Spectral Properties of Accretion Disks around Black Holes. II. Sub-Keplerian Flows with and without Shocks

Close to a black hole, the density of the sub-Keplerian accreting matter becomes higher compared with a spherical flow as a consequence of the presence of a centrifugal barrier, independently of whether or not a standing shock actually forms. This hot, dense flow intercepts soft photons from a cold Keplerian disk and reprocesses them to form high-energy X-rays and gamma rays. We study the spectral properties of various models of accretion disks in which a Keplerian disk on the equatorial plane may or may not be flanked by a sub-Keplerian disk and the sub-Keplerian flow may or may not possess standing shocks. From comparison with the spectra, we believe that the observed properties could be explained better when both components (Keplerian and sub-Keplerian) are simultaneously present close to a black hole, even though the sub-Keplerian halo component may have been produced out of the Keplerian disk itself at larger radii. We are able to understand soft and hard states of black hole candidates, properties of X-ray nova outbursts, and quasi-periodic oscillations of black hole candidates using these two-component models. We fit spectra of X-ray novae GS 1124-68 and GS 2000+25 and satisfactorily reproduce the light curves of these objects.