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

We present Atacama Large Millimeter/submillimeter Array 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2-5.2 hr after the burst. We detect linear polarization at the ≈5σ level, decreasing from Π = (0.87 ±0.13)% to (0.60 ±0.19)%, and evolving in polarization position angle from (10 ±5)° to (-44 ±12)° during the course of the observations. This represents the first detection and measurement of the temporal evolution of polarized radio/millimeter emission in a γ-ray burst. We show that the optical and X-ray observations between 0.03 days and ∼0.3 days are consistent with a fast-cooling forward shock expanding into a wind environment. However, the optical observations at ≲0.03 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 θ _B ≈ 10 ^-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.