Reappearance of the annihilation line source at the Galactic Centre

An intense 511-keV positron annihilation line source was detected from the direction of the Galactic Centre (GC) during the 1970s. Positrons are assumed to be produced by high-energy astrophysical processes at or near the Galactic Centre, and to subsequently annihilate on ambient electrons in the vicinity of the source to generate characteristic 511-keV gamma rays. The source disappeared in the early 1980s and remained in a low state through at least 1984. Here we report the results of new balloon-borne measurements of γ-ray emission from the GC, using a germanium telescope. The 511-keV line was detected at a flux level of (9.8 ± 1.9) x 10-4 photons cm-2s-1 on 1 May 1988 and (12.3±1.6)x10-4 photons cm-2s-1 on 29 October 1988. Apparently the compact object has re-emerged sometime between 1984 and 1988. (author)