Molecular clouds as origin of the Fermi gamma-ray GeV excess
Abstract
The so-called GeV excess of the diffuse Galactic gamma-ray emission, as observed by the Fermi-LAT satellite, is studied with a spectral template fit based on energy spectra for each relevant process of gamma-ray emission. This has the advantage over "conventional" analysis because one includes the spectral knowledge of physical processes into the fit, which allows one to determine simultaneously the standard background processes and contributions from nonstandard processes, like the Fermi bubbles or the GeV excess, in each sky direction. The spectral templates can be obtained in a data-driven way from the gamma-ray data, which avoids the use of emissivity models to subtract the standard background processes from the data. Instead, one can determine these backgrounds simultaneously with any "signals" in any sky direction, including the Galactic disk and the Galactic center. Using the spectral template fit, two hypothesis of the GeV excess were tested: the dark matter (DM) hypothesis assuming the excess is caused by DM annihilation, and the molecular cloud hypothesis assuming the GeV excess is related to a depletion of gamma rays below 2 GeV, as is directly observed in the Central Molecular Zone. The origin of the depletion below 2 GeV is not important but is most likely caused by a magnetic cutoff of cosmic rays approaching molecular clouds (MCs), as will be discussed later. Both hypotheses provide acceptable fits, if one considers a limited field of view centered within 20° around the Galactic center and applies cuts on the energy range and/or excludes low latitudes, cuts typically applied by the proponents of the DM hypothesis. However, if one considers the whole gamma-ray sky and includes gamma-ray energies up to 100 GeV, one can find that the MC hypothesis is preferred over the DM hypothesis for several reasons: i) the MC hypothesis provides significantly better fits; ii) the morphology of the GeV excess follows the morphology of the Carbon-monoxide maps, a tracer of MCs, i.e., there exists a strong GeV excess in the Galactic disk also at large longitudes; iii) the massive Central Molecular Zone with a rectangular field of view of l ×b ≃3.5 °×0.5 ° shows the maximum of the energy flux per log bin in the diffuse gamma-ray spectrum at 2 GeV, i.e., the GeV excess, already in the raw data without any analysis. The rectangular profile contradicts the spherical morphology expected for DM annihilation.
- Publication:
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Physical Review D
- Pub Date:
- August 2017
- DOI:
- 10.1103/PhysRevD.96.043012
- arXiv:
- arXiv:1707.08653
- Bibcode:
- 2017PhRvD..96d3012D
- Keywords:
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- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 53 pages, 8 figures (+ 42 figures in Appendices), extended version of arXiv:1610.08926 accepted for publication in PRD