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Coherence scale of magnetic fields generated in early-time forward shocks of GRBs

Jordana-Mitjans, N, Mundell, CG, Smith, RJ, Guidorzi, C, Marongiu, M, Kobayashi, S, Gomboc, A, Shrestha, M and Steele, IA (2021) Coherence scale of magnetic fields generated in early-time forward shocks of GRBs. Monthly Notices of the Royal Astronomical Society, 505 (2). pp. 2662-2674. ISSN 0035-8711

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We report the earliest ever detection of optical polarization from a GRB forward shock (GRB 141220A), measured 129.5−204.3s after the burst using the multicolour RINGO3 optical polarimeter on the 2-m fully autonomous robotic Liverpool Telescope. The temporal decay gradient of the optical light curves from 86 to ∼2200s post-burst is typical of classical forward shocks with α = 1.091 ± 0.008. The low-optical polarization PBV=2.8+2.0−1.6 per cent (2σ) at mean time ∼168s post-burst is compatible with being induced by the host galaxy dust (⁠AV,HG=0.71±0.15mag), leaving low polarization intrinsic to the GRB emission itself – as theoretically predicted for forward shocks and consistent with previous detections of low degrees of optical polarization in GRB afterglows observed hours to days after the burst. The current sample of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. P∼1−3 per cent⁠), (b) contribution from a polarized reverse shock (GRB deceleration time, jet magnetization) or (c) forward shock intrinsic polarization (i.e. P≤2 per cent⁠), which depends on the magnetic field coherence length-scale and the size of the observable emitting region (burst energetics, circumburst density).

Item Type: Article
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2021 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords: 0201 Astronomical and Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: Oxford University Press
Related URLs:
Date Deposited: 12 Jan 2022 12:32
Last Modified: 12 Jan 2022 12:45
DOI or ID number: 10.1093/mnras/stab1003
URI: https://researchonline.ljmu.ac.uk/id/eprint/16039
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