Speaker
Description
The afterglows of gamma-ray bursts (GRBs) are non-thermal electron synchrotron emissions from relativistic shocks. The amplification mechanism of magnetic field at the shocks is one of the major problems in high-energy astrophysics, which could be solved by polarimetric observations. Two field amplification mechanisms, Weibel instability and magnetohydrodynamic instability, have been proposed so far, and the former one produces field on plasma skin depth scale, smaller than that of the latter one by 7-10 orders of magnitudes. Polarimetric properties of the plasma-scale field model have been well studied (e.g., Rossi et al. 2004; Shimoda & Toma 2021). In this work, we focus on the latter mechanism and build a semi-analytic model of the synchrotron polarization with magnetohydrodynamic turbulence for the first time. We perform numerical calculations and find that for the isotropic turbulence and the zero viewing angle, the observed level of late-phase optical afterglow polarization degrees (∼1-3 %) can be reproduced when the field coherence length scale in the fluid comoving frame is comparable to the thickness of the shocked regions. 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 plasma-scale field model. Simultaneous polarimetric observations of GRB afterglows in the radio and optical bands have recently started, which will help us constrain the magnetic field amplification mechanism.