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 ³-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 ³-ray point sources and inverse Compton scattering are estimated and subtracted. The residual ³-ray intensity exhibits a linear correlation with the atomic gas column density in energy from 100 MeV to 10 GeV. The measured integrated ³-ray emissivity is (1.63 ± 0.05) × 1026 photons s1sr1 H-atom1 and (0.66 ± 0.02) × 1026 photons s1sr1 H-atom1 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%.