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We report the earliest-ever detection of optical polarization from a GRBforward shock (GRB 141220A), measured $129.5-204.3\,$s after the burst usingthe multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous roboticLiverpool Telescope. The temporal decay gradient of the optical light curvesfrom $86\,$s to $\sim 2200\,$s post-burst is typical of classical forwardshocks with $\alpha = 1.091 \pm 0.008$. The low optical polarization $P_{BV} =2.8_{- 1.6} ^{+ 2.0} \, \%$ (2$\sigma$) at mean time $\sim 168\,$s post-burstis 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 emissionitself -- as theoretically predicted for forward shocks and consistent withprevious detections of low degrees of optical polarization in GRB afterglowsobserved hours to days after the burst. The current sample of early-timepolarization data from forward shocks suggests polarization from (a) theGalactic and host galaxy dust properties (i.e. $P \sim 1\%-3\%$), (b)contribution from a polarized reverse shock (GRB deceleration time, jetmagnetization) or (c) forward shock intrinsic polarization (i.e. $P \leq 2\%$),which depends on the magnetic field coherence length scale and the size of theobservable emitting region (burst energetics, circumburst density).

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