The Statistical Polarization Properties of Coherent Curvature Radiation by Bunches: Application to Fast Radio Burst Repeaters

Fast radio bursts (FRBs) are extragalactic radio transients with a millisecond duration and an extremely high brightness temperature. Very recently, some highly circularly polarized bursts were found in a repeater, FRB 20201124A. The significant circular polarization might be produced by coherent curvature radiation by bunches with the line of sight (LOS) deviating from the bunch central trajectories. In this work, we carry out simulations to study the statistical properties of burst polarization within the framework of coherent curvature radiation by charged bunches in the neutron star magnetosphere for repeating FRBs. The flux is almost constant within the opening angle of the bunch. However, when the LOS derives from the bunch opening angle, the larger the derivation, the larger the circular polarization but the lower the flux. We investigate the statistical distribution of circular polarization and flux of radio bursts from an FRB repeater, and find that most of the bursts with high circular polarization have a relatively low flux. Besides, we find that most of the depolarization degrees of bursts have a small variation in a wide frequency band. Furthermore, we simulate the polarization angle (PA) evolution and find that most bursts show a flat PA evolution within the burst phases, and some bursts present a swing of the PA.

Automated summary

It has been found that highly circularly polarized radio bursts exist in Fast Radio Bursts (FRBs). The significant circular polarization may be produced by coherent curvature radiation from bunches with deviating line of sight from the bunch central trajectories. Simulation studies show that radio bursts with high circular polarization typically have a relatively low flux, and most bursts have a small variation in depolarization degrees within a wide frequency band.