SMM detection of diffuse Galactic 511 keV annihilation radiation

Abstract

The γ-ray spectrometer on NASA's Solar Maximum Mission satellite (SMM) has recorded a significant increase in the measured intensity of 511 keV line radiation in each of 5 yr as the Galactic center region passed through its 130° aperture. The overall statistical significance of the detection is in excess of 30σ. Earth occultation and low-backgrond data sets are utilized to demonstrate that this annual increase is due to a celestial source. The time-averaged flux, if attributed to a point source at the Galactic center, is (2.1±0.4)×10<SUP>-3</SUP>γ cm<SUP>-2</SUP>s<SUP>-1</SUP>. However, it is argued that the bulk of the measured radiation arises from an extended region, and may be correlated with the Galactic distribution of interstellar matter. Possible contributors to an interstellar concentration of positrons are discussed.