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Spherical harmonics analysis of Fermi gamma-ray data and the Galactic dark matter halo

Spherical harmonics analysis of Fermi gamma-ray data and the Galactic dark matter halo,Dmitry Malyshev,Jo Bovy,Ilias Cholis

Spherical harmonics analysis of Fermi gamma-ray data and the Galactic dark matter halo  
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We argue that the decomposition of gamma-ray maps in spherical harmonics is a sensitive tool to study dark matter (DM) annihilation or decay in the main Galactic halo of the Milky Way. Using the harmonic decomposition in a window excluding the Galactic plane, we show for one year of Fermi data that adding a spherical template (such as a line-of-sight DM profile) to an astrophysical background significantly reduces chi^2. In some energy bins the significance of this "DM" fraction is above three sigma. Assuming a power-law energy spectrum for the backgrounds and the DM component, we find that sources following the stellar mass density or an NFW annihilation profile are preferred to an NFW decay or a bivariate Gaussian profile. We check that the dependence of the DM fraction on the data analysis choices, such as the window size, the pixel size etc. is not significant. We also calculate the contribution from point sources to the variance matrix. Based on the analysis of the angular power spectrum, we indirectly find the average number of photons coming from the multi-photon sources in energy bins above 3 GeV.
Published in 2010.
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