We report polarization measurements in two prompt emissions of gamma-ray bursts , GRB 110301A and GRB 110721A , observed with the Gamma-ray burst polarimeter ( GAP ) aboard IKAROS solar sail mission .
We detected linear polarization signals from each burst with polarization degree of \Pi = 70 \pm 22 % with statistical significance of 3.7 ~ { } \sigma for GRB 110301A , and \Pi = 84 ^ { +16 } _ { -28 } % with 3.3 ~ { } \sigma confidence level for GRB 110721A .
We did not detect any significant change of polarization angle .
These two events had shorter durations and dimmer brightness compared with GRB 100826A , which showed a significant change of polarization angle , as reported in Yonetoku et al .
( 38 ) .
Synchrotron emission model can be consistent with all the data of the three GRBs , while photospheric quasi-thermal emission model is not favorable .
We suggest that magnetic field structures in the emission region are globally-ordered fields advected from the central engine .