Lowly polarized light from a highly magnetized jet of GRB 190114C

Abstract

We report multi-color optical imaging and polarimetry observations of the afterglow of the first TeV- detected gamma-ray burst, GRB 190114C, using RINGO3 and MASTER II polarimeters. Observations begin 31 s after the onset of the GRB and continue until $\sim 7000$s post-burst. The light curves reveal a chromatic break at $\sim 400-500$s, with initial temporal decay $\alpha =1.669\pm 0.013$ flattening to $\alpha \sim 1$ post-break, which we model as a combination of reverse and forward-shock components, with magnetization parameter $R_{\mathrm{B}}\sim 70$. The observed polarization degree decreases from $7.7\pm 1.1\%$ to $2-4\%$ during $52-109$s post-burst and remains steady at this level for the subsequent $\sim 2000$-s, at constant position angle. Broadband spectral energy distribution modeling of the afterglow confirms GRB 190114C is highly obscured (A${}_{\mathrm{v},\mathrm{HG}}=1.49\pm 0.12$mag; N${}_{\mathrm{H},\mathrm{HG}}=(9.0\pm 0.3)\times 10^{22}$cm${}^{-2}$). We interpret the measured afterglow polarization as intrinsically low and dominated by dust, in contrast to $\mathrm{P}>10\%$ measured previously for other GRB reverse shocks, with a small contribution from polarized prompt photons in the first minute. We test whether 1st and higher-order inverse Compton scattering in a magnetized reverse shock can explain the low optical polarization and the sub-TeV emission but conclude neither is explained in the reverse shock Inverse Compton model. Instead, the unexpectedly low intrinsic polarization degree in GRB 190114C can be explained if large-scale jet magnetic fields are distorted on timescales prior to reverse shock emission.

The full photometry data is available in the journal publication