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ALMA Detection of a Linearly Polarized Reverse Shock in GRB190114C

Laskar, T, Alexander, KD, Gill, R, Granot, J, Berger, E, Mundell, CG, Duran, RB, Bolmer, J, Duffell, P, van Eerten, H, Fong, W-F, Kobayashi, S, Margutti, R and Schady, P (2019) ALMA Detection of a Linearly Polarized Reverse Shock in GRB190114C. The Astrophysical Journal Letters, 878 (1). ISSN 2041-8205

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Abstract

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 lesssim0.03 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.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical and Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society
Related URLs:
Date Deposited: 02 Jul 2019 10:31
Last Modified: 02 Jul 2019 10:45
DOI or Identification number: 10.3847/2041-8213/ab2247
URI: http://researchonline.ljmu.ac.uk/id/eprint/10960

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