HESS Observations of the Galactic Center Region and Their Possible Dark Matter Interpretation

The detection of $\gamma$ rays from the source HESS J1745-290 in the Galactic Center (GC) region with the High Energy Spectroscopic System (HESS) array of Cherenkov telescopes in 2004 is presented. After subtraction of the diffuse $\gamma$-ray emission from the GC ridge, the source is compatible with a point source with spatial extent less than ${1.2}^{\prime }(\mathrm{stat})$ (95% C.L.). The measured energy spectrum above 160 GeV is compatible with a power law with photon index of $2.25\pm 0.04(\mathrm{stat})\pm 0.10(\mathrm{syst})$ and no significant flux variation is detected. It is finally found that the bulk of the very high energy emission must have non-dark-matter origin.

Figure 1

Background-subtracted distribution of the angle $\theta$ between the $\gamma$-ray direction and the position of Sgr ${\mathrm{A}}^{*}$. Circles: all detected $\gamma$-rays events. Open triangles: central object after subtraction of the $\gamma$-ray diffuse emission model (see text). Line: calculated PSF normalized to the number of $\gamma$ rays within 0.1° after subtraction is also shown. The distribution of events after subtraction matches the calculated PSF, while the initial distribution shows a significant tail. The variation of the PSF related to the source energy spectrum, zenith angle, and offset position in the field of view are taken into account. Inset: same distribution for the pointlike source PKS 2155-304 [38]. The calculated PSF (line) also matches the data. From a $dN/d\theta$ excess distribution, the $y$ values have been divided by $2\theta$ ($\theta$ being the bin center).

Figure 2

Spectral energy density $E^2\times dN/dE$ of $\gamma$ rays from the GC source, for the 2004 data (solid points) and 2003 data [3] (open points). Upper limits are 95% C.L. The shaded area shows the power-law fit $dN/dE\sim E^{-\Gamma }$. The dashed line illustrates typical spectra of phenomenological MSSM DM annihilation for best fit neutralino masses of 14 TeV. The dotted line shows the distribution predicted for KK DM with a mass of 5 TeV. The solid line gives the spectrum of a 10 TeV DM particle annihilating into ${\tau }^{+}{\tau }^{-}$ (30%) and $b\overline{b}$ (70%).