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

Understand

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.