The most distant optically polarised GRB afterglow: GRB 240419A at

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.