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

R. Brivio, S. Covino, M. Ferro, A. Saccardi, A. Martin-Carrillo, A. Kuwata, K. Toma, P. D'Avanzo, Y. -D. Hu, L. Izzo, S. Kobayashi, T. Laskar, G. Leloudas, D. B. Malesani, M. Pursiainen, S. Vergani, K. Wiersema, S. Bloemen, S. Campana, V. D'Elia, S. de Wet, M. de Pasquale, P. J. Groot, P. Jakobsson, J. Mao, A. Melandri, G. Pugliese, A. Rossi, R. Salvaterra, B. Schneider, N. R. Tanvir, J. van Roestel, P. M. Vreeswijk, T. Zafar

TL;DR

This study reports the discovery of GRB 240419A at $z=5.178$ with the most distant optical polarised afterglow to date. Through early-time polarimetry and multi-wavelength afterglow analysis, the authors show a high polarization degree ($P\approx$7% at $t-t_0\approx$1740 s) with a nearly constant position angle, while the afterglow evolution at these times is FS-dominated. Modelling favors a forward shock emission in an environment with a globally ordered magnetic field plus a turbulent component amplified by large-scale MHD instabilities, requiring $B_{ord}^2/B_{rnd}^2\sim20$ and a wind-origin field of order $10^{-2}$–$10^{1}$ mG. Bayesian light-curve fitting and afterglow modelling yield $\log_{10}(E_k/{\rm erg})=53.8\pm0.5$, $p=2.4\pm0.2$, $n\sim1\,{ m cm^{-3}}$, $\varepsilon_e\sim0.04$, and $\varepsilon_B\sim2.5\times10^{-4}$, with a lower-limit initial Lorentz factor $\Gamma_0\gtrsim230$. The absence of a jet break and the high redshift of this event provide important constraints on jet magnetisation and afterglow physics at early times in the early universe.

Abstract

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. 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. 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. 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 $t-t_0\lesssim700$ s can be associated with the interplay between the forward and the reverse shocks or with energy injection. 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 ...

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

TL;DR

This study reports the discovery of GRB 240419A at with the most distant optical polarised afterglow to date. Through early-time polarimetry and multi-wavelength afterglow analysis, the authors show a high polarization degree (7% at 1740 s) with a nearly constant position angle, while the afterglow evolution at these times is FS-dominated. Modelling favors a forward shock emission in an environment with a globally ordered magnetic field plus a turbulent component amplified by large-scale MHD instabilities, requiring and a wind-origin field of order – mG. Bayesian light-curve fitting and afterglow modelling yield , , , , and , with a lower-limit initial Lorentz factor . The absence of a jet break and the high redshift of this event provide important constraints on jet magnetisation and afterglow physics at early times in the early universe.

Abstract

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. 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 , its early-time optical polarimetry observations, and the multi-wavelength monitoring of its afterglow. 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. We find a high level of polarisation, \,\%, at 1740~s after the GRB trigger, followed by a slight decrease up to \,\% 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 s can be associated with the interplay between the forward and the reverse shocks or with energy injection. 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 ...

Paper Structure

This paper contains 23 sections, 2 equations, 9 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Gamma and X-rays
  4. Optical/NIR afterglow
  5. Other observations
  6. Data analysis and results
  7. X-rays
  8. Spectral analysis
  9. Polarisation analysis
  10. Observations and data analysis
  11. Results
  12. Light curve analysis
  13. Temporal and spectral fitting
  14. Physical modelling
  15. Discussion
  16. ...and 8 more sections

Figures (9)

  • Figure 1: Acquisition image of the field of GRB 240419A observed by VLT/FORS2 on April 19 2024 at $t-t_0\sim20.9$ min. The blue circle marks the position of the optical afterglow.
  • Figure 2: Swift/XRT PC mode spectrum of GRB 240419A. The best fit is shown in orange (top panel). The ratio between the data and the folded model is displayed in the bottom panel.
  • Figure 3: $Q,U$ plots for the three imaging polarimetric observations. Error bars represent $1\sigma$ uncertainties. The afterglow is marked with the red bars and shows significant polarisation with respect to the (unpolarised) field stars (black dots).
  • Figure 4: Multi-wavelength light curve of GRB 240419A. XRT flux densities were computed at 1.73 keV, the log-mean of the XRT band. The dashed line represents the best fit of the X-ray curve. The shaded light green area marks the time of polarisation observations and the polarisation curve is shown in the top right inset. Upper limits are not shown for display purposes.
  • Figure 5: Expected polarisation degree (top panel) and position angle (bottom panel) time evolution for a globally ordered plus large-scale turbulent magnetic field configuration in the $R$ band (purple line), compared with GRB 240419A observations. The values were calculated with one (left) or averaging over 100 (right) realisation sets of random numbers for the turbulence generation.
  • ...and 4 more figures