Measurement of emission lines by the Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a 0.2-5 MeV Compton telescope with high efficiency, excellent energy resolution, and precise 3D position resolution. These capabilities uniquely equip it to perform imaging, spectroscopy, and polarimetry of various sources in the historically underexplored MeV gap of gamma-ray astrophysics. One such target of study is the 1809 keV signature of decaying Galactic Al-26, which can reveal sites of stellar nucleosynthesis. In 2016, COSI flew on a NASA superpressure balloon for 46 days. This presentation will detail the first measurement of Al-26 in the flight data and thereby establish COSI as a capable probe of stellar nucleosynthesis. Its ability to resolve gamma-ray lines was also demonstrated through the detection and imaging of positron-electron annihilation at 511 keV during the 2016 flight. These studies lay the groundwork for future contributions of COSI as a NASA Small Explorer satellite mission, scheduled to launch in 2025, to the MeV regime.