OBSERVATIONAL SIGNATURE OF THE BOUNDARY-LAYER OF GALACTIC AND EXTRAGALACTIC BLACK-HOLES

We present spectral properties of an accretion disk model, in which a Keplerian accretion disk is flanked by a sub-Keplerian halo component terminating at a standing shock. The post-shock region (which may be c onsidered to be the boundary layer of a black hole) reprocesses the so ft photons emitted from the Keplerian accretion disk. We show that swi tching of states (from hard to soft and vice versa) could be accomplis hed by a change in the accretion rate of the Keplerian disk component. Our consideration, for the first time, resolves a long-standing probl em of identifying the illusive 'Compton cloud' responsible for the swi tching of states.