High‐Energy Gamma Rays from the Massive Black Hole in the Galactic Center

Accreting black holes (BHs) are believed to be sites of possible particle acceleration with conditions that are also favorable for effective gamma-ray production. However, because of photon-photon pair production, only low-energy (MeV) gamma rays can escape these compact objects with typically very large compactness parameters, k1 = (L/LEdd (Rg/R)≧ 0.01, given that in most cases the accretion disks within 10 Schwarzschild radii, Rg, radiate with a power exceeding 10% of the Eddington luminosity, LEdd. Therefore, the high-energy gamma-ray emission of these objects (both of stellar mass and supermassive BHs) is generally suppressed, and consequently, the unique information on possible particle acceleration processes near the event horizon of the BH is essentially lost. Fortunately, this is not the case for the supermassive BH located at the dynamical center of our Galaxy (Sgr A*), which, thanks to its extraordinary low bolometric luminosity (≦10-8LEdd), is transparent for gamma rays up to very high energies, E ± 10 TeV. We discuss different scenarios of gamma-ray production in Sgr A* and show that for a reasonable set of parameters one can expect detectable gamma-ray fluxes of both hadronic and electronic origin. Some of these scenarios are applicable not only for the TeV gamma-ray emission recently reported from the direction of Galactic center, but they may have broader implications relevant to highly variable nonthermal emission of Sgr A* in radio, IR, and X-ray bands.