Abstract
A GeV gamma-ray excess has possibly been individuated in Fermi-LAT data from the Galactic Center and interpreted in terms of Dark Matter (DM) annihilations, either in hadronic (essentially bb̄) or leptonic channels. In order to test this tantalizing interpretation, we address two issues: (i) we improve the computation of secondary emission from DM (Inverse Compton and Bremsstrahlung) with respect to previous works, confirming it to be very relevant for determining the DM spectrum in the leptonic channels, so that any conclusion on the DM nature of the signal critically depends on this contribution; (ii) we consider the constraints from antiprotons on the DM hadronic channel, finding that the uncertainties on the propagation model, and in particular on the halo height, play a major role. Moreover, we discuss the role of solar modulation, taking into account possible charge dependent effects whose importance is estimated exploiting detailed numerical tools. The limits that we obtain severely constrain the DM interpretation of the excess in the hadronic channel, for standard assumptions on the Galactic propagation parameters and solar modulation. However, they considerably relax if more conservative choices are adopted.
Highlights
On the capability of carefully assessing the background over which the excess is supposed to emerge
In order to test this tantalizing interpretation, we address two issues: (i) we improve the computation of secondary emission from Dark Matter (DM) (Inverse Compton and Bremsstrahlung) with respect to previous works, confirming it to be very relevant for determining the DM spectrum in the leptonic channels, so that any conclusion on the DM nature of the signal critically depends on this contribution; (ii) we consider the constraints from antiprotons on the DM hadronic channel, finding that the uncertainties on the propagation model, and in particular on the halo height, play a major role
The antiprotons are a precious cross check in this context for several reasons: first of all, it is common wisdom that –in most non-leptophilic WIMP scenarios– the ratio between the exotic signal and the astrophysical background is noticeably large in this channel; the astrophysical background is known with a considerable level of precision: it depends on the proton flux and, apart from an unavoidable uncertainty on the production cross-section, it is not highly affected by the choice of a diffusion setup among those allowed by light nuclei ratios and other observables
Summary
The DM distribution ρ in the Galaxy is unknown and it is source of significant uncertainty. In this specific case, it is claimed that the morphology of the GC excess signal allows to determine ρ quite precisely. DM emits secondary gamma-rays, namely via the already mentioned IC and Brems processes In the former, the electrons and positrons originating from the annihilation hit an ambient photon (from the CMB, the InfraRed or the Optical galactic radiation field) and up-scatter it to gamma-ray energies. For this work we have cross checked the output of the code, for specific cases, against the semi-analytic calculations mentioned above, finding a very good agreement
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