The High-Energy Tail of the Galactic Center Gamma-Ray Excess

Abstract

Observations by the Fermi-LAT have uncovered a bright, spherically symmetric excess surrounding the center of the Milky Way galaxy. The spectrum of the gamma-ray excess peaks sharply at an energy ~2 GeV, exhibiting a hard spectrum at lower energies, and falls off quickly above an energy ~5 GeV. The spectrum of the excess above ~10 GeV is potentially an important discriminator between different physical models for its origin. We focus our study on observations of the gamma-ray excess at energies exceeding 10 GeV, finding: (1) a statistically significant excess remains in the energy range 9.5-47.5 GeV, which is not degenerate with known diffuse emission templates such as the Fermi Bubbles, (2) the radial profile of the excess at high energies remains relatively consistent with data near the spectral peak, (3) the data above ~5 GeV prefer a slightly greater ellipticity with a major axis oriented perpendicular to the Galactic plane. Using the recently developed non-Poissonian template fit, we find mild evidence for a point-source origin for the high-energy excess, although given the statistical and systematic uncertainties we show that a smooth origin of the high-energy emission cannot be ruled out. We discuss the implication of these findings for pulsar and dark matter models of the gamma-ray excess. Finally we provide a number of updated measurements of the gamma-ray excess, utilizing novel diffuse templates and the Pass 8 dataset.