Magnetic Structures in Gamma-Ray Burst Jets Probed by Gamma-Ray Polarization

TL;DR

This study presents polarization measurements of two gamma-ray bursts, revealing high polarization degrees that support models with globally ordered magnetic fields in the emission regions, contrasting with photospheric models.

Contribution

First polarization measurements of these GRBs with implications for magnetic field structures, supporting globally ordered magnetic fields in GRB jets.

Findings

Detected high polarization degrees (~70-84%) in two GRBs.

Synchrotron emission model is consistent with the data.

Photospheric quasi-thermal emission model is less favorable.

Abstract

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. (2011). 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.