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

N. Jordana-Mitjans, C. G. Mundell, S. Kobayashi, R. J. Smith, C. Guidorzi, I. A. Steele, M. Shrestha, A. Gomboc, M. Marongiu, R. Martone, V. Lipunov, E. S. Gorbovskoy, D. A. H. Buckley, R. Rebolo, N. M. Budnev

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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 ∼ 7000 s post-burst. The light curves reveal a chromatic break at ∼ 400 − 500 s —with initial temporal decay α = 1.669 ± 0.013 flattening to α ∼ 1 post-break —which we model as a combination of reverse and forward-shock components, with magnetization parameter RB ∼ 70. The observed polarization degree decreases from 7.7 ± 1.1% to 2 − 4% during 52 − 109 s post-burst and remains steady at this level for the subsequent ∼ 2000-s, at constant position angle. Broadband spectral energy distribution modeling of the afterglow confirms GRB190114C is highly obscured (Av,HG =1.49±0.12mag; NH,HG =(9.0±0.3)×1022cm−2). We interpret the measured afterglow polarization as intrinsically low and dominated by dust —in contrast to 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.
Original languageEnglish
JournalAstrophysical Journal
DOIs
Publication statusPublished - 2 Apr 2020

Keywords

  • astro-ph.HE

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