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

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 multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous robotic Liverpool Telescope. The temporal decay gradient of the optical light curves from $86$s to $\sim 2200$s post-burst is typical of classical forward shocks 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-burst is compatible with being induced by the host galaxy dust ($A_{V,\mathrm{HG}}=0.71\pm 0.15$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 sample of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. $P\sim 1\%-3\%$), (b) contribution from a polarized reverse shock (GRB deceleration time, jet magnetization) or (c) forward shock intrinsic polarization (i.e. $P\le 2\%$), which depends on the magnetic field coherence length scale and the size of the observable emitting region (burst energetics, circumburst density).

References in corpus (19)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Multi-messenger Observations of a Binary Neutron Star Merger
• Particle acceleration in relativistic collisionless shocks: Fermi process at last?
• Tracing Magnetic Fields with Aligned Grains
• Magnetic acceleration of ultra-relativistic jets in gamma-ray burst sources
• GRB Radiative Efficiencies Derived from the Swift Data: GRBs vs. XRFs, Long vs. Short
• REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
• Ten per cent polarized optical emission from GRB 090102
• On the Mechanism of Gamma-Ray Burst Afterglows
• Bulk Lorentz factors of Gamma-Ray Bursts
• The Konus-Wind catalog of gamma-ray bursts with known redshifts. I. Bursts detected in the triggered mode
• The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs
• Early Optical Polarization of a Gamma Ray Burst Afterglow
• Production of magnetic energy by macroscopic turbulence in GRB afterglows
• Extreme Starlight Polarization in a Region with Highly Polarized Dust Emission
• A detailed study of the optical attenuation of gamma-ray bursts in the Swift era
• Polarimetry and Photometry of Gamma-Ray Bursts with RINGO2
• Early-time polarized optical light curve of GRB 131030A
• Colour variations in the GRB 120327A afterglow

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• Science with a large field-of-view polarization survey: The Large Array Survey Telescope Polarization Node (LAST-P)