* Astronomy

Title: Early Optical Polarisation of Forward Shock Afterglow of GRB 091208B
Authors: T. Uehara, K. Toma, K. S. Kawabata, S. Chiyonobu, Y. Fukazawa, Y. Ikejiri, T. Inoue, R. Itoh, T. Komatsu, H. Miyamoto, T. Mizuno, O. Nagae, H. Nakaya, T. Ohsugi, K. Sakimoto, M. Sasada, H. Tanaka, M. Uemura, M. Yamanaka, T. Yama****a, R. Yamazaki, M. Yoshida

We report that the optical polarisation in the afterglow of GRB 091208B is measured at t = 149 - 706 s after the burst trigger, and the polarisation degree is P = 10.4% ± 2.5%. The optical light curve at this time shows a power-law decay with index -0.75 ± 0.02, which is interpreted as the forward shock synchrotron emission, and thus this is the first detection of the early-time optical polarisation in the forward shock (rather than that in the reverse shock reported by Steele et al. (2009). This detection disfavours the afterglow model in which the magnetic fields in the emission region are random on the plasma skin depth scales, such as amplified by the plasma instabilities, e.g., Weibel instability. We suggest that the fields are amplified by the magnetohydrodynamic instabilities, which would be tested by future observations of the temporal changes of the polarisation degrees and angles for other bursts, and fluorescent H_2 emission are observed in its immediate surroundings.

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