Propagation of a fast radio burst through a birefringent relativistic plasma

TL;DR

This paper investigates how fast radio bursts' polarization is affected by propagation through relativistic plasmas, revealing a new mechanism for circular polarization generation in such extreme environments.

Contribution

It introduces a model explaining the frequency-dependent circular polarization in FRBs via propagation through relativistic plasmas, expanding understanding of plasma effects on radio signals.

Findings

Detected significant frequency-dependent circular polarization in an FRB burst.

Modeled the polarization using a generalized Faraday rotation framework.

Proposed propagation through relativistic plasma as the cause of observed polarization features.

Abstract

The study of impulsive astrophysical radio emission makes it possible to probe the intervening plasma between the emission source and the Earth. In cold electron-ion plasmas, the circular propagating wave modes primarily alter the linear polarization plane that scales with the inverse-square of the emission frequency. In relativistic plasmas, the wave modes are elliptically polarized, and it is possible to convert linearly polarized emission into circular and vice versa. Fast radio bursts (FRBs) enable the study of not only the electron-ion plasma of the intergalactic medium but potentially the extreme magneto-ionic medium in which these intense pulses are produced. Here we report on the polarimetric analysis of a repeat burst from the FRB 20201124A source. The burst displayed a significant frequency-dependent circularly polarized component, unlike other bursts from this source or any other FRB found to date. We model the frequency dependence of the circular polarization using a phenomenological generalized Faraday rotation framework. We find that the observed circular polarization in the burst can be adequately explained by the propagation of an initially linearly polarised burst signal through a relativistic plasma along the line of sight from the progenitor.