GAMMA-RAY OBSERVATIONS OF THE GALACTIC-CENTER - REMOVING THE GALACTIC RIDGE CONTRIBUTION

We combine, for the first time, spectral measurements of the emission from the disk of our Galaxy from keV to GeV energies. This emission is found to have a similar spatial distribution at all energies: a longitude extent of about +/-40-degrees and a latitude extent of less than or similar to 5-degrees. Hence the name ''Galactic ridge'' is appropriate. The energy flux from the ridge is about 10(-7) ergs cm-2 s-1 which gives a luminosity of about 10(38) ergs s-1 assuming an effective distance to the emitting region of 3 kpc. The emission over most of the energy range is dominated by cosmic-ray electrons and protons interacting with interstellar matter. Observations by HEAO A-4 from 1977 to 1979 and by the GRIS balloon instrument in 1988 October give estimates of the ridge spectrum near 100 keV. We subtract this diffuse component from Galactic center observations made by wide-field (approximately 20-degrees) gamma-ray instruments over the past 20 years to determine the emission from point sources in the region. For most of the measurements, the Galactic ridge accounts for between one-third and one-half of the observed flux. This solves the long-standing puzzle of why the wide-field flux is higher than that from the known point sources. Most of the measurements (approximately 80%) have a flux at 100 keV, which, after substraction of our estimate of the diffuse component, is less than or equal to the sum of the normal-state fluxes from the two brightest gamma-ray sources in the region, 1E 1740.7-2942 and GRS 1758-258. These have probably been the dominant gamma-ray point sources in the region since the late 1960s. Subtracting the diffuse emission and the GRS 1758-258 emission from the HEXAGONE observations on 1989 May 22 gives a spectrum that is dominated by lines from positron annihilation.