Constraints on an Annihilation Signal from a Core of Constant Dark Matter Density around the Milky Way Center with H.E.S.S.

An annihilation signal of dark matter is searched for from the central region of the Milky Way. Data acquired in dedicated on-off observations of the Galactic center region with H.E.S.S. are analyzed for this purpose. No significant signal is found in a total of $\sim 9\mathrm{h}$ of on-off observations. Upper limits on the velocity averaged cross section, $⟨\sigma v⟩$, for the annihilation of dark matter particles with masses in the range of $\sim 300\mathrm{GeV}$ to $\sim 10\mathrm{TeV}$ are derived. In contrast to previous constraints derived from observations of the Galactic center region, the constraints that are derived here apply also under the assumption of a central core of constant dark matter density around the center of the Galaxy. Values of $⟨\sigma v⟩$ that are larger than $3\times {10}^{-24}{\mathrm{cm}}^3/\mathrm{s}$ are excluded for dark matter particles with masses between $\sim 1$ and $\sim 4\mathrm{TeV}$ at 95% C.L. if the radius of the central dark matter density core does not exceed 500 pc. This is the strongest constraint that is derived on $⟨\sigma v⟩$ for annihilating TeV mass dark matter without the assumption of a centrally cusped dark matter density distribution in the search region.

Figure 1

Dark matter density as a function of the distance to the Galactic center. The parametrizations for the cusped Einasto and NFW profiles are taken from [10]. The cored Einasto and NFW density profiles follow the respective cusped profiles at distances to the Galactic center that are larger than the core radius of 500 pc.

Figure 2

The signal region close to the Galactic center and the two background regions with a symmetric right ascension offset of $\pm 35\min$ to the signal region. A right ascension-declination coordinate grid is overlaid. The exclusion of the Galactic plane ($|b|<0.3°$) in the signal region and the regions with a $\pm 35\min$ offset in right ascension to $|b|<0.3°$ in the background regions are visible.

Figure 3

Line of sight integral over the squared dark matter density as a function of the angular distance between the direction of the line of sight and the direction of the Galactic center. A 500 pc core radius is assumed for the cored dark matter density profiles. Overlaid is the range of angular distances to the Galactic center covered by the signal and background regions of the off1-on-off2 data sets.

Figure 4

Upper limits on the velocity averaged dark matter self annihilation cross section as a function of the dark matter particle mass. The upper limits for the cored Einasto and NFW density profiles hold for a core radius of 500 pc and the annihilation of dark matter particles into light quarks ([2]). The filled area around the upper limit curve for the cored Einasto dark matter profile shows the $\pm 1\sigma$ variations around the upper limit that is expected for this dark matter density profile when no annihilation signal is detected. The derived upper limit is stronger than the expected upper limit due to the negative significance of the measured excess. For comparison, the velocity averaged annihilation cross section of a thermal relic dark matter particle is shown. Additionally shown are the upper limits that are derived in [9] for cusped Einasto and NFW profiles as well as the upper limit that is derived in [19] for a cored dark matter density distribution around the Sculptor dwarf galaxy.