Germanium detector sees galactic gamma‐ray line

A two‐day balloon flight over Alice Springs, Australia, has brought some good news for the budding field of gamma‐ray line astronomy. The balloon carried a high‐resolution germanium detector, which found a narrow 511‐keV positron annihilation line coming from the direction of the Galactic center. Because positrons are produced in many of the high‐energy processes believed to be occurring in the Galaxy—such as cosmic‐ray induced interactions and explosive nucleosynthesis—the 511‐keV annihilation is one of the most obvious gamma‐ray lines to look for as evidence of these processes. Until these observations, described at the April meeting of The American Physical Society in Washington, D.C. and just published in the 1 October issue of Astrophysical Journal Letters, there had been no unambiguous evidence of a steady, nonsolar gamma‐ray line.