Optics near Kerr black holes: spectra of advection dominated accretion flows

We investigate advection dominated, transsonic accretion flows in the vicinity of a Kerr black hole. We take into account all relativistic effects in the dynamics of the flow and in the propagation of light. We assume the matter to be weakly magnetized and cool via the thermal synchrotron and Bremsstrahlung radiation. We include also the effects of Comptonization. We calculate the spectra of radiation as seen by observers located at different positions relative to the equatorial plane of the disk. The radiation emitted by the accreting matter is anisotropic and observers near the equatorial plane register a higher energy flux. This effect is more pronounced in the case of slowly rotating black holes. We calculate also the shape of a hypothetical gamma line, which may be produced by the thermonuclear reactions in the inner part of the flow. The line is strongly broadened, but the fact that the flow is quasi-spherical removes the two-peak shape of the line seen in the spectra emitted from thin, Keplerian disks. The kinematics of the advection dominated flows is not unique (as opposed to Keplerian disks or spherical free-fall) and it would probably be difficult to find strong limits on source models using the spectral observations.