Accretion processes around supermassive black holes

Abstract

Active Galactic Nuclei (AGNs) are believed to be powered by accretion onto supermassive black holes (BHs). With the development in high resolution observations over a broad frequency range, it is now tenable to study the corresponding physical processes in detail. We find that the emission from the closest supermassive BH candidate, Sagittarius A*, a compact radio source presumably accreting from stellar winds prevailing at the Galactic Center, can be explained as due to a quasi-spherical accretion flow, which circularizes to form a small magnetized accretion disk near the BH's event horizon. The mm/sub-mm and X-ray emissions are produced via thermal synchrotron processes and their self-Comptonization, respectively, in the inner ten Schwarzschild radii of the resultant Keplerian structure. The cm radio emission, however, appears to be produced by non-thermal synchrotron processes in the circularization zone. The recently detected X-ray flare seems to indicate a transient enhancement of mass accretion rate through the inner accretion disk. The 106-day cycle seen at 2.0 cm and 1.3 cm, on the other hand, suggests that the disk is precessing around a spinning BH, whose spin may be determined by timing observation of Sgr A* at mm/sub-mm wavelengths. Our tentative observational result is consistent with this magnetized disk model. The supermassive BH M31*, a compact radio source in the nucleus of M31, has many features in common with Sgr A*, yet their differences are significant. We show that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the capture radius. Sgr A* is presumably a 'hot' BH. While M31* seems to be a member of the 'cold' BH family. The study of the nuclei in radio galaxies reveals many new characteristics of the large scale accretion flows. In NGC 4261, we show that a turbulence-dominated disk, illuminated by its AGN, can not only account for the observed sub-parsec scale radio gap in the core, but also produce the optical broad lines emitted from the region. However, the prominent radio jets distinguish such BHs from those in the compact radio sources. The relativistic jets are probably driven by the action of supermassive, fast spinning BHs. Our study on NGC 6251* indicates that the initial ejection of matter can be associated with the thermal expansion of the accreted gas, which is heated by a spinning BH near its even horizon.