EGRET observations of gamma-ray emission from the interstellar gas in Orion

Abstract

The high-energy diffuse gamma-ray emission from the interstellar gas in Orion was studied using observations from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO) and radio surveys of the H I and CO emission. The good correlation of the gamma-ray emission with the atomic and molecular gas permits determination of the gamma-ray emissivity per nucleon in the interstellar medium and the molecular mass calibrating ratio N(H2)/W(sub co) in Orion. The integral gamma-ray emissivity is (1.7 +/- 0.1) x 10(exp -26)/s/sr for E greater than 100 MeV, in good agreement with expectations from studies of the diffuse emission on larger scales. The N(H2)/W(sub co) ratio is (1.06 +/- 0.14) x 10(exp 20)/sq cm/(K km/s), approximately 40% less than the commonly adopted Galactic average. We find no evidence for variations of the cosmic-ray density or N(H2)/W(sub co) ratio in Orion at the sensitivity and resolution of EGRET.