A HYBRID TWO COMPONENT ACCRETION FLOW SURROUNDING SUPERMASSIVE BLACK HOLES IN AGNs

Abstract

It is commonly believed that the optical/UV and X-ray emissions in luminous active galactic nuclei (AGNs) are produced in an accretion disk and an embedded hot corona, respectively. The inverse Compton scattering of disk photons by hot electrons in the corona can effectively cool the coronal gas if the mass supply is predominantly via a cool disk-like flow as in black hole X-ray binaries (BHXRBs). Thus, the application of such a model to AGNs fails to produce their observed X-ray emission. As a consequence, a fraction of disk accretion energy is usually assumed to be transferred to the corona. To avoid this assumption, we propose that gas in a vertically extended distribution is supplied to a supermassive black hole by the gravitational capture of interstellar medium or stellar wind material. In this picture, the gas partially condenses to an underlying cool disk as it flows toward the black hole, releasing accretion energy as X-ray emission and supplying mass for the disk accretion. Detailed numerical calculations reveal that the X-ray luminosity can reach a few tens of percent of the bolometric luminosity. The value of varies from 0.9 to 1.2 for the mass supply rate ranging from 0.03 to 0.1 times the Eddington value. The corresponding photon index in the 2–10 keV energy band varies from 1.9 to 2.3. Such a picture provides a natural extension of the model for low luminosity AGNs where condensation is absent at low mass accretion rates and no optically thick disk exists in the inner region.