The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV

The ?-ray sky can be decomposed into individually detected sources, diffuse emission attributed to the interactions of Galactic cosmic rays with gas and radiation fields, and a residual all-sky emission component commonly called the isotropic diffuse ?-ray background (IGRB). The IGRB comprises all extragalactic emissions too faint or too diffuse to be resolved in a given survey, as well as any residual Galactic foregrounds that are approximately isotropic. The?first IGRB measurement with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) used 10?months of sky-survey data and considered an energy range between 200?MeV and 100?GeV. Improvements in event selection and characterization of cosmic-ray backgrounds, better understanding of the diffuse Galactic emission (DGE), and a longer data accumulation of 50?months allow for a refinement and extension of the IGRB measurement with the LAT, now covering the energy range from 100?MeV to 820?GeV. The IGRB spectrum shows a significant high-energy cutoff feature and can be well described over nearly four decades in energy by a power law with exponential cutoff having a spectral index of 2.32 ? 0.02 and a break energy of (279 ? 52)?GeV using our baseline DGE model. The total intensity attributed to the IGRB is (7.2 ? 0.6) ? 10?6?cm?2?s?1 sr?1 above 100?MeV, with an additional +15%/?30% systematic uncertainty due to the Galactic diffuse foregrounds.