Narrow Results

The BATSE instrument aboard the Compton Gamma Ray Observatory monitored numerous astronomical sources, including the Galactic black holes Cygnus X-1, GRO J0422+32, GRO J1719-24, GRS 1915+105, and GRO J1655-40, almost continuously from 1991 to 2000 in the energy range 23–1798 keV. We present results for these five black holes from the full nine-year BATSE earth occultation database using the JPL data analysis package EBOP (Enhanced BATSE Occultation Package). Light curves in four broad energy bands with 1-day resolution are presented showing flux histories for each source over the nine years. Spectra during the high flux/outburst states and low flux/quiescent states for these objects are also presented. The differences in the spectra suggest that there may two different classes of Galactic black holes.

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.