  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. Using
germanium detectors, the instrument will provide imaging, spectroscopy, and
polarimetry of astrophysical sources with excellent energy resolution and
degree-scale localization capabilities. 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 bandpass to energies lower
than 200 keV, thus enabling spectral analysis across the shared band of 30
keV--2 MeV range. The BTO instrument will consist of two NaI scintillators and
student-designed readout electronics. Using spectral information from both the
COSI and BTO instruments, physics such as the energy peak turnover in gamma-ray
bursts, the characteristics of magnetar flares, and the event frequency of a
range of transient phenomena will be constrained. In this paper, we present the
expected science returnables from BTO and comment on the shared returnables
from the COSI and BTO missions. We include simulations of gamma-ray bursts,
magnetar giant flares, and terrestrial gamma-ray flashes using BTO's spectral
response. Additionally, we estimate BTO's gamma-ray burst detection rate and
find that BTO will detect ~150 gamma-ray bursts per year, with most of these
events being long bursts.
