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 considered to be the boundary layer of a black hole) reprocesses the soft photons emitted from the Keplerian accretion disk. We show that switching of states (from hard to soft and vice versa) could be accomplished by a change in the accretion rate of the Keplerian disk component. Our consideration, for the first time, resolves a long-standing problem of identifying the illusive `Compton cloud responsible for the switching of states.