High-energy emission from relativistic particles in Sagittarius A

EGRET onboard the Compton Gamma Ray Observatory has recently detected a high-energy source at the Galactic center. However, it is not yet clear whether the gamma-rays are produced by the hypothesized massive black hole, Sgr A*, or whether the emitter is diffuse. The lack of variability in the gamma-ray flux, up to an amplitude of about 20%, seems to suggest the latter. But hydrodynamical simulations of the accretion process point to a fluctuation amplitude in the inflowing mass rate below this level on a timescale comparable to the current observation time line. Thus, if the gamma-ray flux is directly tied to the dissipation of gravitational energy, e.g., if it is produced by relativistic particles energized by a shock within the infalling plasma, we cannot rule Sgr A* out as the source of the gamma-rays on this basis alone. In earlier work, we demonstrated that the gamma-ray spectrum from the Galactic center may be consistent with the radiative decay of pions produced via proton-proton interactions in this shock acceleration scenario. However, this study was incomplete for several reasons. In this paper, we extend this work significantly by making several improvements to our treatment of the particle physics. We also make use of the new EGRET data published since our last paper. Our analysis now indicates that the hypothesized massive black hole is probably not the source of the gamma-rays. Understanding the nature of the Galactic center gamma-ray source may ultimately depend on whether or not future gamma-ray detectors will have the capability of resolving it. In this paper we consider a point source, but if the gamma-rays are in fact associated with diffuse emission, GLAST may be able to image the source with a spatial resolution of similar to 30" to 5'.