A possible periodic RM evolution in the repeating FRB 20220529

TL;DR

This study analyzes nearly three years of RM time-series data for FRB 20220529, obtained with FAST, to search for periodic RM modulation. Using both Lomb-Scargle periodograms and phase-folding analyses, the authors identify a plausible ~$P \approx 200$ days RM cycle with significances of $4.1σ$ and $3.1σ$ respectively, and observe hints of a similar periodicity in the burst rate. They discuss three physical interpretations—binary orbital motion, an outflow/disk around an intermediate-mass black hole, and magnetized turbulence—concluding that extended monitoring is needed to confirm the periodicity and constrain the progenitor system. The work highlights how RM time evolution provides a powerful diagnostic of the local magneto-ionic environment near FRB sources, potentially revealing orbital dynamics or strong outflows in their immediate vicinity.

Abstract

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of extragalactic origin. Some of them repeat, while others apparently do not. Investigations of periodic activity in repeating FRB have been conducted to probe their origins. While periodicity in the burst rate has been reported, studies of periodicities in other properties, such as dispersion measure (DM) and rotation measure (RM), are sparse. FRB~20220529 was monitored by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) for nearly three years, providing an opportunity to investigate periodicity in its observed properties. Here we report a possible period of $\sim 200$ days in the RM evolution, with a significance of {4.1 $σ$} estimated via the Lomb-Scargle algorithm and {3.1 $σ$} with the phase-folding method. Periodicity in the burst rate was also investigated. It may indicate that the FRB progenitor is in a binary system, which is consistent with the significant RM increase and prompt recovery of this FRB on a week-timescale. Other scenarios, such as a system with an intermediate-mass black hole, are also explored.

Figures (6)

• Figure 1: Temporal Evolution of RM, DM, and Burst Rate in FRB 20220529. (A) FRB Daily Burst Rate. Grey vertical dashed lines indicate observation days (bursts detected and undetected). The y-axis of the plot is logarithmic. (B) Dispersion measure of bursts. The blue points represent the detected DM values with their uncertainties. (C) Rotation measure of bursts. The filled blue region delineates the RM variation range excluding the RM flare epoch, with the y-axis range scaled to compress the RM flare region and enhance the visibility of the primary dataset. The red points correspond to the daily-binned RM values. The orange lines mark the phases corresponding to the peak of the 200-day period, approximately at phase 0.25.
• Figure 2: Results of Periodicity examinations. Upper Left: LSP of RM variations for FRB 20220529, derived from FAST observations. The red dashed line marks the most significant peak at 199 days. The x-axis of the plot is logarithmic. Upper right: Simulation results of the LSP. The red dashed line represents the maximum power obtained with the observed RM. In the simulation, RM values were randomly selected and assigned to the TOA before applying the LSP. $10^6$ simulations were performed. The probability of obtaining a maximum power greater than or equal to the observed maximum power is $2\times10^{-5}$. Lower Left: Results of phase folding analysis. The red dashed line indicates the period corresponding to the maximum $\chi^2$ value, which is 204 days. The x-axis of the plot is logarithmic. Lower Right : Simulation results of phase folding analysis. $10^6$ simulations were performed. The probability of obtaining a maximum $\chi^2$ greater than or equal to the observed maximum $\chi^2$ is $1.0\times10^{-3}$.
• Figure 3: Phase-folded RM variations with a 200-day period. The blue points represent the observed RM values and their uncertainties. The red points show the averaged RM values within each phase bin. The orange line depicts the fit result to a sine function.
• Figure 4: Significance for different bin sizes and periods in phase-folding periodicity search. The x-axis represents the bin size, and the y-axis shows the period. The color intensity indicates the corresponding significance, which reflects the significance of the periodicity detection for each trial. It is evident that periods around $\sim$ 200 days are relatively significant for reasonable bin sizes.
• Figure 5: False Alarm Probability versus trial period in LSP. Orange points show FAP values for each trial period in the RM periodicity search, while blue points denote results from the Burst Rate periodicity analysis. Gray dashed lines indicate statistical significance thresholds corresponding to $3\sigma$ and $5\sigma$ confidence levels. The y-axis is inverted, with FAP values decreasing from bottom to top.