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

TL;DR

This paper reports the first detection of gamma-ray polarization in the prompt emission of GRB 100826A, revealing a significant change in polarization angle and a high polarization degree, which challenges simple jet models and suggests complex magnetic structures.

Contribution

The study provides the first polarization measurement of GRB prompt emission with careful systematic error analysis, highlighting the need for non-axisymmetric magnetic field models.

Findings

Detected a polarization degree of 27 ± 11%

Observed a significant change in polarization angle during emission

Systematic errors were carefully included in the analysis

Abstract

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.