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arXiv · 2607.20604

Revealing the Structure and Magnetization of GRB Jets with ALMA Polarization Observations

Tanmoy Laskar·Collin T. Christy·Noah Franz·Gavin Farley·Kate D. Alexander·Jonathan Granot·Ramandeep Gill·Tarraneh Eftekhari·Shiho Kobayashi·Hendrik van Eerten·Raffaella Margutti·Edo Berger
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Abstract

We present a systematic study of the currently available ALMA millimeter polarimetric sample of gamma-ray burst (GRB) afterglows. Our sample comprises 24 observations (20 new) of 11 long-duration GRBs spanning $\approx0.1$-87 days after the burst. We detect significant linear polarization in 8 observations across 6 events, with polarization degrees ranging from $Π_L\approx0.6\%$ to $2.4\%$. For the remaining observations, we place deep upper limits (median $Π_L\lesssim1\%$). Multi-epoch observations reveal diverse polarization evolution. GRB 190114C yields the best-sampled $Π_L$ for a radio afterglow to date, with early evolution favoring patchy magnetic fields in the reverse shock (RS) and later polarization broadly consistent with forward-shock (FS) models. GRB 220921A exhibits a rapid rise in polarization from $Π_L\lesssim0.4\%$ to $2.4\%$ over 1.9-6.8 days, inconsistent with toroidal RS magnetic-field models but broadly consistent with several FS random-field models. GRB 221009A yields the highest-significance polarization detections in the sample ($Π_L\approx1.4$-$1.6\%$), yet neither existing FS nor RS polarization models reproduce both the observed polarization evolution and the viewing geometry inferred from broadband afterglow modeling, with the exception of patchy fields in the RS. Deep upper limits for GRB 171205A rule out RS toroidal-field models for the published off-axis geometry, while a strong, single-epoch detection of GRB 190829A, if RS-dominated, requires a nearly on-axis geometry for toroidal-field configurations. Interpreting the observations within a patchy-field framework implies magnetic-field coherence scales of order $θ_B\sim10^{-3}$-$10^{-2}$ rad. These observations demonstrate the diagnostic power of radio/mm polarimetry for probing the magnetic-field structure, emission region, and viewing geometry of relativistic GRB jets.

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Tanmoy Laskar, Collin T. Christy, Noah Franz, Gavin Farley, Kate D. Alexander, Jonathan Granot, Ramandeep Gill, Tarraneh Eftekhari, Shiho Kobayashi, Hendrik van Eerten, Raffaella Margutti, Edo Berger. 2026-07-22. Revealing the Structure and Magnetization of GRB Jets with ALMA Polarization Observations. https://arxiv.org/abs/2607.20604

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