Chien-Ying Yang(National Tsing-Hua University) Alex Lowell (UCSD);Jeng-Lun Chiu (NTHU); Che-Yen Chu (NTHU); Yi-Chi Chang (NTHU); Hung-Hsiang Liang (NTHU); Hsiang-Kuang Chang (NTHU); Steven E. Boggs (UCSD, UCB); the COSI Collaboration (UCB, LBNL, NTHU, AS, NCU, IRAP)
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 balloon flight, where 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 (see publications by Lowell et al.). In order to determine COSI’s polarization response and to identify systematic deviations from a sinusoidal modulation, the polarization performance of COSI was validated in the laboratory before the 2016 flight with a partially polarized beam created by scattering emission from a radioactive source off a scintillator. Also, 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 when including $\sim$10$^4$ events in this work, where the upper bound on the systematic error is 3\%-4\%. In this talk, I will present the results used to validate COSI’s polarimetric performance, which is further determined and estimated for current and future COSI missions based on the minimum detectable polarization.