First Detection of Radio Linear Polarization in a Gamma-Ray Burst Afterglow

We report the first detection of radio polarization of a gamma-ray burst (GRB) afterglow with the first intensive combined use of telescopes in the millimeter and submillimeter ranges for GRB 171205A. The linear polarization degree in the millimeter band at the subpercent level (0.27% ± 0.04%) is lower than those observed in late-time optical afterglows (weighted average of ∼1%). The Faraday depolarization by nonaccelerated, cool electrons in the shocked region is one of the possible mechanisms for the low value. This scenario requires a total energy that is larger by a factor of ∼10 than ordinary estimates without considering nonaccelerated electrons. The polarization position angle varies by at least 20° across the millimeter band, which is not inconsistent with this scenario. This result indicates that polarimetry in the millimeter and submillimeter ranges is a unique tool for investigating GRB energetics, and coincident observations with multiple frequencies or bands would provide more accurate measurements of the nonaccelerated electron fraction.