The Galactic Center GeV Excess from a Series of Leptonic Cosmic-Ray Outbursts

TL;DR

This paper explores whether a series of leptonic cosmic-ray outbursts can explain the GeV gamma-ray excess observed near the Galactic Center, proposing specific outburst scenarios and their implications for the observed emission morphology and spectrum.

Contribution

It introduces a model of multiple leptonic cosmic-ray outbursts that can reproduce the spatial and spectral features of the Galactic Center gamma-ray excess.

Findings

Two outbursts can explain the emission outside 1-2 degrees from the Galactic Center.

Additional recent outbursts or millisecond pulsars are needed for the inner region.

Older outbursts inject higher energy electrons, shaping the spectrum after energy losses.

Abstract

It has been proposed that a recent outburst of cosmic-ray electrons could account for the excess of GeV-scale gamma rays observed from the region surrounding the Galactic Center. After studying this possibility in some detail, we identify scenarios in which a series of leptonic cosmic-ray outbursts could plausibly generate the observed excess. The morphology of the emission observed outside of $\sim1^{\circ}-2^{\circ}$ from the Galactic Center can be accommodated with two outbursts, one which took place approximately $\sim10^6$ years ago, and another (injecting only about 10\% as much energy as the first) about $\sim10^5$ years ago. The emission observed from the innermost $\sim1^{\circ}-2^{\circ}$ requires one or more additional recent outbursts and/or a contribution from a centrally concentrated population of unresolved millisecond pulsars. In order to produce a spectrum that is compatible with the measured excess (whose shape is approximately uniform over the region of the excess), the electrons from the older outburst must be injected with significantly greater average energy than those injected more recently, enabling their spectra to be similar after $\sim10^6$ years of energy losses.