Presentation #309.04 in the session “Instrumentation”.
The MeV gamma-ray sky is alight with emission from objects ranging from accreting black holes to supernova remnants and pulars, with emission from an excess of positrons annihilating in the Galactic plane, and with emission from radioactive decay of Al-26, Fe-60, and Ti-44 tracing the lives and deaths of massive stars in our Galaxy. Transient gamma-ray bursts (GRBs) erupt across the sky while MeV data on the sources of gravitational waves and neutrinos will revolutionize multimessenger astrophysics. Yet characterizing these objects at MeV energies and understanding the energetic phenomena powering them has proved challenging over the last several decades. Building on missions such as CGRO and INTEGRAL, our team has developed the Compton Spectrometer and Imager (COSI) to investigate the MeV sky. The COSI instrument operates from 0.2-5MeV using Germanium detectors (GeDs) in a Compton telescope. This technology, developed over the last ~15 years, provides unprecedented angular and spectral resolution and polarization detection capability at MeV energies. COSI combines efficient GeDs with a >25% instantaneous field of view. Active shields surround the GeDs, both rejecting background and detecting GRBs and other gamma-ray flares across most of the sky. We have demonstrated the power of the COSI instrument on NASA’s high altitude balloons. We are now refining our concept for flying the COSI instrument on a Small Explorer (SMEX) satellite mission (COSI SMEX, see arXiv:1908.04334) in Phase A of NASA’s competitive mission development. I will discuss COSI’s exciting science and technology demonstrated on the balloon platform and the audacious future enabled by COSI SMEX.