The most distant optically polarised GRB afterglow: GRB 240419A at z = 5.178

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
² Como Lake Centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
³ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁴ School of Physics and Centre for Space Research, University College Dublin, Belfield, D04 V1W8 Dublin, Ireland
⁵ Astronomical Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
⁶ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
⁷ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
⁸ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
¹⁰ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹¹ Department of Physics & Astronomy, University of Utah, Salt Lake City, UT 84112, USA
¹² DTU Space, Technical University of Denmark, Elektrovej 327-328, DK-2800 Lyngby, Denmark
¹³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
¹⁴ Department of Astrophysics/IMAPP, Radboud University, 6525 AJ, Nijmegen, The Netherlands
¹⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹⁶ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
¹⁷ Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hertfordshire AL10 9AB, UK
¹⁸ Space Science Data Center (SSDC) – Agenzia Spaziale Italiana (ASI), 00133 Roma, Italy
¹⁹ University of Messina, Mathematics, Informatics, Physics and Earth Science Department, Via F.S. D’Alcontres 31, Polo Papardo, 98166 Messina, Italy
²⁰ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
²¹ South African Astronomical Observatory, P.O. Box 9 Observatory 7935, South Africa
²² The Inter-University Institute for Data Intensive Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
²³ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland
²⁴ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China
²⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, (RM), Italy
²⁶ Anton Pannekoek Institute for Astronomy, University of Amsterdam, P.O. Box 94249 1090 GE, Amsterdam, The Netherlands
²⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
²⁸ INAF – IASF Milano, Via Alfonso Corti 12, 20133 Milano, Italy
²⁹ Aix Marseille University, CNRS, CNES, LAM, Marseille, France
³⁰ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA
³¹ School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
³² School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia

^⋆ Corresponding author: riccardo.brivio@inaf.it

Received: 31 May 2025
Accepted: 6 September 2025

Abstract

Context. Gamma-ray bursts (GRBs) are extremely bright phenomena powered by relativistic jets arising from explosive events at cosmological distances. The nature of the jet and the configuration of the local magnetic fields are still unclear, with the distinction between different models possibly provided by the detection of early-time polarisation.

Aims. Past observations do not agree on a universal scenario describing early-time polarisation in GRB afterglows, and new studies are necessary to investigate this open question. We present here the discovery of GRB 240419A, its redshift determination of z = 5.178, its early-time optical polarimetry observations, and the multi-wavelength monitoring of its afterglow.

Methods. We analysed three epochs of polarimetric data to derive the early-time evolution of the polarisation. The multi-wavelength light curve from the X-rays to the near-infrared band was also investigated to give a broader perspective on the whole event.

Results. We find a high level of polarisation, P = 6.97^+1.84_−1.52%, at 1740 s after the GRB trigger, followed by a slight decrease up to P = 4.81^+1.87_−1.53% at 3059 s. On the same timescale, the polarisation position angle is nearly constant. The multi-band afterglow at the time of the polarisation measurements is consistent with a forward shock (FS), while the earlier evolution at t − t₀ ≲ 700 s can be associated with the interplay between the forward and the reverse shocks or with energy injection.

Conclusions. The detected polarised radiation when the afterglow is FS-dominated and the stable position angle are consistent with an ordered magnetic field plus a turbulent component driven by large-scale magnetohydrodynamic instabilities. The lack of a jet break in the light curve prevents a comparison of the polarisation temporal evolution with theoretical expectations from magnetic fields amplified by microscopic-scale turbulence, limiting our ability to constrain the observer’s viewing angle. Notably, GRB 240419A is the most distant GRB with a detected polarised optical afterglow, extending the redshift range for such measurements.