(A. Zoglauer, S.E. Boggs, T.J. Brandt, J.-L. Chiu, A. Lowell, C. Sleator, J.A. Tomsick, J. Roberts, M. Amman, P. Jean, P. von Ballmoos)
The 511 keV line from positron annihilation was first detected from the Galactic Center region in the 1970’s, but the source of these positrons is still unconfirmed and remains one of the pioneering topics in gamma-ray astronomy. Positrons from beta decay of stellar nucleosynthesis products, such as 26Al, can account for a significant fraction of the annihilation emission, but the observed spatial distribution, in particular the excess in the Galactic bulge, is difficult to explain. The Compton Spectrometer and Imager (COSI) is a soft-gamma ray telescope (0.2-5 MeV) designed to study astrophysical sources and further our understanding of Galactic positrons by directly imaging the spatial distribution for the first time. In May-July 2016 COSI had a successful 46 day flight on NASA’s new Super Pressure Balloon, and significantly detected the Galactic 511 keV line in the spectrum and image. In this presentation, we will give a brief overview of the COSI instrument and the 2016 balloon flight, and describe the results of the Galactic positron annihilation line as detected by COSI.