Polarization of magnetospheric curvature radiation in repeating fast radio bursts

Abstract

Fast radio bursts (FRBs) can exhibit a wide variety of polarization properties, not only between sources but also from burst to burst for a same one. In this work, we revisit the polarization 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 polarization features, such as high levels of circular polarization (CP) or a sign change of CP. High linear polarization (LP) would appear when the line of sight is inside the emission beam (the on-beam geometry), whereas high CP would be present when it is outside (the off-beam geometry). By considering two scenarios of the 'bulk shapes' (thick versus thin), we apply the model to explain the polarization features of four repeating FRBs (FRB 20121102A, FRB 180916B, FRB 20190520B and FRB 20201124A). Most bursts are dominated by LP and negligible events have sign changes in CP, 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.