Faraday rotation in fast radio bursts

TL;DR

This paper investigates how Faraday rotation in highly magnetized pair plasma near magnetars influences the polarization of Fast Radio Bursts, predicting unique wavelength dependencies that can help identify propagation effects.

Contribution

It introduces a model of polarization propagation in magnetar winds, highlighting specific PA($\\lambda$) scaling laws and their implications for interpreting FRB polarization data.

Findings

PA scales as λ or λ³ depending on magnetic dominance

Faraday rotation effects depend on propagation angle and plasma parameters

Distinct polarization signatures can differentiate propagation regimes

Abstract

Fast Radio Bursts (FBRs) show highly different polarization properties: high/small RMs, high/small circular/linear fractions. We outline a complicated picture of polarization propagation in the inner parts of the magnetars' winds, at scales $\sim$ few to hundreds of light cylinder radii. The key point is the Faraday rotation of linear polarization in highly magnetized symmetric pair plasma, a $\propto B^2$ effect. Position angle (PA) rotation rate is maximal for propagation across the magnetic field and disappears only for parallel propagation. In the highly magnetized regime, $\omega \ll \omega_B$, it becomes independent of the magnetic field. Very specific properties of PA($\lambda$) (scaling of the rotation angle with the observed wavelength $\lambda$) can help identify/sort out the propagation effects. Two basic regimes in pair plasma predict PA $\propto \lambda$ and $\propto \lambda^3$ (depending on the magnetic dominance); both are different from the conventional plasma's PA = RM $ \lambda^2$. This is the main prediction of the model. A number of effects, all sensitive to the underlying parameters, contribute to the observed complicated polarization patterns: streaming of plasma along magnetic field lines near the light cylinder, Faraday depolarization, effects of limiting polarization, the associated effect of linear-circular conversion, and synchrotron absorption.