The Galactic Center as a Dark Matter Gamma-Ray Source

TL;DR

The paper investigates whether the EGRET gamma-ray excess toward the Galactic Center can be explained by WIMP annihilation and assesses GLAST's ability to test this hypothesis. It develops a two-component flux model combining a conventional diffuse background with a WIMP-induced signal, linking the DM flux to particle physics via $σv$, $Mχ$, and $dN_f/dE$ and to astrophysics via the halo integral $J(ψ)$. Through a generic toy-model and a concrete mSUGRA neutralino scenario, it shows that GLAST's superior spectral and angular capabilities could distinguish a WIMP signal and map the GC DM density, or alternatively constrain weaker DM sources. The study highlights that even with moderate DM cusps, substantial regions of the SUSY parameter space could be probed, though current EGRET data impose only weak constraints on these models.

Abstract

The EGRET telescope has identified a gamma-ray source at the Galactic center. We point out here that the spectral features of this source are compatible with the gamma-ray flux induced by pair annihilations of dark matter weakly interacting massive particles (WIMPs). We show that the discrimination between this interpretation and other viable explanations will be possible with GLAST, the next major gamma-ray telescope in space, on the basis of both the spectral and the angular signature of the WIMP-induced component. If, on the other hand, the data will point to an alternative explanation, we prove that there will still be the possibility for GLAST to single out a weaker dark matter source at the Galactic center. The potential of GLAST has been explored both in the context of a generic simplified toy-model for WIMP dark matter, and in a more specific setup, the case of dark matter neutralinos in the minimal supergravity framework. In the latter, we find that even in the case of moderate dark matter densities in the Galactic center region, there are portions of the parameter space which will be probed by GLAST.

Figures (12)

• Figure 1: In the left panel: differential $\gamma$-ray yield per annihilation (see Eq.(\ref{['gammafluxcont']})) for a fixed annihilation channel ($b\bar{b}$) and for a few sample values of WIMP masses. For comparison we also show the emissivity, with an arbitrarily rescaled normalization, from the interaction of primaries with the interstellar medium. In the right panel: differential yield per annihilation for a few sample annihilation channels and a fixed WIMP mass ($200\; \rm{GeV}$). The solid lines are the total yields, while the dashed lines are components not due to $\pi^0$ decays.
• Figure 2: Fit of the EGRET GC $\gamma$-ray data for two sample WIMP models. We fix the WIMP mass ($M_\chi = 50$ GeV in the upper panel, $M_\chi = 80.3$ GeV in the lower panel) and select a single annihilation channel in each of the two cases ($b\bar{b}$ in the upper panel, $W^-W^+$ in the lower one). Signal and background components are indicated separately, while their sum is shown with a solid line. For both models the value of the reduced statistical $\chi^2$ variable obtained from the fit is around 5.
• Figure 3: Reduced $\chi^2$ corresponding to the best fits of the EGRET excess for a WIMP model with fixed mass $M_\chi$ and a single annihilation channel allowed ($b\bar{b}$ is shown with a dotted line, $W^-W^+$ with a solid line); the $\chi^2$ has been minimized over the normalizations of both the signal and the background (with restrictions as explained in the text). Reduced $\chi^2$ values should be compared with the value obtained in case data are fitted with a background component only, marked with a horizontal line in the upper part of the figure.
• Figure 4: Lines of constant reduced $\chi^2$ corresponding to best fits of the EGRET GC excess, in the plane WIMP mass $M_\chi$ versus normalization of the WIMP-induced signal $N_\chi$. The plot applies to our toy-model with a single annihilation channel allowed, i.e. $b\bar{b}$ in the case displayed.
• Figure 5: Simulation of the data set which will be obtained with GLAST in 2 years, in case the EGRET GC excess is due to the WIMP-induced flux shown in one of the sample fits in Fig. \ref{['egretfit']} (lower panel). The error bars refer to statistical errors for the chosen energy binning and for the angular acceptance $\Delta\Omega=10^{-3}\;\rm{sr}$.