Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy

Abstract

We present a new method to characterize unresolved point sources (PSs), generalizing traditional template fits to account for non-Poissonian photon statistics. We apply this method to Fermi Large Area Telescope gamma-ray data to characterize PS populations at high latitudes and in the Inner Galaxy. We find that PSs (resolved and unresolved) account for ~50% of the total extragalactic gamma-ray background in the energy range ~1.9 to 11.9 GeV. Within 10${}^{\circ }$ of the Galactic Center with $|b|\ge 2^{\circ }$, we find that ~5-10% of the flux can be accounted for by a population of unresolved PSs, distributed consistently with the observed ~GeV gamma-ray excess in this region. The excess is fully absorbed by such a population, in preference to dark-matter annihilation. The inferred source population is dominated by near-threshold sources, which may be detectable in future searches.

7+22 pages, 4+18 figures; v2, minor changes, new Pass 8 data analyzed (conclusions unchanged); v3, PRL version, substantive improvements and additional checks (conclusion unchanged)