Evidence for Spatial Variation in the High-Energy Spectrum of the Galactic Center Excess

Fermi Large Area Telescope observations towards the Milky Way center have revealed a spatially extended source of gamma rays in excess of the modeled astrophysical backgrounds. Possible explanations for this `galactic center excess' include weakly-interacting massive particle dark matter annihilations, unresolved milllisecond pulsars, and cosmic-ray outbursts from the galactic center. I will discuss an analysis comparing the the spatial morphology and spectrum of the excess signal in the innermost few degrees of the galactic center versus the outlying sky regions. We find that the excess spectrum above $\sim$10 GeV is spatially varying: the spectrum extends above these energies outside of $\sim5^\circ$ in galactocentric radius, but cuts off sharply by $\sim$10 GeV in the innermost few degrees. If interpreted as a real feature of the excess, this radial variation in the spectrum has important implications for both astrophysical and dark matter interpretations of the galactic center excess. Single-component dark matter annihilation models face challenges in reproducing this variation; on the other hand, a population of unresolved millisecond pulsars contributing both prompt and secondary inverse Compton emission may be able to explain the spectrum as well as its spatial dependency.