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


Laskar T, Alexander KD, Gill R, Granot J, Berger E, Mundell C G, Barniol Duran R, Bolmer J, Duffell P, van Eerten H, et al. ALMA Detection of a Linearly Polarized Reverse Shock in GRB 190114C. The Astrophysical Journal Letters. 2019;878 (1) :L26.

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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.

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Last updated on 02/16/2020