Investigating the Uniformity of the Excess Gamma rays towards the Galactic Center Region

TL;DR

This study analyzes the spatial and spectral properties of the gamma-ray excess near the Galactic Center, highlighting challenges for dark matter models and suggesting unresolved pulsars as a plausible source, with implications for background modeling.

Contribution

It provides a detailed spatial-spectral analysis of the gamma-ray excess, revealing radial spectral variations and systematic uncertainties affecting source interpretation.

Findings

The excess spectrum extends above 10 GeV outside 5° radius.

The spectrum steeply cuts off between 10-20 GeV in the innermost region.

Unresolved pulsars could explain the spectral and spatial features.

Abstract

We perform a composite likelihood analysis of subdivided regions within the central $26^\circ\times20^\circ$ of the Milky Way, with the aim of characterizing the spectrum of the gamma-ray galactic center excess in regions of varying galactocentric distance. Outside of the innermost few degrees, we find that the radial profile of the excess is background-model dependent and poorly constrained. The spectrum of the excess emission is observed to extend upwards of 10 GeV outside $\sim5^\circ$ in radius, but cuts off steeply between 10--20 GeV only 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. We show that the expected differences in the photon-count distributions of a smooth dark matter annihilation signal and an unresolved point source population are an order of magnitude smaller than the fluctuations in residuals after fitting the data, which implies that mismodeling is an important systematic effect in point source analyses aimed at resolving the gamma-ray excess.