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تسجيل مستخدم جديدAstrophysical interpretation of the anisotropies in the unresolved gamma-ray background
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Recently, a new measurement of the auto- and cross-correlation angular power spectrum (APS) of the isotropic gamma-ray background was performed, based on 81 months of data of the Fermi Large-Area Telescope (LAT). Here, we fit, for the first time, the new APS data with a model describing the emission of unresolved blazars. These sources are expected to dominate the anisotropy signal. The model we employ in our analysis reproduces well the blazars resolved by Fermi LAT. When considering the APS obtained by masking the sources in the 3FGL catalogue, we find that unresolved blazars under-produce the measured APS below $sim$1 GeV. Contrary to past results, this suggests the presence of a new contribution to the low-energy APS, with a significance of, at least, 5$sigma$. The excess can be ascribed to a new class of faint gamma-ray emitters. If we consider the APS obtained by masking the sources in the 2FGL catalogue, there is no under-production of the APS below 1 GeV, but the new source class is still preferred over the blazars-only scenario (with a significance larger than 10$sigma$). The properties of the new source class and the level of anisotropies induced in the isotropic gamma-ray background are the same, independent of the APS data used. In particular, the new gamma-ray emitters must have a soft energy spectrum, with a spectral index ranging, approximately, from 2.7 to 3.2. This complicates their interpretation in terms of known sources, since, normally, star-forming and radio galaxies are observed with a harder spectrum. The new source class identified here is also expected to contribute significantly to the intensity of the isotropic gamma-ray background.
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The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. We analyze the angular power spectrum of the diffuse emission measured by the Fermi LAT at Galactic latitud
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The small angular scale fluctuations of the (on large scale) isotropic gamma-ray background (IGRB) carry information about the presence of unresolved source classes. A guaranteed contribution to the IGRB is expected from the unresolved gamma-ray AGN
while other extragalactic sources, Galactic gamma-ray source populations and dark matter Galactic and extragalactic structures (and sub-structures) are candidate contributors. The IGRB was measured with unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi gamma-ray observatory, and these data were used for measuring the IGRB angular power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky observations were performed to provide a reference against which to compare the results obtained for the real data set. The Monte Carlo simulations are also a method for performing those detailed studies of the APS contributions of single source populations, which are required in order to identify the actual IGRB contributors. We present preliminary results of an anisotropy search in the IGRB. At angular scales <2deg (e.g. above multipole 155), angular power above the photon noise level is detected, at energies between 1 and 10 GeV in each energy bin, with statistical significance between 7.2 and 4.1 sigmas. The energy not dependence of the fluctuation anisotropy is pointing to the presence of one or more unclustered source populations, while the energy dependence of the intensity anisotropy is consistent with source populations having average photon index 2.40pm0.07.
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