Kavli Affiliate: Kejia Lee
| First 5 Authors: , , , ,
| Summary:
In this study, we report multi-year polarization measurements of four
repeating FRBs initially discovered by CHIME: FRBs~20190117A, 20190208A,
20190303A, and 20190417A. We observed the four repeating FRBs with 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 sigma_mathrmRM = 2.78 pm 0.05 rad m$^-2$ and 5.19 pm 0.09 rad
m$^-2$, respectively. The linear polarization distributions among five
repeating FRB–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 $gamma sim (0-0.8)$, corresponding to a
shallow power spectrum $alpha = -(gamma + 2) sim -(2.0-2.8)$ 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 comparing 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.
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