Gamma-ray emission from the inner Galactic ridge

We present measurements of the 0.05-10 MeV gamma-ray spectra as a function of longitude from the inner Galactic ridge using the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory. The d ifferential continuum emission relative to intensities at +/-10 degrees Gal actic latitude appears to be a composite of at least three independent comp onents: a soft low-energy component with a broad longitude distribution and with spectra well approximated by an exponentially absorbed power law; a h ard component with a similarly broad longitude distribution modeled by a po wer law from similar to 200 keV to 10 MeV with photon index similar to-1.75 ; and strong positron annihilation line and continuum contributions observe d toward the center with intensities that decrease rapidly with longitude d istance from the center. Although OSSE cannot distinguish between a simple one-component latitude distribution and a more complicated one with, for ex ample, broad and narrow latitude components, an "effective" 5 degrees-6 deg rees FWHM Gaussian latitude width gives a spectrum and intensity for the po wer-law component that agrees with extrapolations of measurements at higher energies using a cosmic-ray interaction model. However, the latitude distr ibution of the emission is not well measured. Near the Galactic center, bri ght variable sources contribute significantly to the low-energy spectrum. W hen account is taken of these variable-source contributions, both the soft low-energy and hard power-law components show a consistent longitude distri bution that follows the Galactic matter distribution as evidenced by the Ga lactic CO distribution. These results, in conjunction with previous measure ments, provide new information for determining the Galactic cosmic-ray elec tron spectrum at lower energies.