Propagation-induced Frequency-dependent Polarization Properties of Fast Radio Burst

TL;DR

This paper presents an analytical model of polarization propagation in magnetized plasma to explain frequency-dependent polarization features of FRBs, offering insights into their magnetic environments and radiation mechanisms.

Contribution

It introduces an analytical solution for polarization transfer in FRBs, capturing frequency-dependent effects and providing a more physical interpretation than previous empirical methods.

Findings

Precession of polarization spectrum on the Poincaré sphere due to strong Faraday effects.

Dense plasma can produce highly circularly polarized waves.

Model successfully fits observed polarization data of specific FRBs.

Abstract

Frequency-dependent polarization properties provide crucial insights into the radiation mechanisms and magnetic environments of fast radio bursts (FRBs). We explore an analytical solution of radiative transfer of the polarization properties of FRBs as a strong incoming wave propagates in a homogeneous magnetized plasma. The cases of a thermal plasma is studied in detail. The rotational axis of the polarization spectrum undergoes precession with frequency on the Poincar\'e sphere when the medium has both strong Faraday rotation and conversion. Such precession on the Poincar\'e sphere could occur in hot or cold plasma with a strong magnetic field component perpendicular to the line of sight. Significant absorption can exist in a dense plasma medium, which may give rise to a highly circularly polarized outgoing wave. We apply the analytical solution with the mixing Faraday case to fit the observations of frequency-dependent Stokes parameters for FRB 20180301A and FRB 20201124A. The analytical solution offers a more physical description of FRBs' magnetic environment properties than the empirical ``generalized Faraday rotation'' method commonly adopted in the literature. The frequency-dependent Stokes parameters may be associated with reversing rotation measures or the presence of a persistent radio source around an FRB.