On the Circular Polarisation of Repeating Fast Radio Bursts

TL;DR

This paper investigates the origin of circular polarisation in repeating fast radio bursts, suggesting that magnetospheric propagation effects and plasma asymmetries influence polarisation levels, which can help identify their source mechanisms.

Contribution

It provides a theoretical analysis of how magnetospheric propagation effects and plasma asymmetries affect circular polarisation in FRBs from magnetars.

Findings

Wave mode coupling unlikely to produce strong circular polarisation.

Cyclotron absorption depends on plasma density.

High circular polarisation requires electron-positron asymmetries.

Abstract

Fast spinning (e.g., sub-second) neutron star with ultra-strong magnetic fields (or so-called magnetar) is one of the promising origins of repeating fast radio bursts (FRBs). Here we discuss circularly polarised emissions produced by propagation effects in the magnetosphere of fast spinning magnetars. We argue that the polarisation-limiting region is well beyond the light cylinder, suggesting that wave mode coupling effects are unlikely to produce strong circular polarisation for fast spinning magnetars. Cyclotron absorption could be significant if the secondary plasma density is high. However, high degrees of circular polarisation can only be produced with large asymmetries in electrons and positrons. We draw attention to the non-detection of circular polarisation in current observations of known repeating FRBs. We suggest that the circular polarisation of FRBs could provide key information on their origins and help distinguish different radiation mechanisms.