2023/06/01 by Rogelio Medina Covarrubias, Fabio De Colle, Covarrubias, Rogelio Medina +5
Physics and Astronomy · #Astro and Planetary Science #FOS: Physical sciences #Gamma-ray bursts and supernovae #High Energy Astrophysical Phenomena (astro-ph.HE) #Solar and Space Plasma Dynamics
paper · pdf · doi:10.48550/arxiv.2306.01136
openalex publication_date 2023/06/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We compute the linear polarisation during the afterglow phase of gamma-ray bursts, for both on-axis and off-axis observers. We use numerical simulations of the deceleration of a relativistic jet, and compute the polarisation by post-processing the results of the numerical simulations. In our simulations, we consider a magnetic field that is chaotic in the plane of the shock, in addition to a magnetic field component that is parallel to the shock velocity. While the linear polarisation computed for on-axis observers is consistent with previous analytical estimates, we found that lateral expansion, which is accurately handled in our simulations, plays a crucial role in determining the linear polarisation for off-axis observers. Our results show that the off-axis linear polarisation, as seen by off-axis observers, exhibits a single peak, in contrast to the two peaks inferred by previous analytical studies. The maximum polarisation degree is 40% at an observing angle θ\rm obs=0.4 rad, and it decreases as the observing angle increases, which is opposite to what predicted by analytical models, where polarisation increases with larger observing angles. From the upper limit of 12% in the linear polarisation obtained at 244 days for the GRB 170817A, we also infer an anisotropy factor of B_∥/B_⊥ = 0.5-0.9, consistent with the post-shock magnetic field being amplified by turbulence.