Across the soft gamma-ray regime: utilizing simultaneous detections in the Compton Spectrometer and Imager (COSI) and the Background and Transient Observer (BTO) to understand astrophysical transients

The Compton Spectrometer and Imager (COSI) is a NASA funded Small Explorer (SMEX) mission slated to launch in 2027. COSI will house a wide-field gamma-ray telescope designed to survey the entire sky in the 0.2-5 MeV range, providing imaging, spectroscopy, and polarimetry of astrophysical sources. In addition to the main instrument, COSI will fly with a Student Collaboration Project known as the Background and Transient Observer (BTO). BTO will extend the COSI science to lower energies by providing spectral information in the 30 keV–2 MeV range. Using spectral information from both the COSI and BTO instruments, physics such as the energy peak turnover in gamma-ray bursts, the properties of soft gamma-ray repeaters, and the rates of transient phenomena will be constrained. In this talk, we present on the shared science returnables from the COSI and BTO missions. This will include an overview of the instrument and survey designs, simulations of several transient phenomena as observed with both COSI and BTO, and a final analysis of the event rates expected in the BTO instrument.