The characteristics of the Galactic center excess measured with 11 years of Fermi-LAT data

Abstract

The excess of $γ$ rays in the data measured by Fermi-LAT from the Galactic center region is one of the most intriguing mysteries in Astroparticle Physics. This Galactic center excess (GCE), has been measured with respect to different interstellar emission models (IEMs), source catalogs, data selections and techniques. Although several proposed interpretations have appeared in the literature, there are not firm conclusions as to its origin. The main difficulty in solving this puzzle lies in modeling a region of such complexity and thus precisely measuring the characteristics of the GCE. In this paper, we use 11 years of Fermi-LAT data, state of the art IEMs, and the newest 4FGL source catalog to provide precise measurements of the energy spectrum, spatial morphology, position, and sphericity of the GCE. We find that the GCE has a spectrum which is peaked at a few GeV and is well fit with a log-parabola. The normalization of the spectrum changes by roughly $60\%$ when using different IEMs, data selections and analysis techniques. The spatial distribution of the GCE is compatible with a dark matter (DM) template produced with a generalized NFW density profile with slope $γ= 1.2-1.3$. No energy evolution is measured for the GCE morphology between $0.6-30$ GeV at a level larger than $10\%$ of the $γ$ average value, which is 1.25. The analysis of the GCE modeled with a DM template divided into quadrants shows that the spectrum and spatial morphology of the GCE is similar in different regions around the Galactic center. Finally, the GCE centroid is compatible with the Galactic center, with best-fit position between $l=[-0.3^{\circ},0.0^{\circ}],b=[-0.1^{\circ},0.0^{\circ}]$, and it is compatible with a spherical symmetric morphology. In particular, fitting the DM spatial profile with an ellipsoid gives a major-to-minor axis ratio between 0.8-1.2.