Imaging of the MeV gamma-ray universe with COSI

Gamma-ray line observations in the MeV band are crucial to understanding cosmic matter production, e.g., 511 keV lines from positron annihilation, nuclear lines originating from nucleosynthesis, and signatures of MeV-scale dark matter. The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer satellite mission and will be planned for launch in 2027. Owing to its wide field-of-view and excellent energy resolution, COSI can achieve an unprecedented gamma-ray line sensitivity, allowing us to enter a new era of MeV gamma-ray observations. One of the keys to the gamma-ray line measurement is to reconstruct gamma-ray images quantitatively from complex Compton scattering events. Generally, an event of Compton telescopes is described with a Compton scattering angle and a scattered gamma-ray direction, and the gamma-ray direction is constrained to a circle on the sky, not uniquely determined from a single event. Therefore, a statistical method is essential to recover the gamma-ray source distribution quantitatively, which can solve such an inverse and ill-posed problem. In this presentation, I will give an overview of the gamma-ray line observations enabled by COSI. Then, I will present an image analysis framework being developed for COSI. We plan to analyze the COSI data using iterative deconvolution and spatial model fitting methods as two complementary approaches. They will be provided publicly to a broad astronomical community through the COSI pipeline tools and yearly data challenges. I will show the proof of concept of the gamma-ray imaging with COSI by referring to Data Challenge 2, scheduled for release in early 2024, as an example.