Time varying polarized gamma-rays from GRB 160821A: evidence for ordered magnetic fields

TL;DR

This paper presents evidence of time-varying, highly polarized gamma-ray emission from GRB 160821A, indicating the presence of ordered magnetic fields and synchrotron radiation during the burst.

Contribution

It provides the most conclusive evidence to date of high polarization and polarization angle variation in a gamma-ray burst, supporting models with ordered magnetic fields.

Findings

High linear polarization ($66^{+26}_{-27}\%$) detected at 5.3 sigma

Polarization angle varies twice during the burst at 3.5 and 3.1 sigma

Supports synchrotron radiation in ordered magnetic fields

Abstract

GRB 160821A is the third most energetic gamma ray burst observed by the {\it Fermi} gamma-ray space telescope. Based on the observations made by Cadmium Zinc Telluride Imager (CZTI) on board {\it AstroSat}, here we report the most conclusive evidence to date of (i) high linear polarization ($66^{+26}_{-27} \%$; $5.3 \sigma$ detection), and (ii) variation of its polarization angle with time happening twice during the rise and decay phase of the burst at $3.5 \sigma$ and $3.1 \sigma$ detections respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines which are highly ordered on angular scales of $1/\Gamma$, where $\Gamma $ is the Lorentz factor of the outflow.