Thermal Comptonization models for gamma-ray emission from black-hole sources

Black holes power the variable luminous emission observed from quasars and the nuclei of active galaxies; at least it is assumed so here. The list of black-hole candidates detected at gamma-ray energies includes the galactic binary X-ray source Cygnus X-1, the Galactic Center, and the extragalactic sources Cen A (NGC 5128), NGC 4151, MCG 8-11-11 and 3C 273. The luminosity of these objects peaks in the photon energy range 0.1 equal to or less than epsilon equal to or less than 10 MeV and dominates the total energy output in the observable wavebands. SAS-2 and COS-B upper limits and observations indicate a cutoff or softening in their spectra between approx. 1 and approx. 30 MeV, which is probably due to absorption of gamma rays by photon-photon pair production or a break in the underlying particle spectrum. Indeed, spectral softening at MeV energies seems to be required of active galactic nuclei (AGN) in general; otherwise the superposition of the emission from these sources would conflict with observations of the diffuse gamma-ray background radiation. The salient features associated with gamma-ray emission from the above-mentioned sources are summarized in this paper, beginning with Cygnus X-1, the best-studied black-hole candidate. Because of similarity in properties, models developed to explain the X-ray and gamma-ray observations of Cygnus X-1 can be equally applied to AGN. In this review, successes and limitations of thermal Comptonization models for the high-energy emission of black-hole sources are considered.