Chandra, GLAST, and the Galactic Center

Abstract

Two-temperature spherical accretion flows produce $\approx 100$ Mev gamma-rays from the decay of neutral pions created in proton-proton collisions close to the black hole; they also produce $\sim 10$ keV X-rays by bremsstrahlung emission at large radii. The gamma-ray to X-ray luminosity ratio is nearly independent of black hole mass and accretion rate. It does depend sensitively on the radial density profile of the accretion flow through the parameter a, where $n \propto r^{-a}$. For the canonical Bondi value of a = 3/2, the gamma-ray to X-ray luminosity ratio is $\approx 30$. We interpret a recent Chandra detection coincident with the massive black hole at the Galactic Center as being thermal bremsstrahlung emission from the accretion flow. With this normalization, the expected gamma-ray luminosity is $\approx 10^{35}$ ergs s$^{-1}$ if a = 3/2. This is nearly two orders of magnitude above the detection threshold of the GLAST telescope. For $a \approx 1/2$, however, (a value suggested by recent theoretical arguments), the expected gamma-ray luminosity is only $\approx 10^{29}$ ergs s$^{-1}$; GLAST should therefore provide an important probe of the true accretion rate and radial density profile of the accretion flow onto Sgr A*.