Presentation #404.01 in the session “MeV Gamma Rays and Multi-messenger Astronomy”.
The Compton Spectrometer and Imager (COSI) is a Small Explorer (SMEX) satellite mission recently selected for development and scheduled for launch in late-2025. COSI is a wide-field telescope designed to survey the gamma-ray sky at 0.2-5 MeV. It provides imaging, spectroscopy, and polarimetry of astrophysical sources. Such capabilities are made possible by COSI’s germanium cross-strip detectors, which provide high efficiency, high resolution spectroscopy and precise 3D positioning of photon interactions. Science goals for COSI include studies of 511 keV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization, and detecting and localizing multimessenger sources. The instantaneous field of view for the germanium detectors is 25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts or other gamma-ray flares over most of the sky. At the time of the HEAD meeting, COSI will be in its formulation phase (“Phase B”), and I will provide an overview of the science goals, observational requirements, and instrument design.