Gamma-ray Emission from the Inner Galactic Ridge

An improved measurement of the composite 0.05 to 10 MeV gamma-ray spectra as a function of longitude from the inner galactic ridge has been made using the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory. The continuum appears to be a composite of 3 independent components. A strong positron annihilation contribution is observed toward the center with an intensity which decreases rapidly with distance from the center (FWHM 12 ). A second contribution is a soft low-energy component with a broad longitude distribution and spectra well-approximated by an exponentially absorbed power-law. The third is a hard component with with an approximately longitude-independent power-law spectral shape (photon index 1:75) from 200 keV to 10 MeV. When positron annihilation radiation is subtracted, this is the the dominant feature above 0.2 MeV at all observed longitudes. For an assumed 5 FWHM latitude width at the center, the spectrum and intensity of this component agrees with extrapolations of measurements above 30 MeV using a cosmic-ray interaction model. When account is taken of bright source contributions, both these continuum components' distributions are consistent with the galactic CO longitude distribution along the plane. These results, in conjunction with previous measurements, provide new infomation for determining the galactic cosmic-ray electron spectrum down to the MeV region. Subject headings: galaxies: individual (Milky Way) | gamma rays: spectra