Challenges in Detecting Gamma-Rays From Dark Matter Annihilations in the Galactic Center

TL;DR

The paper analyzes how a bright, astrophysical TeV gamma-ray source at the Galactic Center complicates detecting gamma rays from dark matter annihilations with instruments like GLAST (Fermi) and HESS. It combines HESS GC data with DM annihilation flux models and uses a J-factor framework to project continuum and line-detection prospects, highlighting the strong dependence on inner-halo profiles. The findings show that, once current constraints are applied, only a narrow window in annihilation cross section and halo profile space remains potentially observable by GLAST, and monoenergetic gamma-ray lines are unlikely except for special halo configurations. These results underscore the challenges of GC-based DM searches and motivate pursuing other targets, such as dwarf spheroidal galaxies or DM density spikes around intermediate-mass black holes.

Abstract

Atmospheric Cerenkov Telescopes, including HESS and MAGIC, have detected a spectrum of gamma-rays from the galactic center region which extends from $\sim$200 GeV or lower, to at least $\sim$10 TeV. Although the source of this radiation is not yet known, the spectrum appears to behave as a simple power-law, which is not the expectation for gamma-rays generated through the annihilation of dark matter particles. If instead we conclude that the source of these gamma-rays is astrophysical in origin, this spectrum will constitute a background for future dark matter searches using gamma-rays from this region. In this paper we study how this background will affect the prospects for experiments such as GLAST to detect dark matter in the galactic center. We find that only a narrow range of dark matter annihilation rates are potentially observable by GLAST given this newly discovered background and considering current constraints from EGRET and HESS. We also find that a detection of line emission, while not completely ruled out, is only possible for a very narrow range of dark matter models and halo profiles.

Figures (6)

• Figure 1: The gamma-ray spectrum observed by HESS (points and error bars) and the spectrum predicted by several astrophysical models ahrodermer. Also shown is the spectrum as extrapolated from the HESS data, as well as the projected sensitivity of GLAST when the background is neglected (95% CL, following Ref. stat). See text for more details.
• Figure 2: The gamma-ray spectrum (per annihilation) generated through dark matter annihilations for a variety of channels. A 500 GeV dark matter mass has been used in this example.
• Figure 3: The maximum flux of gamma-rays from dark matter in the galactic center consistent with the HESS data. The two frames correspond to two dark matter masses: 500 GeV (left) and 3 TeV (right). For a 500 GeV WIMP, the quantity $(<\sigma v>/10^{-26}$cm$^3$/s)$\times J(\Delta \Omega) \Delta \Omega$ can be as large as $\sim$100 without exceeding the HESS data. For a 3 TeV WIMP, this quantity can be as large as $\sim$20.
• Figure 4: The minimum flux of gamma-rays from dark matter in the galactic center needed to be detected by GLAST, assuming a power-law background extrapolated from the HESS data (see Fig. \ref{['astro']}). The two frames correspond to two dark matter masses: 50 GeV (left) and 3 TeV (right). For a 50 GeV WIMP, the quantity $(<\sigma v>/(3 \times 10^{-26}$cm$^3$/s))$\times J(\Delta \Omega) \Delta \Omega$ needs to be larger than $\sim$0.2 to be identified by GLAST. For a 3 TeV WIMP, this quantity must be larger $\sim$50 to be detected. For GLAST, we have used an effective area $\times$ exposure time of 1 m$^2$ yr.
• Figure 6: The range of annihilation rates to gamma-ray lines and WIMP masses within the (5$\sigma$) reach of GLAST or HESS. The cross section shown in the y-axis is that to $\gamma \gamma$. If a $\gamma Z$ line is being considered, the cross section must be larger by a factor of 2 to be detected.