Polarisation of magnetospheric curvature radiation in repeating fast radio bursts

TL;DR

This paper investigates the polarization properties of coherent curvature radiation in magnetospheres of neutron stars to explain the diverse polarization features observed in repeating fast radio bursts, linking them to emission geometry and bulk shape.

Contribution

It introduces a model considering bulk shapes to explain FRB polarization features, connecting polarization types to emission geometry and bulk configurations.

Findings

Most bursts show high linear polarization.

Negligible circular polarization sign changes suggest 'thin' bulk emission.

'Thin' bulks with large opening angles are likely responsible for repeating FRBs.

Abstract

Fast radio busts (FRBs) can exhibit a wide variety of polarisation properties, not only between sources but also from burst to burst for a same one. In this work, we revisit the polarisation characters of coherent curvature radiation from a bulk of charged bunches in the magnetosphere of a highly magnetized neutron star. FRBs have been observed to have a variety of polarisation features, such as high levels of circular polarisation or a sign change of circular polarisation. High linear polarisation would appear when the line of sight is inside the emission beam (the on-beam geometry), whereas high circular polarisation would be present when it is outside (the off-beam geometry). By considering two scenarios of the ``bulk shapes'' (thick vs. thin), we apply the model to explain the polarisation features of three repeating FRBs (FRB 20201124A, FRB 20190520B and FRB 20121102A). Most bursts are dominated by linear polarisation and negligible events have sign changes in circular polarisation, suggesting that such FRBs are most likely to be emitted by the ``thin'' bulks with large opening angles. The higher probability of ``thin'' bulks could be meaningful for understanding repeating FRB central engine, i.e., the sparking dynamics to produce different bulks of energetic bunches on a neutron star surface.