# Detailed polarization measurements of the prompt emission of five Gamma-Ray Bursts

## TL;DR

This study presents precise polarization measurements of five gamma-ray bursts using the POLAR instrument, providing insights into jet structures and magnetic fields, with results indicating low polarization levels and evolving polarization angles.

## Contribution

First statistically meaningful polarization measurements of gamma-ray bursts with the POLAR instrument, clarifying polarization levels and evolution during bursts.

## Key findings

- Gamma-ray emission is at most weakly polarized.
- Polarization angle evolves within bursts.
- Results challenge some existing models predicting higher polarization.

## Abstract

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst.