Observational constraints on the models of disk accretion onto a black hole

Abstract

Observations of Galactic black hole candidates performed with the ART‐P telescope aboard Granat in 1990–1992 focused our attention on studying of the sources spectral variability vs the accretion rate. We show that in contrast with the current scenario of disk accretion onto a black hole the hard spectral state of the sources always occurs at smaller accretion rate than the soft state. The arguments are presented that hard x‐rays detected during the ‘‘very high’’ and ‘‘low’’ states of black hole candidates have a distinct origin.