Observation of annihilation radiation from the galactic center: The point and ridge components

Using the all-sky map obtained with the Medium Energy Detectors (80 keV-2 MeV) of the UCSD/MIT Hard X-ray and Low-Energy Gamma Ray Experiment on HEAO-1, a joint determination of the point source and the galactic ridge components of 511 keV annihilation radiation has been made. The ridge determination assumes the longitudinal distribution measured from 90-270 keV, which has a strong central concentration of width about 120°. Spectral fits assumed an unresolved annihilation line centered at 511 keV and orthopositronium continuum with positronium fraction f=0.9. Detector resolution was 9% at 511 keV, and field of view was 16° FWHM. The point source flux is significant at 95% confidence, and is in reasonable agreement with that measured with HEAO-3 one to two years later in 1980. The ridge component is not significantly measured, but is consistent with a measurement by GRIS at 25° longitude, and the derived two sigma limit lies well below the ~2.5×10-3 photon/(cm2-s-rad), averaged over the central radian, obtained with SMM observations.