Fermi LAT Observation of Diffuse Gamma Rays Produced Through Interactions Between Local Interstellar Matter and High-energy Cosmic Rays

Observations by the Large Area Telescope (LAT) on the Fermi mission of diffuse gamma-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 gamma-ray point sources and inverse Compton scattering are estimated and subtracted. The residual gamma-ray intensity exhibits a linear correlation with the atomic gas column density in energy from 100 MeV to 10 GeV. The measured integrated gamma-ray emissivity is (1.63 0.05) 10<SUP>-26</SUP> photons s<SUP>-1</SUP>sr<SUP>-1</SUP> H-atom<SUP>-1</SUP> and (0.66 0.02) 10<SUP>-26</SUP> photons s<SUP>-1</SUP>sr<SUP>-1</SUP> H-atom<SUP>-1</SUP> 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%.