Publication
Coherence scale of magnetic fields generated in early-time forward shocks of GRBs
Publisher:
Oxford University Press (OUP)
Date:
10-04-2021
DOI:
10.1093/MNRAS/STAB1003
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 $\\sim 2200\\,$s post-burst is typical of classical forward shocks with α = 1.091 ± 0.008. The low-optical polarization $P_{ BV} = 2.8 _{- 1.6} ^{+ 2.0} \\, {{\\ \\rm per\\ cent}}$ (2σ) at mean time $\\sim 168\\,$s post-burst is compatible with being induced by the host galaxy dust ($A_{V, {\\rm HG}}= 0.71 \\pm 0.15 \\,$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 s le of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. $P \\sim 1-3{{\\ \\rm per\\ cent}}$), (b) contribution from a polarized reverse shock (GRB deceleration time, jet magnetization) or (c) forward shock intrinsic polarization (i.e. $P \\le 2{{\\ \\rm per\\ cent}}$), which depends on the magnetic field coherence length-scale and the size of the observable emitting region (burst energetics, circumburst density).