Daisuke Yonetoku, Toshio Murakami, Shuichi Gunji, Tatehiro Mihara, Kenji Toma, Tomonori Sakashita, Yoshiyuki Morihara, Takuya Takahashi, Noriyuki Toukairin, Hirofumi Fujimoto, Yoshiki Kodama, Shin Kubo, IKAROS Demonstration Team
We report the polarization measurement in prompt $\gamma$-ray emission of GRB
100826A with the Gamma-Ray Burst Polarimeter (GAP) aboard the small solar power
sail demonstrator IKAROS. We detected the firm change of polarization angle
(PA) during the prompt emission with 99.9% ($3.5 \sigma$) confidence level, and
the average polarization degree ($\Pi$) of $27 \pm 11$% with 99.4% ($2.9
\sigma$) confidence level. Here the quoted errors are given at 1 $\sigma$
confidence level for two parameters of interest. The systematic errors have
been carefully included in this analysis, unlike any previous reports. Such a
high $\Pi$ can be obtained in several emission models of gamma-ray bursts
(GRBs), including synchrotron and photospheric models. However, it is difficult
to explain the observed significant change of PA within the framework of
axisymmetric jet as considered in many theoretical works. The non-axisymmetric
(e.g., patchy) structures of the magnetic fields and/or brightness inside the
relativistic jet are therefore required within the observable angular scale of
$\sim \Gamma^{-1}$. Our observation strongly indicates that the polarization
measurement is a powerful tool to constrain the GRB production mechanism, and
more theoretical works are needed to discuss the data in more details.
View original:
http://arxiv.org/abs/1111.1779
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