Abstract
Recent observations of gamma rays with the Fermi Large Area Telescope (LAT) in the direction of the inner galaxy revealed a mysterious excess of GeV. 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 that it is due to either spherically distributed annihilating dark matter (DM) or an abnormal population of millisecond pulsars. We suggest 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. This depletion of CRs inside the clouds may be a reason for the deficit of gamma rays from the Central Molecular Zone (CMZ) at energies below a few GeV. This in turn changes the ratio between various emission components at those energies and may potentially absorb the GeV excess by a simple renormalization of key components.
Highlights
Introduction and MotivationThe physical nature of dark matter (DM) remains one of the biggest puzzles in modern physics despite the extensive searches for DM with colliders, direct detectors and indirect observations
We elaborated the mechanism of the modification of emission spectra of molecular cloud (MC), which, according to [16], can fit the mysterious GeV excess around Galactic Center (GC)
Our mechanism represents a viable alternative to other possible excess explanations such as annihilating DM, millisecond pulsars, etc
Summary
The physical nature of dark matter (DM) remains one of the biggest puzzles in modern physics despite the extensive searches for DM with colliders, direct detectors and indirect observations. Searches for narrow lines in the FermiLAT data were not successful so far—only upper limits were derived; see, e.g., [3] This justifies a motivation to develop new gamma-ray telescopes with better spectroscopic capabilities. It is expected to have better sensitivity to narrow spectral lines around GC in the energy range of ∼(0.1–100) GeV in comparison with Fermi-LAT due to better energy resolution This and other aspects of DM searches by GAMMA-400 are described in detail in [5]. Almost all of the mentioned opportunities were challenged at some point by a nontrivial observational fact: abnormal excessive emission was identified in other regions of the galactic plane [15]. The section describes in detail our model of CR interaction with MCs, which may modify the MC emission spectra through a peculiar way leading to the imitation of some excessive emission at GeV energies
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