A possible periodic RM evolution in the repeating FRB 20220529

Abstract: Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients from the distant universe. Some of them repeat, while others do not. In order to explore their origin, periodic examinations have been conducted on repeating FRBs. Most of them show irregular properties, including burst rate, dispersion measure (DM), and rotation measure (RM). A notable exception is FRB~20180916B, which shows a significant 16-day periodic burst rate. Possible periodic activities have also been reported in FRB~20121102A and FRB~20240209A. However, periodic studies of other properties are sparse. FRB~20220529 was monitored by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) for nearly three years, enabling periodic examinations of its properties. Here we report a possible period of $\sim 200$ days in the RM evolution, with a significance of 4.2 $\sigma$ estimated via the Lomb-Scargle algorithm and 3.5 $\sigma$ with a phase-folding method. The burst rate was also examined for periodicity. This is consistent with the binary origin indicated by the significant RM increase and its prompt recovery.