Author: Kenji Toma
Abstract: In spite of extensive light-curve and spectral observations of GRB prompt emission, the emission mechanism is still open. Polarimetric observations can give us some critical information for solving the problem. Recently we have measured gamma-ray polarizations in three bursts with the Gamma-ray Polarimeter (GAP) on board the small solar-power-sail demonstrator IKAROS
(Yonetoku, Murakami, Gunji, Mihara, Toma, et al. 2011; 2012). We review the observational results of the GAP (and other instruments) with discussing some theoretical implications for the prompt emission mechanism. We also report a strict observational verification of CPT invariance in the photon sector as a result of the GRB polarization measurement (Toma, Mukohyama, Yonetoku et al. 2012).