GRB 091208B: First detection of the optical polarization in early forward shock emission of a gamma-ray burst afterglow

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. YamashitaR. Yamazaki, M. Yoshida

Research output: Contribution to journalArticle

28 Citations (Scopus)

Abstract

We report that the optical polarization in the afterglow of GRB 091208B is measured at t = 149-706s after the burst trigger, and the polarization 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 polarization in the forward shock (rather than that in the reverse shock reported by Steele et al.). This detection disfavors the afterglow model in which the magnetic fields in the emission region are random on the plasma skin depth scales, such as those 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 polarization degrees and angles for other bursts.

Original languageEnglish
Article numberL6
JournalAstrophysical Journal Letters
Volume752
Issue number1
DOIs
Publication statusPublished - 2012 Jun 10
Externally publishedYes

Keywords

  • gamma-ray burst: individual (GRB 091208B)
  • magnetic fields
  • polarization
  • shock waves

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Uehara, T., Toma, K., Kawabata, K. S., Chiyonobu, S., Fukazawa, Y., Ikejiri, Y., Inoue, T., Itoh, R., Komatsu, T., Miyamoto, H., Mizuno, T., Nagae, O., Nakaya, H., Ohsugi, T., Sakimoto, K., Sasada, M., Tanaka, H., Uemura, M., Yamanaka, M., ... Yoshida, M. (2012). GRB 091208B: First detection of the optical polarization in early forward shock emission of a gamma-ray burst afterglow. Astrophysical Journal Letters, 752(1), [L6]. https://doi.org/10.1088/2041-8205/752/1/L6