Faraday conversion in pair-symmetric winds of magnetars and Fast Radio Bursts

TL;DR

This paper explores how polarization propagates in pair-symmetric magnetar winds near the light cylinder, revealing that such plasmas can cause significant polarization changes and explain observed features in Fast Radio Bursts.

Contribution

It introduces a model of Faraday conversion in magnetar winds that accounts for large polarization variations and explains observed FRB polarization phenomena.

Findings

Plasmas act as phase retarders, altering polarization on the Poincare sphere.

Large polarization changes can occur with low dispersion measure plasma screens.

Frequency scaling of RM can vary between λ^2 and λ, affecting interpretation of observations.

Abstract

We consider propagation of polarization in the inner parts of pair-symmetric magnetar winds, close to the light cylinder. Pair plasmas in magnetic field is birefringent, a $\propto B^2$ effect. As a result, such plasmas work as phase retarders: Stokes parameters follow a circular trajectory on the Poincare sphere. In the highly magnetized regime, $\omega, \, \omega_p \ll \omega_B$, the corresponding rotation rates are independent of the magnetic field. A plasma screen with dispersion measure DM $\sim 10^{-6}$ pc cm$^{-3}$ can induce large polarization changes, including large effective Rotation Measure (RM). The frequency scaling of the (generalized) RM, $ \propto \lambda ^\alpha $, mimics the conventional RM with $\alpha =2$ for small phase shifts, but can be as small as $\alpha =1$. In interpreting observations the frequency scaling of polarization parameters should be fitted independently. The model offers explanations for (i) large circular polarization component observed in FRBs, with right-left switching; (ii) large RM, with possible sign changes; (iii) time-depend variable polarization. Relatively dense and slow wind is needed - the corresponding effect in regular pulsars is small.