A new Determination of the Extragalactic Background of Diffuse Gamma Rays taking into accou.pdf

a r X i v : 0 7 0 5 .0 0 9 4 v 2 [ a s t r o - p h ] 1 1 M a y 2 0 0 7 Astronomy Astrophysics manuscript no. paper February 5, 2008 (DOI: will be inserted by hand later) A new Determination of the Extragalactic Background of Diffuse Gamma Rays taking into account Dark Matter Annihilation W. de Boer, A. Nordt, C. Sander, V. Zhukov Institut fu?r Experimentelle Kernphysik, Universita?t Karlsruhe (TH), P.O. Box 6980, 76128 Karlsruhe, Germany e-mail: Wim.de.Boer@cern.ch, sander@ekp.uni-karlsruhe.de, zhukov@ekp.uni-karlsruhe.de Received November 30, 2006; accepted May 2, 2007 Abstract. The extragalactic background (EGB) of diffuse gamma rays can be determined by subtracting the Galactic contri- bution from the data. This requires a Galactic model (GM) and we include for the first time the contribution of dark matter annihilation (DMA), which was previously proposed as an explanation for the EGRET excess of diffuse Galactic gamma rays above 1 GeV. In this paper it is shown that the newly determined EGB shows a characteristic high energy bump on top of a steeply falling soft contribution. The bump is shown to be compatible with a contribution from an extragalactic DMA signal from weakly interacting massive particles (WIMPs) with a mass between 50 and 100 GeV in agreement with the EGRET excess of the Galactic diffuse gamma rays and in disagreement with earlier analysis. The remaining soft contribution of the EGB is shown to resemble the spectra of the observed point sources in our Galaxy. Key words. —Gamma Rays: diffuse, extragalactic —Milky Way: halo, dark matter — elementary particles: dark matter anni- hilation 1. Introduction In a previous paper (De Boer et al. (2005), in the following called Paper I, we discussed the possibility to explain the observed excess in the EGRET data (Hunter et al. 1997) in the diffuse Galactic gamma rays above 1 GeV as a dark matter annihilation (DMA) signal. The excess was found to be compatible with a DMA signal from Weakly Interac