Fermi LAT measurements of diffuse gamma-ray emission: results at the first-year milestone

TL;DR

This paper reports on Fermi LAT's first-year measurements of diffuse gamma-ray emission, confirming previous findings, refining cosmic-ray interaction models, and providing new insights into Galactic and extragalactic gamma-ray backgrounds.

Contribution

It presents the first-year results of Fermi LAT on diffuse gamma-ray emission, improving models and confirming the absence of previously reported excesses.

Findings

No excess of diffuse GeV emission confirmed

Measured gamma-ray emissivity spectrum of local interstellar gas

Studied cosmic-ray density gradients and X(CO) ratio in the outer Galaxy

Abstract

For more than one year the Fermi Large Area Telescope has been surveying the gamma-ray sky from 20 MeV to more than 300 GeV with unprecedented statistics and angular resolution. One of the key science targets of the Fermi mission is diffuse gamma-ray emission. Galactic interstellar gamma-ray emission is produced by interactions of high-energy cosmic rays with the interstellar gas and radiation field. We review the most important results on the subject obtained so far: the non-confirmation of the excess of diffuse GeV emission seen by EGRET, the measurement of the gamma-ray emissivity spectrum of local interstellar gas, the study of the gradient of cosmic-ray densities and of the X(CO)=N(H2)/W(CO) ratio in the outer Galaxy. We also catch a glimpse at diffuse gamma-ray emission in the Large Magellanic Cloud. These results allow the improvement of large-scale models of Galactic diffuse gamma-ray emission and new measurements of the extragalactic gamma-ray background.