Spectral states of Galactic black hole candidates: Results of observations with ART-P/Granat

Abstract

Observations of Galactic black hole candidates performed with the ART-P telescope aboard Granat in 1990–1992 focused our attention on the problem of studying spectral variability of these sources as a function of the accretion rate. We show that in contrast with the current models of disk accretion onto a black hole the hard spectral state of the sources in which they exhibit a Cyg X-1 type hard X-ray spectrum always occurs at smaller accretion rate than the soft state. The arguments that hard X-rays detected during the “very high” and “low” states of black hole candidates have a distinct origin are presented.