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

Understand

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.