Abstract

Contributed Talk - Splinter MeVSky

Tuesday, 08 September 2026, 16:00   (MW-0337)

Nuclear Astrophysics with the Compton Spectrometer and Imager

Thomas Siegert and the COSI collaboration
JMU Würzburg

Observations in the MeV gamma-ray band provide huge potential for obtaining fundamental results on topics ranging from nucleosynthesis via studies of nuclear emission lines to multimessenger astrophysics (MMA) through detections of gamma-ray bursts (GRBs) and other gamma-ray transients. Due to the challenges of operating in the MeV band, it is one of the least explored regions of the electromagnetic spectrum, leaving a large potential for discovery. In order to take advantage of this scientific opportunity, the Compton Spectrometer and Imager (COSI) satellite mission is planned to launch as a NASA Small Explorer (SMEX) mission in 2027. Using the Compton technique, COSI will survey the entire sky at 0.2-5 MeV. COSI provides imaging, spectroscopy, and polarimetry of astrophysical sources, and its germanium detectors provide excellent energy resolution for gamma-ray line measurements. The instantaneous field of view is >25% of the sky for the germanium detectors, which are surrounded by an active anti-coincidence shield, providing background rejection and at the same time allowing for detections of GRBs and transients over most of the sky. COSI will be less than a year from launch at the time of this meeting, which is the ideal time for community engagement: COSI’s end user software cosipy for the analysis of simulated data in pre-launch “data challenges” and later for actual on-orbit observations is publicly available, allowing the community to play a part in optimising the software and to prepare for their scientific investigations. Two of the major questions addressed by COSI are the origin of Galactic positrons as traced by the positron annihilation line at 511 keV, and the formation of heavy elements that are directly observed through nuclear gamma-ray lines, such as the 1.809 MeV line from radioactive Al-26 and the 1.173 and 1.332 MeV lines from Fe-60. Stellar evolution of massive stars beyond their final fate as core-collapse supernovae in the remnant phase can further be studied for even century old remnants with the gamma-ray lines from the isotope Ti-44. This talk will present an overview of the COSI mission and recap on the current status of observational nuclear astrophysics towards an order of magnitude improvement in sensitivity with COSI.