Ilias Cholis (Johns Hopkins University)

The Fermi Galactic Center excess; A signal of Dark Matter Annihilation, Millisecond Pulsars or Bursts of Cosmic-Rays?

The possible gamma-ray excess in the inner Galaxy and the Galactic center suggested by Fermi-LAT observations has triggered great interest in the astro-particle physics community. Among its various interpretations have been WIMP dark matter annihilations, gamma-ray emission from a population of millisecond pulsars, or emission from cosmic rays injected in a sequence of burst-like events or continuously at the GC. Given that the galactic diffuse emission is the dominant (by an order of magnitude or more) at any direction greater than 2 degrees from the GC understanding the background systematics has been a vital missing piece in the discussion. I will present the first comprehensive study of model systematics coming from the Galactic diffuse emission in the inner part of our Galaxy and their impact on the inferred properties of the excess emission at Galactic latitudes between 2 and 20 degrees and energies 300 MeV to 500 GeV. I will show both theoretical and empirical model systematics, which are deduced from a large range of Galactic diffuse emission models and a principal component analysis of residuals in numerous test regions along the Galactic plane. The hypothesis of an extended spherical excess emission with a uniform energy spectrum is compatible with the Fermi-LAT data in the region of interest at 95% CL. Assuming that this excess is the extended counterpart of the one seen in the inner few degrees of the Galaxy, a lower limit of 10 degrees (95% CL) can be derived on its extension away from the GC.

Finally, in light of these data, I will also discuss interpretations of this excess, based on annihilating DM, leptonic CR outburst and a population of millisecond pulsars.