# ALMA Detection of a Linearly Polarized Reverse Shock in GRB 190114C

## TL;DR

This paper reports the first detection of polarized millimeter emission from a gamma-ray burst, revealing details about magnetic fields in the reverse shock and ejecta through ALMA observations.

## Contribution

It presents the first measurement of linear polarization in millimeter emission from a GRB, constraining magnetic field coherence and structure in the ejecta.

## Key findings

- Detected linear polarization at ~0.6-0.9% in GRB 190114C's afterglow
- Constrained magnetic field coherence scale to about 10^-3 radians
- Ruled out large-scale ordered magnetic fields in the jet

## Abstract

We present ALMA 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the burst. We detect linear polarization at the $\approx 5\,\sigma$ level, decreasing from $\Pi=(0.87\pm0.13)\%$ to $(0.60\pm0.19)\%$, and evolving in polarization position angle from $(10\pm5)^\circ$ to $(-44\pm12)^\circ$ during the course of the observations. This represents the first detection of polarized millimeter emission in a $\gamma$-ray burst. We show that the optical and X-ray observations between $0.03$ days and $\sim0.3$ days are consistent with a fast cooling forward shock expanding into a wind environment. However, the optical observations at $\lesssim0.03$ days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked ejecta. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the ejecta to an angular size of $\theta_{\rm B} \approx10^{-3}$ radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular a large scale toroidal field, in the jet.