Abstract
AN intense 511-keV positron annihilation line source was detected from the direction of the Galactic Centre (GC) during the 1970s1,2. 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 19843–5. Here we report the results of new balloon-borne measurements ofγ-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 cnT^-1 on 1 May 1988 and (12.3 ± 1.6) x 10-4 photons cm-2s-1 on 29 October 1988. Apparently the compact object has re-emerged sometime between 1984 and 1988. Flux was also measured from a point in the Galactic Plane (GP) 25° west of the GC, and was found to be (2.4 ± 1.7) x 10-4 photons cm-2s-1. Diffuse flux from the GP has been reported6, but these new observations suggest that the model used in ref. 6 may not be correct.
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Leventhal, M., MacCallum, C., Barthelmy, S. et al. Reappearance of the annihilation line source at the Galactic Centre. Nature 339, 36–38 (1989). https://doi.org/10.1038/339036a0
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DOI: https://doi.org/10.1038/339036a0
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