Abstract
The center of the Galaxy is one of the prime targets in the search for a signal of annihilating (or decaying) dark matter. If such a signal were to be detected, it would shed light on one of the biggest mysteries in physics today: What is dark matter? Fundamental properties of the particle nature of dark matter, such as its mass, annihilation cross section, and annihilation final states, could be measured for the first time. Several experiments have searched for such a signal, and some have measured excesses that are compatible with it. A long-standing and compelling excess is observed in γ-rays by the Fermi Large Area Telescope ( Fermi–LAT). This excess is consistent with a dark matter particle with a mass of approximately 50 (up to ∼200) GeV annihilating with a velocity-averaged cross section of ∼10−26 cm3 s−1. Although a dark matter origin of the excess remains viable, other interpretations are possible. In particular, there is some evidence that the excess is produced by a population of unresolved point sources of γ-rays—for example, millisecond pulsars. In this article, I review the current status of the observation of the Fermi–LAT Galactic center excess, the possible interpretations of the excess, the evidence and counterevidence for each, and the prospects for resolving its origin with future measurements.
Highlights
The GC excess is a subdominant component of the total γ -ray flux from the direction of the GC
PS), disk-like (Disk PS), and DM annihilation (NFW PS, which is chosen to have a spatial morphology that is compatible with the GC excess—i.e., the square of the NFW density distribution with a slope γ = 1.25)
Uncertainties in modeling the Galactic γ -ray fore/background have a strong impact on the NPTF results: The non-Poissonian template used to model the unresolved source component—and, in particular, the fraction that originates from faint sources—strongly depends on the assumptions in the interstellar emission model (IEM) that describes the fore/background emission, as shown in Reference 96
Summary
The prominent feature that characterizes the GC excess is a spectrum that peaks at a few GeV in energy, as shown in Figure 1 for a representative set of analyses. The peak energy is generally accepted as a robust feature of the GC excess spectrum, it has been observed that, for a subset of IEM assumptions and parameterization of the GC excess spectrum, it can shift upward by a few GeV to about 5 GeV (e.g., 13) These results, which are not shown, do not yield as good an agreement with the data as those included in the figure. The spectrum might be harder in regions farther from the Galactic plane [16, 17] (see Section 2.2)
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