Anisotropies in the Diffuse Gamma-Ray Background Measured by the Fermi LAT
NAVAL RESEARCH LAB WASHINGTON DC SPACE SCIENCE DIV
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The origin of the all-sky diffuse gamma-ray emission remains one of the outstanding questions in high-energy astrophysics. First detected by OSO-3 1, the isotropic gamma-ray background IGRB was subsequently measured by SAS-2 2, the Energetic Gamma Ray Experiment Telescope 3,4, and most recently by the Large Area Telescope LAT on board the Fermi Gamma-Ray Space Telescope Fermi 5. The term IGRB is used to refer to the observed diffuse gamma-ray emission which appears isotropic on large angular scales but may contain anisotropies on small angular scales. The IGRB describes the collective emission of unresolved members of extragalactic source classes and Galactic source classes that contribute to the observed emission at high latitudes, and gamma-ray photons resulting from the interactions of ultra-high-energy cosmic rays with intergalactic photon fields 6.