Search for gamma rays from dark matter annihilations around intermediate mass black holes with the HESS experiment

The HESS array of Cherenkov telescopes has performed, from 2004 to 2007, a survey of the inner galactic plane at photon energies above 100 GeV. About 400 hours of data have been accumulated in the region between $-30$ and $+60\mathrm{degrees}$ in galactic longitude, and between $-3$ and $+3\mathrm{degrees}$ in galactic latitude. Assuming that dark matter is composed of weakly interacting massive particles, we calculate here the HESS sensitivity map for dark matter annihilations, and derive the first experimental constraints on the (”minispikes”) scenario, in which a gamma-ray signal arises from dark matter annihilation around intermediate mass black holes. The data exclude the proposed scenario at a 90% confidence level for dark matter particles with velocity-weighted annihilation cross section $\sigma v$ above ${10}^{-28}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ and mass between 800 GeV and 10 TeV.

Figure 1

Number of gammas per annihilation above the energy threshold $E_{\mathrm{th}}=100\mathrm{GeV}$, $N_{\gamma }(E_{\gamma }>E_{\mathrm{th}})$, as a function of the neutralino mass, $m_{\chi }$, for 100% BR in the $b\overline{b}$ channel (solid black line) and 100% BR in the ${\tau }^{+}{\tau }^{-}$ (solid red line). This number corresponds to the annihilation spectrum integrated from $E_{\mathrm{th}}$ up to $m_{\chi }$. Below 2 TeV neutralino masses, more gammas per annihilation are expected in the case of the ${\tau }^{+}{\tau }^{-}$ channel than for $b\overline{b}$, thus the latter will provide a conservative value. Above 2 TeV, the $b\overline{b}$ channel yields more gammas than the ${\tau }^{+}{\tau }^{-}$ one.

Figure 2

Integrated luminosity function of IMBHs for the scenario A, i.e. for IMBH masses of $\sim {10}^2{\mathrm{M}}_{\odot }$ (left), and the scenario B, i.e. for IMBH masses of $\sim {10}^5{\mathrm{M}}_{\odot }$ (right), for a Milky Way-sized halo obtained from an average of 200 stochastic realizations (see text for details). Neutralino masses of 300, 500 GeV and 1 TeV, respectively, and a velocity-weighted annihilation cross section $\sigma v=3\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ are considered. The integrated luminosity corresponds to the number of black holes yielding an integrated gamma-ray flux higher than a given integrated flux, $N_{\mathrm{BH}}(>{\Phi }_{\gamma })$, as a function of the integrated flux ${\Phi }_{\gamma }$. The gamma-ray flux is integrated above 100 GeV. The nominal $5\sigma$ HESS point source sensitivity (25 hours) is plotted (black dashed-dotted line) for comparison.

Figure 3

Integrated flux ${\Phi }_{\gamma }$ expressed in ${\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ above the energy threshold as a function of the neutralino mass $m_{\chi }$ for thresholds of 1, 10, and 100 GeV, respectively, and a velocity-weighted annihilation cross section $\sigma v=3\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. The quoted error bars correspond to the r.m.s. of the integrated flux distribution. The maximum flux is obtained for neutralino masses well above the energy threshold of the instrument (see text for details).

Figure 4

HESS sensitivity map in galactic coordinates, i.e. 90% C.L. limit on the integrated gamma-ray flux above 100 GeV, for dark matter annihilation assuming a dark matter particle of mass $m_{\chi }=500\mathrm{GeV}$ and annihilation into the $b\overline{b}$ channel. The flux sensitivity is correlated to the exposure and acceptance maps. In the galactic latitude band between $-2°$ and 2°, the gamma-ray flux sensitivity reaches ${10}^{-12}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$.

Figure 5

Distribution of the number of IMBHs for the scenario B in the survey range corresponding to a field of view between $\pm 3°$ in galactic latitude and from $-30°$ to 60° in galactic longitude. The distribution is obtained from 200 stochastic realizations of Milky Way-like halos (see Sec. 2a for details). The mean of the distribution is 4.3 with an r.m.s of 2.3. The distribution is well fitted to a Poisson distribution with mean 4.4 (black dashed line).

Figure 6

Constraints on the IMBH gamma-ray production scenario for different neutralino parameters, shown as upper limits on the annihilation cross section $\sigma v$ as a function of the mass of the dark matter particle $m_{\mathrm{DM}}$, but with a number of implicit assumptions about the IMBH initial mass function and halo profile (see text for details). For scenario B, the probability of having no observable halos in our galaxy is 10% from Poisson statistics making these limits essentially 90% confidence level exclusion limits for this one particular (albeit optimistic) scenario.

Figure 7

90% C.L. exclusion limits on the annihilation cross section $\sigma v$ as a function of the mass of the dark matter particle $m_{\mathrm{DM}}$ for a Kaluza-Klein boson ${\tilde{B}}^{(1)}$ (solid blue line) and a pure wino $\tilde{W}$ in AMSB (solid red line). The limit is derived from the HESS flux sensitivity in the galactic plane survey within the minispike scenario. Kaluza-Klein models in UED scenario (blue dashed line) and AMSB models (red dotted line) are plotted together with those satisfying the WMAP constraints on the dark matter particle relic density (pink dashed line and green dotted line, respectively).