Search for $\gamma$-Ray Line Signals from Dark Matter Annihilations in the Inner Galactic Halo from 10 Years of Observations with H.E.S.S.

Spectral lines are among the most powerful signatures for dark matter (DM) annihilation searches in very-high-energy $\gamma$ rays. The central region of the Milky Way halo is one of the most promising targets given its large amount of DM and proximity to Earth. We report on a search for a monoenergetic spectral line from self-annihilations of DM particles in the energy range from 300 GeV to 70 TeV using a two-dimensional maximum likelihood method taking advantage of both the spectral and spatial features of the signal versus background. The analysis makes use of Galactic center observations accumulated over ten years (2004–2014) with the H.E.S.S. array of ground-based Cherenkov telescopes. No significant $\gamma$-ray excess above the background is found. We derive upper limits on the annihilation cross section $⟨\sigma v⟩$ for monoenergetic DM lines at the level of $4\times {10}^{-28}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ at 1 TeV, assuming an Einasto DM profile for the Milky Way halo. For a DM mass of 1 TeV, they improve over the previous ones by a factor of 6. The present constraints are the strongest obtained so far for DM particles in the mass range 300 GeV–70 TeV. Ground-based $\gamma$-ray observations have reached sufficient sensitivity to explore relevant velocity-averaged cross sections for DM annihilation into two $\gamma$-ray photons at the level expected from the thermal relic density for TeV DM particles.

Figure 1

Schematic of the background measurement technique for ROI 2 and two different telescope-pointing positions, in Galactic coordinates. The off region is taken symmetrically to the on region from a given observational pointing position (black cross). Two off regions are shown, each one corresponding to a specific pointing position. On and off regions have the same angular size and shape. The positions of Sgr A* (red star), $\mathrm{G}0.9+0.1$ (red dot), and HESS J1745-303 (red dot) are shown. The gray-filled box with Galactic latitudes from $-0.3°$ to $+0.3°$ and the gray-filled disk are excluded for signal and background measurements. The color scale gives the $J$-factor value per spatial bin of $0.02°\times 0.02°$ for the Einasto DM profile.

Figure 2

Constraints on the flux $\mathrm{\Phi }$ (left panel) and on the velocity-weighted annihilation cross section $⟨\sigma v⟩$ (right panel) for the prompt annihilation into two photons derived from H.E.S.S. observations taken over ten years (254 h of live time) of the inner 300 pc of the GC region. The constraints are expressed in terms of 95% C.L. upper limits as a function of the DM mass $m_{\mathrm{DM}}$ for the Einasto profile. The observed limits are shown as red dots. Expected limits are computed from 1000 Poisson realizations of the expected background derived from blank-field observations at high Galactic latitudes. The mean expected limit (black solid line) together with the 68% (green band) and 95% (yellow band) C.L. containment bands are shown. The bands include the statistical and the systematic uncertainties. The observed limits derived in the analysis of four years (112 h of live time) of GC observations by H.E.S.S. [13] are shown as blue squares, together with the mean expected limit (blue solid line) and the 68% containment band (blue shaded area) in the left panel. The natural scale for monochromatic $\gamma$-ray line signal is highlighted as a gray-shaded area in the right panel.

Figure 3

Comparison of constraints for prompt annihilation into two photons obtained by H.E.S.S. for the Einasto (red dots) and NFW (cyan dots) profiles, respectively, with the limits from the observations of the Milky Way halo by Fermi-LAT [35] (black triangles) as well as the limits from 157 h of MAGIC observations of the dwarf galaxy Segue 1 [36] (green triangles). The gray-shaded area shows the natural scale for a monochromatic $\gamma$-ray line signal.