THE SPECTRUM OF ISOTROPIC DIFFUSE GAMMA-RAY EMISSION BETWEEN 100 MeV AND 820 GeV

Abstract

The {\\gamma}-ray sky can be decomposed into individually detected sources,\ndiffuse emission attributed to the interactions of Galactic cosmic rays with\ngas and radiation fields, and a residual all-sky emission component commonly\ncalled the isotropic diffuse {\\gamma}-ray background (IGRB). The IGRB comprises\nall extragalactic emissions too faint or too diffuse to be resolved in a given\nsurvey, as well as any residual Galactic foregrounds that are approximately\nisotropic. The first IGRB measurement with the Large Area Telescope (LAT) on\nboard the Fermi Gamma-ray Space Telescope (Fermi) used 10 months of sky-survey\ndata and considered an energy range between 200 MeV and 100 GeV. Improvements\nin event selection and characterization of cosmic-ray backgrounds, better\nunderstanding of the diffuse Galactic emission, and a longer data accumulation\nof 50 months, allow for a refinement and extension of the IGRB measurement with\nthe LAT, now covering the energy range from 100 MeV to 820 GeV. The IGRB\nspectrum shows a significant high-energy cutoff feature, and can be well\ndescribed over nearly four decades in energy by a power law with exponential\ncutoff having a spectral index of $2.32\\pm0.02$ and a break energy of\n$(279\\pm52)$ GeV using our baseline diffuse Galactic emission model. The total\nintensity attributed to the IGRB is $(7.2\\pm0.6) \\times 10^{-6}$ cm$^{-2}$\ns$^{-1}$ sr$^{-1}$ above 100 MeV, with an additional $+15$%/$-30$% systematic\nuncertainty due to the Galactic diffuse foregrounds.\n