On a possible origin of the gamma-ray excess around the Galactic Center

TL;DR

This paper proposes that the gamma-ray excess observed around the Galactic Center can be explained by cosmic ray interactions with molecular clouds, where CR depletion inside clouds alters gamma-ray emission, offering an alternative to dark matter or pulsar hypotheses.

Contribution

It introduces a novel mechanism of cosmic ray depletion inside molecular clouds via MHD turbulence barriers, providing an alternative explanation for the gamma-ray excess.

Findings

CR depletion inside molecular clouds can reduce gamma-ray emission below a few GeV.

The proposed mechanism can potentially account for the observed gamma-ray excess.

CR interactions with clouds influence the gamma-ray spectrum and its interpretation.

Abstract

Recent observations of gamma-rays with the Fermi Large Area Telescope (LAT) in the direction of the inner Galaxy revealed a mysterious GeV excess. Its intensity is significantly above predictions of the standard model of cosmic rays (CRs) generation and propagation with a peak in the spectrum around a few GeV. Popular interpretations of this excess are due to either spherically distributed annihilating dark matter (DM) or abnormal population of millisecond pulsars. We suggested an alternative explanation of the excess through the CR interactions with molecular clouds in the Galactic Center (GC) region. We assumed that the excess could be imitated by the emission of molecular clouds with depleted density of CRs with energies below ~ 10 GeV inside. A novelty of our work is in detailed elaboration of the depletion mechanism of CRs with the mentioned energies through the "barrier" near the cloud edge formed by the self-excited MHD turbulence. Such depletion of CRs inside the clouds may be a reason of deficit of gamma rays from the Central Molecular Zone (CMZ) at energies below few GeV. This in turn changes the ratio between various emission components at those energies, and may potentially absorb the GeV excess by simple renormalization of key components.