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We report the polarization measurement in prompt $\gamma$-ray emission of GRB100826A with the Gamma-Ray Burst Polarimeter (GAP) aboard the small solar powersail demonstrator IKAROS. We detected the firm change of polarization angle(PA) during the prompt emission with 99.9% ($3.5 \sigma$) confidence level, andthe 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 havebeen carefully included in this analysis, unlike any previous reports. Such ahigh $\Pi$ can be obtained in several emission models of gamma-ray bursts(GRBs), including synchrotron and photospheric models. However, it is difficultto explain the observed significant change of PA within the framework ofaxisymmetric jet as considered in many theoretical works. The non-axisymmetric(e.g., patchy) structures of the magnetic fields and/or brightness inside therelativistic jet are therefore required within the observable angular scale of$\sim \Gamma^{-1}$. Our observation strongly indicates that the polarizationmeasurement is a powerful tool to constrain the GRB production mechanism, andmore theoretical works are needed to discuss the data in more details.

Detection of Gamma-Ray Polarization in Prompt Emission of GRB 100826A