EGRET Observations of the Diffuse Gamma-Ray Emission in Orion: Analysis Through Cycle 6

Abstract

We present a study of the high-energy diffuse emission observed toward Orion by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory. The total exposure by EGRET in this region has increased by more than a factor of two since a previous study. A simple model for the diffuse emission adequately fits the data; no significant point sources are detected in the region studied ($l=195^{\circ }$ to $220^{\circ }$ and $b=-25^{\circ }to-10^{\circ }$) in either the composite dataset or in two separate groups of EGRET viewing periods considered. The gamma-ray emissivity in Orion is found to be $(1.65\pm 0.11)\times 10^{-26}{ssr}^{-1}$ for E > 100 MeV, and the differential emissivity is well-described as a combination of contributions from cosmic-ray electrons and protons with approximately the local density. The molecular mass calibrating ratio is $N(H_2)/W_{CO}=(1.35\pm 0.15)\times 10^{20}c{m}^{-2}(Kkm/s{)}^{-1}$.

16 pages, including 5 figures. 3 Tables as three separate files. Latex document, needs AASTEX style files. Accepted for publication in ApJ