The signature of positron annihilation, namely the 511~keV $\gamma$-ray line, was first detected coming from the direction of the Galactic center in the 1970's, but the source of Galactic positrons still remains a puzzle. The measured flux of the annihilation corresponds to an intense steady source of positron production, with an annihilation rate of $\sim$10$^{43}$~e$^+$/s. Spatially, the 511~keV intensity is strongest in the Galactic center region, with an additional component that is consistent with the Galactic disk; however, the unique morphology is not well constrained.
The Compton Spectrometer and Imager (COSI) is a balloon-borne soft $\gamma$-ray (0.2--5~MeV) telescope designed to perform wide-field imaging and high-resolution spectroscopy, with a goal of furthering our understanding of Galactic positrons. COSI employs a compact Compton telescope design, using 12 cross-strip germanium detectors to track the trajectory of incident photons, where position and energy deposits from Compton interactions allow for a reconstruction of the source sky position and significant background reduction.
COSI had a record-breaking 46-day balloon flight from Wanaka, New Zealand, in May--July 2016, and here we report on the detection and analyses of the 511~keV emission from those observations. To isolate the Galactic positron annihilation emission, we have developed a background subtraction technique utilizing the COMPTEL Data Space. With this new method, we find a 7.2~$\sigma$ detection of the 511~\keV line and a broader spatial distribution of the emission than has been previously reported.
