An alternative Explanation for the Fermi GeV Gamma-Ray Excess

Abstract

The "GeV-excess" of the diffuse gamma-rays in the halo is studied with a template fit based on energy spectra for each possible process of gamma-ray emission. Such a fit allows to determine the background and signal simultaneously, so the Galactic Disk can be included in the analysis. We find evidence that the "excess", characterised by a gamma-ray spectrum peaking at 2 GeV, is much stronger in Molecular Clouds in the disk than the "GeV-excess" observed up to now in the halo. The possible reason why the emissivity of Molecular Clouds peaks at 2 GeV are the energy losses and magnetic cutoffs inside MCs, thus depleting the low energy part of the CR spectra and shifting the maximum of the gamma-ray spectra to higher energies. This peaking of the emissivity in Molecular Clouds at 2 GeV was clearly observed from the spectrum of the Central Molecular Zone, which dominates the emission in the inner few degrees of the Galactic Centre. Although the spectrum of the Central Molecular Zone peaks at 2 GeV, it cannot be responsible for the "GeV-excess" observed in the halo, since the latitude extension of the Zone is below $|b|<0.5^{\circ }$. However, lines-of-sight into the halo cross Molecular Clouds in the disk, so the emissivity of clouds in the disk will be observed in the halo as an apparent "GeV-excess". The fact, that this "GeV-excess" has the same morphology in the disk {\it and} in the halo as the column density of Molecular Clouds, as traced by the CO map from the Planck satellite resembling an NFW-like latitude profile, and the fact the MCs have an emissivity peaking at 2 GeV shows that the "GeV-excess" originates from Molecular Clouds in the disk, not from a process surrounding the Galactic Centre.

arXiv comment37 pager, 35 figures

References in corpus (14)

• PAMELA Measurements of Cosmic-ray Proton and Helium Spectra
• Dark Matter Annihilation in The Galactic Center As Seen by the Fermi Gamma Ray Space Telescope
• Detection of a Gamma-Ray Source in the Galactic Center Consistent with Extended Emission from Dark Matter Annihilation and Concentrated Astrophysical Emission
• Dark Matter and Pulsar Model Constraints from Galactic Center Fermi-LAT Gamma Ray Observations
• A comment on the emission from the Galactic Center as seen by the Fermi telescope
• Stringent and Robust Constraints on the Dark Matter Annihilation Cross Section From the Region of the Galactic Center
• Interacting Cosmic Rays with Molecular Clouds: A Bremsstrahlung Origin of Diffuse High Energy Emission from the Inner 2deg by 1deg of the Galactic Center
• Radial distribution of the diffuse gamma-ray emissivity in the galactic disk
• The Contribution of Cosmic Rays Interacting With Molecular Clouds to the Galactic Center Gamma-Ray Excess
• The Gamma-Ray Luminosity Function of Millisecond Pulsars and Implications for the GeV Excess
• Galactic Gamma-Ray Background Radiation from Supernova Remnants
• New evidence for charge-sign dependent modulation during the solar minimum of 2006 to 2009
• The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF
• On the GeV excess in the diffuse γ-ray emission towards the Galactic Center

Cited by in corpus (2)

• The Fermi Galactic Center GeV Excess and Implications for Dark Matter
• Gravitational probes of dark matter physics