The polarimetric performance of the Compton Spectrometer and Imager (COSI)

The Compton Spectrometer and Imager (COSI) is a compact Compton telescope which is inherently sensitive to gamma-ray polarization in the energy range of 0.2-2.0 MeV. A long duration gamma-ray burst, GRB 160530A, was detected by COSI during its 2016 COSI’s balloon flight. The polarization of GRB 160530A was constrained based on the distribution of azimuthal scattering angles from each incident photon inside COSI’s germanium detector array.1 In order to determine COSI’s polarization response and to identify systematic deviations from an ideal sinusoidal modulation, the polarization performance of COSI was validated in the laboratory prior to the 2016. A partially polarized beam was created by scattered emission from a radioactive source off a scintillator. In addition, measurements and simulations of unpolarized radioactive sources were compared to validate our capability of capturing the instrument systematics in the simulations. No statistically significant differences exist between the measured and simulated modulations and polarization angle, where the upper bound on the systematic error is 3%-4%.2 In this talk, I will present the measurements used to validate COSI’s polarimetric performance. Furthermore, I will use these results to estimate the minimum detectable polarization levels for current and future COSI missions.