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 .