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

TL;DR

This paper refines the measurement of the isotropic diffuse gamma-ray background (IGRB) spectrum over 100 MeV to 820 GeV using Fermi LAT data, revealing a high-energy cutoff and providing detailed intensity estimates.

Contribution

The study extends the IGRB measurement to higher energies and improves modeling, revealing a spectral cutoff and more precise intensity measurements.

Findings

IGRB spectrum shows a high-energy cutoff.

The spectrum is well described by a power law with exponential cutoff.

Total IGRB intensity is approximately 7.2 x 10^{-6} cm^{-2} s^{-1} sr^{-1} above 100 MeV.

Abstract

The {\gamma}-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 {\gamma}-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, 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 $2.32\pm0.02$ and a break energy of $(279\pm52)$ GeV using our baseline diffuse Galactic emission model. The total intensity attributed to the IGRB is $(7.2\pm0.6) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ above 100 MeV, with an additional $+15$%/$-30$% systematic uncertainty due to the Galactic diffuse foregrounds.