The gamma-ray spectrum of the galactic-center region

The HEAO-3 High Resolution Gamma-Ray Spectrometer observed the galactic center region in fall 1979 and in the spring of 1980. Variation of the positron annihilation line at 511 keV has been reported previously. The fall1979 observations show a significant high-energy continuum at energies above 511 keV. The intensity of a possible positronium triplet-state continuum is found to be less than that expected for direct positron annihilation and positronium decay in an ionized, warm (T<~105 K) plasma; depending on assumption for the shape of the high-energy continuum spectrum, positronium fractions between 0.0 and 0.75 (at 90% statistical confidence level) are consistent with the observations.