MG14 - Talk detail

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The Fermi Large Area Telescope (LAT) has recently released a precise measurement of the isotropic diffuse gamma-ray background (IGRB), the integrated emission of all unresolved sources, characterizing its intensity between 0.1 GeV and 820 GeV. At these energies, the IGRB spectrum is found compatible with a power law with exponent 2.32 and an exponential cutoff at 280 GeV. Such a cutoff has been observed for the first time and could be due to the absorption of high energy gamma rays by extragalactic background light. We show that the gamma-ray emission from unresolved extragalactic sources, such as active galactic nuclei and star forming galaxies, is consistent, within the uncertainties, with IGRB data.The Fermi-LAT Collaboration is working on a catalog of sources detected above 50 GeV (2FHL). The new Pass 8 event reconstruction which improves significantly the acceptance and point-spread function of the instrument allows the LAT to detect in the 2FHL catalog about 360 sources in 80 months of exposure. Using detailed Monte Carlo simulations and the pixel counting method we measure, for the first time, the source count distribution of gamma-ray sources detected for energies larger than 50 GeV. The resulting intrinsic source count distribution, associated with extragalactic sources, resolves about 80-100% of the IGRB confirming the previous results obtained with population analysis.

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