Uncovering the origin of Galactic positrons with the Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX) satellite mission scheduled for launch in 2027. COSI is a wide-field Compton telescope designed to survey the entire gamma-ray sky at 0.2-5 MeV, providing imaging, spectroscopy, and polarimetry of astrophysical sources. One of COSI’s key science goals is to uncover the origin of Galactic positrons. The production of positrons and their annihilation in the interstellar medium is one of the pioneering topics of gamma-ray astronomy. Despite five decades of study since the initial detection of the 511 keV annihilation line from the inner Galaxy the origin of these positrons and the distribution of the positron annihilation is not understood. COSI combines unprecedented soft gamma-ray diffuse imaging capabilities and high spectral resolution to explore the mysterious origin of Galactic positrons. We will present an overview of COSI’s positron science objectives and measurement requirements that will enable these positron studies.