paper

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

arXiv:1911.08499 · doi:10.3847/1538-4357/ab7248

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 s post-burst. The light curves reveal a chromatic break at s, with initial temporal decay flattening to post-break, which we model as a combination of reverse and forward-shock components, with magnetization parameter . The observed polarization degree decreases from to during s post-burst and remains steady at this level for the subsequent -s, at constant position angle. Broadband spectral energy distribution modeling of the afterglow confirms GRB 190114C is highly obscured (Amag; Ncm). We interpret the measured afterglow polarization as intrinsically low and dominated by dust, in contrast to 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.

arXiv commentThe full photometry data is available in the journal publication

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