EGRET Observations of the Extragalactic Gamma‐Ray Emission

TLDR

The all‑sky survey in high‑energy gamma rays (E > 30 MeV) by EGRET provides a unique opportunity to examine diffuse gamma‑ray emission, which includes a Galactic component from cosmic‑ray interactions with interstellar gas and radiation and an almost uniformly distributed component generally believed to originate outside the Galaxy. By carefully modeling and removing the Galactic diffuse emission, the authors deduce the flux, spectrum, and uniformity of the extragalactic emission. The extragalactic emission is well described by a power‑law photon spectrum with index –(2.10 ± 0.03) from 30 MeV to 100 GeV, showing no large‑scale spatial anisotropy or spectral changes.

Abstract

The all-sky survey in high-energy gamma rays (E$>$30 MeV) carried out by the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma-Ray Observatory provides a unique opportunity to examine in detail the diffuse gamma-ray emission. The observed diffuse emission has a Galactic component arising from cosmic-ray interactions with the local interstellar gas and radiation as well an almost uniformly distributed component that is generally believed to originate outside the Galaxy. Through a careful study and removal of the Galactic diffuse emission, the flux, spectrum and uniformity of the extragalactic emission is deduced. The analysis indicates that the extragalactic emission is well described by a power law photon spectrum with an index of -(2.10+-0.03) in the 30 MeV to 100 GeV energy range. No large scale spatial anisotropy or changes in the energy spectrum are observed in the deduced extragalactic emission. The most likely explanation for the origin of this extragalactic high-energy gamma-ray emission is that it arises primarily from unresolved gamma-ray-emitting blazars.