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

Tanmoy Laskar, Kate D. Alexander, Ramandeep Gill, Jonathan Granot, E. Berger, Carole Mundell, Rodolfo Barniol Duran, Jan Bolmer, Paul C. Duffell, Hendrik Van Eerten, Wen-fai Fong, Shiho Kobayashi, R. Margutti, Patricia Schady

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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.

Original languageEnglish
Article numberL26
JournalAstrophysical Journal Letters
Issue number1
Publication statusPublished - 13 Jun 2019


  • gamma-ray burst: general
  • gamma-ray burst: individual (GRB 190114C)
  • polarization

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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