Multi-year polarimetric monitoring of four CHIME-discovered repeating fast radio bursts with FAST

Abstract

Fast radio bursts (FRBs) are bright, millisecond-duration radio emissions originating from cosmological distances. In this study, we report multi-year polarization measurements of four repeating FRBs initially discovered by the Canadian Hydrogen Intensity Mapping Experiment (CHIME): FRBs 20190117A, 20190208A, 20190303A, and 20190417A. We observed the four repeating FRBs with the Five-hundred-meter Aperture Spherical Radio Telescope (FAST), detecting a total of 66 bursts. Two bursts from FRB 20190417A exhibit a circular polarization signal-to-noise ratio greater than 7, with the highest circular polarization fraction recorded at 35.7%. While the bursts from FRBs 20190208A and 20190303A are highly linearly polarized, those from FRBs 20190117A and 20190417A show depolarization due to multi-path propagation, with σ_RM = 2.78 ± 0.05 and 5.19 ± 0.09 rad m⁻², respectively. The linear polarization distributions among five repeating FRBs—FRBs 20190208A, 20190303A, 20201124A, 20220912A, and 20240114A—are nearly identical but show distinct differences from those of non-repeating FRBs. FRBs 20190117A, 20190303A, and 20190417A exhibit substantial rotation measure (RM) variations between bursts, joining other repeating FRBs in this behavior. Combining these findings with published results, 64% of repeating FRBs show RM variations greater than 50 rad m⁻², and 21% exhibit RM reversals. A significant proportion of repeating FRBs reside in a dynamic magneto-ionic environment. The structure function of RM variations shows a power-law index of γ ∼ (0–0.8), corresponding to a shallow power spectrum α = −(γ + 2) ∼ −(2.0–2.8) of turbulence, if the RM variations are attributed to turbulence. This suggests that the variations are dominated by small-scale RM density fluctuations. We perform K-S tests to compare the RMs of repeating and non-repeating FRBs, which reveal a marginal dichotomy in the distribution of their RMs. We caution that the observed dichotomy may be due to the small sample size and selection biases.