EGRET Observations of the Extragalactic Gamma Ray Emission
P. Sreekumar
TL;DR
This study analyzes EGRET's all-sky survey to measure the extragalactic diffuse gamma-ray background at high latitudes by carefully subtracting instrumental and Galactic foregrounds. It finds a robust power-law spectrum with a photon index Γ ≈ 2.10 from 30 MeV to 10 GeV, with no significant large-scale anisotropy. The data strongly support unresolved gamma-ray emitting blazars as the dominant origin, consistent with blazar spectral indices and evolution, and implying that blazar spectra extend to at least 50–100 GeV in the same power-law form. These results refine prior measurements and place important constraints on exotic diffuse sources while informing models of blazar populations and high-energy particle acceleration.
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.