2001ApJ...555...12D

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)x10^–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)x10²⁰ 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 (215°.5, 0°.5) with a flux of (1.4±0.5)x10^–7 cm^–2.s^–1 (>100 MeV) is suggested by the data.