Molecular clouds as the origin of the Fermi gamma-ray GeV-excess
August 16, 2017
August 03, 2018
The Fermi-LAT data reveal an excess of diffuse gamma-rays at energies of around 2 GeV in the
direction of the Galactic center. The excess has been studied by many groups and is observed
above the expectation for diffuse gamma-ray emission from cosmic ray interactions with the in-
terstellar material (π0 production from cosmic ray protons and bremstrahlung from electrons in
the interstellar gas) and the interstellar radiation field (inverse Compton scattering of electrons in
the interstellar radiation field). In addition to these standard components in diffuse gamma-rays
we find evidence for two additional processes: π 0 production in sources during acceleration and
π 0 production in molecular clouds.
The first one is characterized by nuclear cosmic rays with a hard E −2 spectrum, expected from
diffusive shockwave acceleration and can be traced by the 1.8 MeV gamma-ray line from radioac-
tive 26 Al decays, which is synthesized in sources. The second one is characterized by nuclear
cosmic rays inside molecular clouds with a sharp cutoff below 6-14 GV, which is most clearly
observed in the dense Central Molecular Zone encircling the Galactic center in the Galactic disk.
The cutoff leads to a suppression of low energy cosmic ray interactions in molecular clouds, which
causes a shift in the maximum of the gamma-ray spectrum to higher energies, the hall-mark of
the GeV-excess. This was previously interpreted as a dark matter annihilation signal.
No spatial information is provided to our fit. As a result we obtain an uncorrelated and spatially
highly resolved distribution of the GeV-excess. We show that a shift in the maximum of the
gamma-ray spectrum, or equivalently the GeV-excess, is observed in all directions, where molec-
ular clouds are present; these directions are available from the high resolution all-sky CO maps
from the Planck satellite.
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