloud gradually rises from very soft class to the very hard class. 
Partha Sarathi Pal, Sandip K. Chakrabarti and Anuj Nandi
astro-ph.HE