Fermilat Observation of Diffuse Gamma Rays Produced Through Interactions Between Local Interstellar Matter and High-Energy Cosmic Rays

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Observations by the Large Area Telescope (LAT) on the Fermi mission of diffuse γ-rays in a mid-latitude region in the third quadrant (Galactic longitude l from 200° to 260° and latitude | b | from 22° to 60°) are reported. The region contains no known large molecular cloud and most of the atomic hydrogen is within 1 kpc of the solar system. The contributions of γ-ray point sources and inverse Compton scattering are estimated and subtracted. The residual γ-ray intensity exhibits a linear correlation with the atomic gas column density in energy from 100 MeV to 10 GeV. The measured integrated γ-ray emissivity is (1.63 ± 0.05) × 10 −26 photons s −1 sr −1 H-atom −1 and (0.66 ± 0.02) × 10 −26 photons s −1 sr −1 H-atom −1 above 100 MeV and above 300 MeV, respectively, with an additional systematic error of ∼10%. The differential emissivity from 100 MeV to 10 GeV agrees with calculations based on cosmic ray spectra consistent with those directly measured, at the 10% level. The results obtained indicate that cosmic ray nuclei spectra within 1 kpc from the solar system in regions studied are close to the local interstellar spectra inferred from direct measurements at the Earth within ∼10%.