# Faraday conversion and magneto-ionic variations in Fast Radio Bursts

## TL;DR

This paper investigates the magneto-ionic environment of FRB 121102, using Faraday effects to constrain the properties of its surrounding medium and persistent source, revealing a complex, mixed plasma with specific magnetic field strengths.

## Contribution

It introduces constraints on the plasma composition and magnetic field in the environment of FRB 121102 based on Faraday rotation and conversion observations.

## Key findings

- Persistent source likely contains relativistic and non-relativistic electrons.
- Magnetic field in the confining medium is between 10 and 30 mG.
- Model explains observed Faraday effects with a self-consistent plasma environment.

## Abstract

The extreme, time-variable Faraday rotation observed in the repeating fast radio burst (FRB) 121102 and its associated persistent synchrotron source demonstrates that some FRBs originate in dense, dynamic and possibly relativistic magneto-ionic environments. Here we show that besides rotation of the linear-polarisation vector (Faraday rotation), such media can generally convert linear to circular polarisation (Faraday conversion). We use non-detection of Faraday conversion, and the temporal variation in Faraday rotation and dispersion in bursts from FRB\,121102 to constrain models where the progenitor inflates a relativistic nebula (persistent source) confined by a cold dense medium (e.g. supernova ejecta). We find that the persistent synchrotron source, if composed of an electron-proton plasma, must be an admixture of relativistic and non-relativistic (Lorentz factor $\gamma<5$) electrons. Furthermore we independently constrain the magnetic field in the cold confining medium, which provides the Faraday rotation, to be between $10$ and $30\,$mG. This value is close to the equipartition magnetic field of the confined persistent source implying a self-consistent and over-constrained model that can explain the observations.