K. Spekkens, B. S. Mason, J. E. Aguirre, B. Nhan
We present deep radio observations of four nearby dwarf spheroidal (dSph) galaxies, to search for extended synchrotron emission resulting from weakly interacting massive particle (WIMP) dark matter annihilations in their halos. We map a total of 40.5 deg^2 around the Draco, Ursa Major II, Coma Bernices, and Willman 1 dSphs with the GBT at 1.4 GHz to detect the extended annihilation halos of the type predicted by Colafrancesco et al. (CPU07) for Draco, greatly reducing discrete-source confusion using the NVSS catalog. We achieve a sensitivity of ~ 7 mJy/beam in our discrete source-subtracted maps, implying that the NVSS is highly effective at removing background sources from GBT maps. We construct radial surface brightness profiles from each of the subtracted maps, and jackknife the data to quantify the significance of the features therein. At the ~10 arcmin resolution of our observations, foregrounds contribute a standard deviation of 1.8 - 5.7 mJy/beam to our high-latitude maps, with the emission in the Draco and Coma dominated by foregrounds. On the other hand, we find no significant emission in the Ursa Major II and Willman 1 fields, and explore the implications of our upper limits for particle dark matter using the fiducial models of CPU07. For a WIMP mass of 100 GeV annihilating into b-bbar final states and B = 1 muG, the individual dSphs place an upper limit on the annihilation cross-section log(, cm^3/s) <~ -25 for one set of charged particle propagation parameters adopted by CPU07; this is comparable to that inferred at gamma-ray energies from the two-year Fermi-LAT data. We discuss three avenues for improving the constraints on presented here, which may yield an additional order of magnitude in sensitivity for some models. We conclude that deep radio observations of dSphs are highly complementary to indirect WIMP searches at higher energies.
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http://arxiv.org/abs/1301.5306
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