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 differential continuum emission relative to intensities at ±10° Galactic latitude appears to be a composite of at least three independent components: a soft low-energy component with a broad longitude distribution and with spectra well approximated by an exponentially absorbed power law; a hard component with a similarly broad longitude distribution modeled by a power law from ~200 keV to 10 MeV with photon index ~-1.75; and strong positron annihilation line and continuum contributions observed toward the center with intensities that decrease rapidly with longitude distance from the center. Although OSSE cannot distinguish between a simple one-component latitude distribution and a more complicated one with, for example, broad and narrow latitude components, an "effective" 5°-6° FWHM Gaussian latitude width gives a spectrum and intensity for the power-law component that agrees with extrapolations of measurements at higher energies using a cosmic-ray interaction model. However, the latitude distribution of the emission is not well measured. Near the Galactic center, bright variable sources contribute significantly to the low-energy spectrum. When account is taken of these variable-source contributions, both the soft low-energy and hard power-law components show a consistent longitude distribution that follows the Galactic matter distribution as evidenced by the Galactic CO distribution. These results, in conjunction with previous measurements, provide new information for determining the Galactic cosmic-ray electron spectrum at lower energies.