Periodic variation of magnetoionic environment of a fast radio burst source

TL;DR

This study reports a 26.24-day periodic variation in the Faraday rotation measure of FRB 20201124A, providing evidence for its binary system origin and suggesting such systems may be common among repeating FRBs.

Contribution

First detection of a periodic RM variation in an FRB, indicating binary system dynamics and advancing understanding of FRB environments.

Findings

26.24-day RM periodicity detected with high significance

Supports binary system model for FRB 20201124A

Suggests binary systems may be common in repeating FRBs

Abstract

Fast radio bursts (FRBs) are luminous, dispersed millisecond-duration radio bursts whose origin is poorly known. Recent observations suggest that some FRBs may reside in binary systems, even though conclusive evidence remains elusive. Here we report the detection of a 26.24$\pm$0.02 day periodicity in Faraday rotation measure (RM) of an actively repeating source named FRB 20201124A. The detection was made from 3,106 bursts collected with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) over ~365 days. The RM periodicity is coherently phase-connected across ~14 cycles over a 1-year duration. Our detection of RM periodicity corresponds to a natural logarithmic Bayesian factor of 1,168. The detection significances vary between 5.9-34 {\sigma} under different assumptions. Such a periodicity provides evidence for the binary nature of FRB 20201124A, where the periodic RM variations arise from the orbital motion of the FRB source within the magnetoionic environment of the system. Together with previous observations, our result suggests that being in binary systems may be a common feature for actively repeating FRB sources.