Abstract
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 sample of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. $P \sim 1-3{{\ \rm per\ cent}}$), (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).
Highlights
Gamma-ray bursts (GRBs) are the brightest flashes of γ-ray emission in the Universe
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 observed polarization PBV = 2.8+−21..06 % belongs to the pt = 0.97 upper quantile of the overall distribution; there is only 3% probability that this measurement is consistent with zero polarization
Summary
Gamma-ray bursts (GRBs) are the brightest flashes of γ-ray emission in the Universe. After the collapse of a massive star or coalescence of two compact stellar objects (Woosley 1993; Berger 2014; Abbott et al 2017a,b), the accretion into a new-born compact object powers ultrarelativistic jetted emission that —via internal dissipation processes— produces the highly variable and characteristic γ-ray prompt emission. The forward shock is powered by shocked ambient medium and tangled magnetic fields are locally generated in shocks and amplified by plasma instabilities (e.g., via Weibel instability; Weibel 1959; Nishikawa et al 2003; Silva et al 2003; Medvedev et al 2005). The emission is expected to be intrinsically unpolarized when the jet is on-axis (Medvedev & Loeb 1999). It can be significantly polarized if the random field is anisotropic (e.g. the averaged strength in the shock normal direction is stronger or weaker) and the line-of-sight runs almost along the jet edge. The emission can show few percents of polarization due to differential dust extinction (Hiltner 1949; Lazarian 2007) along the Galactic line-of-sight and in the GRB environment (e.g., Lazzati et al 2003; Klose et al 2004)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
