COSI: Pioneering Exploration in the MeV Gap

The Compton Spectrometer and Imager, better known as COSI, is a Small Explorer class mission destined for launch in 2027. It will survey the soft gamma-ray sky in 0.2-5 MeV, with a primary field of view of >25% of the sky instantaneously. This provides good coverage for gamma-ray bursts and transients, which is important to contribute to multimessenger localization efforts. Its germanium detectors were specifically designed for high energy resolution to focus on the study of annihilation and nuclear lines. COSI will image the Galaxy in 511 keV and several nuclear lines to determine substructure, especially in the Galactic Bulge to inform the conditions of electron-positron annihilation. Compton event reconstruction will provide gamma-ray polarization information. MeV polarization is important to constraining theoretical predictions for blazar jet composition, and can provide additional insights into other high-energy sources. COSI's science portfolio includes AGN, dark matter, GRBs, classical novae, and galactic diffuse emission.