High-energy transient events can be used to probe the extreme physics that power them, as well as the properties of matter in violent astrophysical environments such as in the vicinity of compact objects or the birthplaces of black holes. X-ray binary outbursts provide important insights into the physics of accretion and the nature of black holes and neutron stars. Meanwhile, gamma-ray bursts (GRBs) carry signatures of the powerful progenitors that produce them, yet the origins of their prompt emission and their jet structure remain unclear. Spectral and timing analyses have been effective investigative tools for these extreme settings, especially as telescopes advance and models increase in predictive power. Pairing them with linear polarization analyses of high-energy emissions can add even more information about the emission mechanisms and source geometries of accreting black holes and GRBs.

The Compton Spectrometer and Imager (COSI) is a soft gamma-ray (0.2-5 MeV) telescope designed to study astrophysical sources including accreting black holes and GRBs. It has significant heritage as a balloon-borne telescope, and was selected as a NASA Small Explorer (SMEX) slated to launch on a satellite in 2027. COSI employs a compact Compton telescope designed to conduct high-resolution spectroscopy, imaging over a wide field-of-view, polarization studies, and effective suppression of background events. Compton telescopes detect multiple interactions from individual incoming photons, allowing for polarization information to be captured through measurements of the distribution of azimuthal angles. While the standard method relies on binning the photons to produce and fit an azimuthal scattering angle distribution, improved polarization sensitivity is obtained by using additional information to more accurately weigh each event’s contribution to the likelihood statistics. In this work, we report validations of COSI’s capabilities as a polarimeter. Furthermore, we develop tools that enable future spectral and polarimetric analyses of COSI GRB observations.

We also present the first observed outburst from the transient X-ray binary source MAXI J0637-430, based on observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT). We study the source's transition from a soft state dominated by disk-blackbody emission to a hard state dominated by a power-law or thermal Comptonization component, with NuSTAR providing the first coverage of MAXI J0637-430 above 10 keV. These broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test alternative excess emission models such as blackbody emission from the plunging region, a reflection component with a Comptonization continuum, and a reflection component of a blackbody illuminating the disk. The study demonstrates the importance of broadband spectral analyses of accreting compact objects. We include a discussion on how joint spectral and polarization analyses could be conducted for long-duration transient sources with COSI in the future.