Coherence scale of magnetic fields generated in early-time forward shocks of GRBs
Date Issued
2021
Author(s)
Jordana-Mitjans, N.
•
Mundell, C. G.
•
Smith, R. J.
•
Guidorzi, C.
•
•
Kobayashi, S.
•
Gomboc, A.
•
Shrestha, M.
•
Steele, I. A.
Abstract
We report the earliest ever detection of optical polarization from a GRB forward shock (GRB 141220A), measured 129.5 - 204.3 s 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 ~ 2200 s post-burst is typical of classical forward shocks with α = 1.091 ± 0.008. The low-optical polarization P_BV = 2.8_{- 1.6}^{+ 2.0} per cent (2σ) at mean time ~ 168 s post-burst is compatible with being induced by the host galaxy dust (A_{V, HG} = 0.71 ± 0.15 mag), 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).
Volume
505
Issue
2
Start page
2662
Issn Identifier
0035-8711
Ads BibCode
2021MNRAS.505.2662J
Rights
open.access
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