GRB 091208B: FIRST DETECTION OF THE OPTICAL POLARIZATION IN EARLY FORWARD SHOCK EMISSION OF A GAMMA-RAY BURST AFTERGLOW

Abstract

We report that the optical polarization in the afterglow of GRB 091208B is\nmeasured at t = 149 - 706 s after the burst trigger, and the polarization\ndegree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a\npower-law decay with index -0.75 +/- 0.02, which is interpreted as the forward\nshock synchrotron emission, and thus this is the first detection of the\nearly-time optical polarization in the forward shock (rather than that in the\nreverse shock reported by Steele et al. (2009). This detection disfavors the\nafterglow model in which the magnetic fields in the emission region are random\non the plasma skin depth scales, such as amplified by the plasma instabilities,\ne.g., Weibel instability. We suggest that the fields are amplified by the\nmagnetohydrodynamic instabilities, which would be tested by future observations\nof the temporal changes of the polarization degrees and angles for other\nbursts.\n