OSSE observations of Galactic 511 keV annihilation radiation

The Oriented Scintillation spectrometer Experiment (OSSE) on the Compton Gamma‐Ray Observatory has performed numerous observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation and to search for time variability of the emission. These observations show conclusive evidence for a narrow 511 keV line and positronium continuum. For the first OSSE galactic center observation, the fitted 511 keV line flux was (2.3±0.3)×10−4 γ cm−2 s−1 and the positronium continuum flux was (8.8±0.7)×10−4 γ cm−2 s−1, corresponding to a positronium fraction (0.96±0.04). The quoted uncertainty in the positronium flux does not include the effect of the underlying continuum model on the fitted positronium flux. No significant time variability of the line flux has been observed; the 3σ upper limit to daily variations from the mean is 3×10−4 γ cm−2 s−1. The galactic distribution of the 511 keV line emission is found to be most consistent with a 2‐compon...