Revisit of GeV Gamma-Ray Emission from Orion B with the Fermi Large Area Telescope

Abstract We revisit the γ -ray emission above 300 MeV towards the massive star-forming region of Orion B by adopting 14 yr observations with the Fermi Large Area Telescope and utilizing the updated software tools. The extended γ -ray emission region around Orion B is resolved into two components (region I and region II). The γ -ray spectrum of region I agrees with the predicted γ -ray spectrum assuming the cosmic ray (CR) density is the same as that of Alpha Magnetic Spectrometer (AMS-02) measured locally. The γ -ray emissivity of region II appears to be deficit at low energy band ( E < 3 GeV). Through modeling we find that CR densities exhibit a significant deficit below 20 GeV, which may be caused by a slow diffusion inside the dense region. This is probably caused by an increased magnetic field whose strength increases with the gas density.