Accretion around active galactic nuclei

In bright AGN, at distances of several Schwarzschild radii, the accretion onto a black hole proceeds predominantly in a form of a cool accretion disk. However, very close to a black hole the accreting material forms a multi-phase medium (coexisting cold optically thick gas, very hot optically thin plasma and an intermediate warm partially ionized material), and the geometrical arrangement is not clear. Several different geometries satisfy the current observational constraints. In faint AGN the observationally estimated Bondi accretion rate is orders of magnitudes higher than expected at the basis of the observed luminosity so again the accretion pattern is not understood, with angular momentum problem, outflow of material and/or nonstationarity possibly playing a role. Better theoretical models are being developed but still a number of important physical processes are to be introduced. Such better models, and extensive monitoring of the variability are needed to understand such a complex phenomenon as the accretion onto a supermassive black hole.