High-energy spectrum of the galactic center black hole Sgr A

The massive blackhole candidate Sgr A* may be accreting from an ambient Galactic center wind at a rate ≈ 10²² g s^-1. Dissipational processes within the large-scale quasi-spherical infall, from 50 r_g to 10⁵ r_g (where r_g = 3 × 10¹¹ cm is the Schwarzschild radius for M = 10⁶ M_⊙) can account well for the observed radio spectrum and flux (due to cyclotron/synchrotron emission) and its X-ray/Gamma-ray luminosity (apparently due to Bremsstrahlung emission). Small scale instabilities associated with the stagnation region of the flow produce fluctuations in the accretion rate with an amplitude of up to 30% and a time scale of several months to over a year. This may account for the long term variability of the high-energy (and radio) luminosity observed from Sgr A*. Although the average accreted angular momentum is approximately zero, these instabilities also induce fluctuations in the specific angular momentum of the accreted gas that lead to a circularized flow at distances less than about 20 r_g. Optically thick emission from this "disk" (roughly the size of Mercury's orbit) is the origin of the IR flux recently detected from the location of Sgr A*, but is not itself a significant source of high-energy emission. Published by Elsevier Science on behalf of COSPAR.