EGRET Observations of Monoceros: Diffuse Gamma-Ray Emission in the Outer Galaxy

We present an analysis of the diffuse gamma-ray emission observed toward Monoceros by the Energetic Gamma-Ray Experiment Telescope (EGRET). The region studied, l = 210°-250°, b = -15° to +20°, has kinematically well-defined segments of the local and Perseus arms and is one of the best regions for the study of variations of cosmic-ray density across the outer Galaxy. The local gamma-ray emissivity is (1.81 ± 0.17) × 10^-26 s^-1 sr^-1 for energies greater than 100 MeV. For energies greater than 300 MeV, the gamma-ray emissivity in Monoceros is consistent with that of other local clouds studied with EGRET, which suggests that the density of GeV cosmic-ray protons does not vary significantly on scales of less than 1 kpc. As with other local clouds studied with EGRET, an excess of emissivity is observed above 1 GeV relative to that predicted from the cosmic-ray spectrum at the Earth. The gamma-ray emissivity is apparently enhanced in the Perseus arm relative to the interarm regions, although to a much lesser degree than predicted by the model of Hunter et al. The molecular mass calibrating ratio, X = N(H₂)/W_CO, is (1.64 ± 0.31) × 10²⁰ cm^-2 (K km s^-1)^-1 in the local clouds and is apparently about the same (to within 50%) in Maddalena's cloud. An uncataloged, marginally significant (~4.5 σ) unidentified point source near (2155, 05) with a flux of (1.4 ± 0.5) × 10^-7 cm^-2 s^-1 (>100 MeV) is suggested by the data.