OSSE observations of positron annihilation in the galactic plane

OSSE has measured the galactic longitude distribution of both the 511 keV annihilation line radiation and the three-photon positronium continuum within 40 of the center. They have similar shapes, with a composite longitude distribution well represented by an 11 FWHM Gaussian central bulge together with a possible broad disk component comprising as much as 15% of the integral ux between 40 and 320 longitude. Contributions from one or more discrete source(s) at or near the center with total annihilation radiation (line plus continuum) intensities of up to 4 10 4 photons cm 2 sec 1 (2 upper limit) are compatible with the observed distribution. The best estimate of the positronium fraction is 0.97 0.03. No evidence for variation in the positronium fraction is observed.