Cosmic Ray Protons in the Inner Galaxy and the Galactic Center Gamma-Ray Excess

TL;DR

The paper argues that the gamma-ray excess observed in the inner Galaxy can be explained by cosmic ray protons from local supernova remnants, challenging the dark matter annihilation hypothesis and emphasizing the importance of background modeling.

Contribution

It demonstrates that cosmic ray protons can produce gamma-ray features similar to the observed excess, providing an astrophysical explanation rather than a dark matter origin.

Findings

Gamma-ray excess can be explained by cosmic ray protons from supernova remnants.

A recent cosmic-ray injection burst explains the Galactic center excess.

Extended inner Galaxy excess suggests older cosmic-ray injection episodes.

Abstract

A gamma-ray excess over background has been claimed in the inner regions of the Galaxy, triggering some excitement about the possibility that the gamma rays originate from the annihilation of dark matter particles. We point out that the existence of such an excess depends on how the diffuse gamma-ray background is defined, and on the procedure employed to fit such background to observations. We demonstrate that a gamma-ray emission with spectral and morphological features closely matching the observed excess arises from a population of cosmic ray protons in the inner Galaxy, and provide proof of principle and arguments for the existence of such a population, most likely originating from local supernova remnants. Specifically, the "Galactic center excess" is readily explained by a recent cosmic-ray injection burst, with an age in the 1-10 kilo-year range, while the extended inner Galaxy excess points to mega-year old injection episodes, continuous or impulsive. We conclude that it is premature to argue that there are no standard astrophysical mechanisms that can explain the excess.