Synchrotron Polarization of Gamma-Ray Burst Afterglow Shocks with Hydrodynamic-scale Turbulent Magnetic Field

TL;DR

This paper models the polarization of gamma-ray burst afterglows assuming hydrodynamic-scale turbulent magnetic fields, predicting polarization degrees and behaviors that can distinguish between different magnetic field amplification mechanisms.

Contribution

It introduces a semi-analytic model of GRB afterglow polarization based on hydrodynamic-scale turbulence, providing predictions to differentiate magnetic field amplification processes.

Findings

Polarization degrees are around 1% for certain coherence lengths.

Radio polarization can be comparable or higher than optical polarization.

Polarization angles vary randomly and continuously.

Abstract

Afterglows of gamma-ray bursts (GRBs) are emitted from expanding forward shocks, which are expected to have magnetic field much stronger than the interstellar field, although the origin of the field is a long-standing problem. Two field amplification mechanisms, plasma kinetic instabilities and magnetohydrodynamic instabilities, have been discussed so far. The coherence length scales of the fields amplified by these two processes are different by $7-10$ orders of magnitudes, and the polarimetric observations may distinguish them. We construct a semi-analytic model of the forward shock afterglow polarization under the assumption of hydrodynamic-scale turbulent magnetic field. We perform numerical calculations of synchrotron polarization for the isotropic turbulence and the zero viewing angle. We find that the polarization degrees are $ \sim1~\%$ when the field coherence length scale in the fluid comoving frame is of the order of the thickness of the shocked regions. This range of polarization degree is comparable to that of the observed late-phase optical afterglows. Our model also shows that the radio polarization degrees are comparable to the optical ones on average but can be higher than the optical ones at some time intervals. The polarization angles are shown to vary randomly and continuously. These polarimetric properties are clearly different from the case of plasma kinetic instability. Simultaneous polarimetric observations of GRB afterglows at the radio and optical bands have recently started, which will help us constrain the magnetic field amplification mechanism.