Dark Matter Annihilation in The Galactic Center As Seen by the Fermi Gamma Ray Space Telescope

TL;DR

This paper analyzes the first two years of Fermi-LAT gamma-ray data toward the Galactic Center to search for dark matter annihilation signals. By modeling astrophysical backgrounds and testing a spherically symmetric, DM-like excess within ≈1° of the GC, the authors find a central component whose spectrum peaks at 1–4 GeV and is well described by DM with Mass ≈ 7–10 GeV annihilating mainly to τ^+τ^−, requiring ⟨σv⟩ on the order of 10^{−27}–10^{−26} cm^3 s^{−1} depending on the inner halo slope γ ≈ 1.18–1.33 and halo normalization. The inferred DM distribution is consistent with a cusped profile possibly enhanced by adiabatic contraction and with a thermal relic cross section, and the results have potential connections to low-mass DM hints from direct-detection experiments. The authors also consider astrophysical alternatives, such as unresolved pulsars or Sgr A* contributions, but argue that these face significant challenges in explaining both the spectral shape and the strong central concentration observed.

Abstract

We analyze the first two years of data from the Fermi Gamma Ray Space Telescope from the direction of the inner 10 degrees around the Galactic Center with the intention of constraining, or finding evidence of, annihilating dark matter. We find that the morphology and spectrum of the emission between 1.25 degrees and 10 degrees from the Galactic Center is well described by a the processes of decaying pions produced in cosmic ray collisions with gas, and the inverse Compton scattering of cosmic ray electrons in both the disk and bulge of the Inner Galaxy, along with gamma rays from known points sources in the region. The observed spectrum and morphology of the emission within approximately 1.25 degrees (~175 parsecs) of the Galactic Center, in contrast, departs from the expectations for by these processes. Instead, we find an additional component of gamma ray emission that is highly concentrated around the Galactic Center. The observed morphology of this component is consistent with that predicted from annihilating dark matter with a cusped (and possibly adiabatically contracted) halo distribution (density proportional to r^{-gamma}, with gamma=1.18 to 1.33). The observed spectrum of this component, which peaks at energies between 1-4 GeV (in E^2 units), can be well fit by a 7-10 GeV dark matter particle annihilating primarily to tau leptons with a cross section in the range of 4.6 x 10^-27 to 5.3 x 10^-26 cm^3/s, depending on how the dark matter distribution is normalized. We also discuss other sources for this emission, including the possibility that much of it originates from the Milky Way's supermassive black hole.

Figures (16)

• Figure 1: The locations of the 69 known point sources included in our study. The size of each $X$ is proportional to the intensity of that source in the range of 1-100 GeV, as described in the Fermi First Source Catalog. The region between the two solid circles is that shown in Figs. \ref{['angular1']}-\ref{['angular4']}.
• Figure 2: The best fit model for the emission in the four lowest energy bins from the region between 9 and 10 degrees from the Galactic Center (the region between the two circles shown in Fig. \ref{['ptsources']}), compared to the observations of the FGST. See text for details.
• Figure 3: The same as Fig. \ref{['angular1']}, but for higher energy bins, as labeled.
• Figure 4: The same as Fig. \ref{['angular1']}, but for higher energy bins, as labeled.
• Figure 5: The same as Fig. \ref{['angular1']}, but for higher energy bins, as labeled.