Rapidly‐Responding Optical Polarimetry of GRB afterglows with Hiroshima 1.5‐m Telescope and One‐shot Wide‐field Polarimeter

We have developed an optical wide‐field polarimeter, HOWPol, which employs a wedged double Wollaston prism and enables us to derive linear polarization (i.e., Stokes Q/I and U/I) from only a single exposure. We successfully performed observation for the afterglow of GRB 091208B. Its first exposure started ∼150 seconds after the gamma‐ray trigger. Our preliminary result suggests that the early afterglow was significantly polarized, larger than a few percent.