1 research outputs found
Numerical simulations of polarization in structured jets: application to GRB 170817 afterglow
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 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
, consistent with the post-shock magnetic field
being amplified by turbulence.Comment: 11 pages, 9 figures, accepted for publication in MNRA