Magnetic structures in gamma-ray burst jets probed by gamma-ray polarization

Abstract

We report polarization measurements in two prompt emissions of gamma-ray bursts, GRB110301A and GRB110721A, observed with the gamma-ray burst polarimeter (GAP) on borad the IKAROS solar sail mission. We detected linear polarization signals from each burst with polarization degree of Π = 70 ± 22% with statistical significance of 3.7σ for GRB110301A, and Π = 84 +16- 28% with 3.3σ confidence level for GRB110721A. We did not detect any significant change of polarization angle. These two events had shorter durations and dimmer brightness compared with GRB100826A, which showed a significant change of polarization angle, as reported in Yonetoku etal. Synchrotron emission model can be consistent with the data of the three GRBs, while the photospheric quasi-thermal emission model is not favored. We suggest that magnetic field structures in the emission region are globally ordered fields advected from the central engine. © © 2012. The American Astronomical Society. All rights reserved..