Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes

TL;DR

The Fermi LAT measured diffuse Galactic gamma-ray emission at intermediate latitudes, finding it consistent with cosmic ray models and resolving the previously observed EGRET GeV excess.

Contribution

This study provides improved measurements of diffuse gamma-ray emission that challenge the existence of the EGRET GeV excess and align with cosmic ray spectra.

Findings

LAT measurements match cosmic ray-based models

EGRET GeV excess is not confirmed by LAT data

Diffuse gamma-ray emission is consistent with local CR spectra

Abstract

The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs) interacting with the interstellar gas and radiation field. Measurements by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to diffuse Galactic gamma-ray emission models consistent with directly measured CR spectra (the so-called ``EGRET GeV excess''). The excess emission was observed in all directions on the sky, and a variety of explanations have been proposed, including beyond-the-Standard-Model scenarios like annihilating or decaying dark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray Space Telescope has measured the diffuse gamma-ray emission with improved sensitivity and resolution compared to EGRET. We report on LAT measurements of the diffuse gamma-ray emission for energies 100 MeV to 10 GeV and Galactic latitudes 10 deg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky is well reproduced by a diffuse Galactic gamma-ray emission model that is consistent with local CR spectra and inconsistent with the EGRET GeV excess.