  Measurements of the linear polarization of high-energy emission from pulsars,
accreting black holes, and gamma-ray bursts (GRBs) provide an opportunity for
constraining the emission mechanisms and geometries (e.g., of the accretion
disk, jet, magnetic field, etc.) in the sources. For photons in the soft (MeV)
gamma-ray band, Compton scattering is the most likely interaction to occur in
detectors. Compton telescopes detect multiple interactions from individual
incoming photons, allowing for scattering angles to be measured. After many
photons are detected from a source, the distribution of azimuthal angles
provides polarization information. 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 weight each event's contribution to the likelihood
statistic. In this chapter, we describe the Compton Spectrometer and Imager
(COSI) and its capabilities for polarization measurements. We also describe the
maximum likelihood technique, its application to COSI data analysis, and plans
for its future use.
