Faraday depolarization and induced circular polarization by multi-path propagation with application to FRBs

TL;DR

This paper analyzes how multi-path scattering with magnetic fields affects the polarization of astronomical signals, explaining depolarization and circular polarization in FRBs and pulsars considering observational limitations.

Contribution

It provides a theoretical framework linking polarization changes to scattering screen properties and observational resolutions, with applications to FRBs and pulsars.

Findings

Polarization depends on bandwidth and temporal resolution ratios.

Linearly polarized waves can develop circular polarization after scattering.

Conditions for depolarization and polarization variability are outlined.

Abstract

We describe how the observed polarization properties of an astronomical object are related to its intrinsic polarization properties and the finite temporal and spectral resolutions of the observing device. Moreover, we discuss the effect that a scattering screen, with non-zero magnetic field, between the source and observer has on the observed polarization properties. We show that the polarization properties are determined by the ratio of observing bandwidth and coherence bandwidth of the scattering screen and the ratio of temporal resolution of the instrument and the variability time of screen, as long as the length over which the Faraday rotation induced by the screen changes by $\sim\pi$ is smaller than the size of the screen visible to the observer. We describe the conditions under which a source that is 100\% linearly polarized intrinsically might be observed as partially depolarized, and how the source's temporal variability can be distinguished from the temporal variability induced by the scattering screen. In general, linearly polarized waves passing through a magnetized scattering screen can develop a significant circular polarization. We apply the work to the observed polarization properties of a few fast radio bursts (FRBs), and outline potential applications to pulsars.