Super-Orbital Variations in Magnetar Rotation Measure Arising from the Precession of Companion Star: Implications for FRB 20220529

TL;DR

This work addresses the puzzling RM variations of FRB 20220529 by introducing a binary magnetar model in which the companion star's magnetic axis precesses around the orbital axis. The authors develop a RM/DM calculation incorporating a toroidal wind field and explore disc-wind versus isotropic-wind scenarios, showing that RM evolution can exhibit super-orbital behavior with a beat structure relative to the orbital period. Fitting to FRB 20220529 yields a precession period of $P_{\mathrm{pre}} = 181.60^{+1.50}_{-1.33}$ days and a magnetic-axis inclination of $i_{\mathrm{m}} = 18.96^{\circ +3.10}_{-2.46}$, with a notable improvement over a non-precessing model; the DM signal remains small, arguing against a dense equatorial disc. The model predicts a future super-orbital cycle of approximately $1.8\times 10^{3}$ days, offering a testable signature and providing constraints on the companion wind geometry and magnetic configuration, relevant for interpreting FRB environments in binaries.

Abstract

Recent observations of FRB 20220529 reveal significant variation and a partial reversal in its rotation measure (RM), suggesting the presence of a dynamically evolving magnetized environment, which could be caused by the orbital motion of the magnetar within the binary system. Here we develop the binary model by suggesting that the spin and magnetic axis of the companion star could undergo precession around the orbital axis. It is then investigated how the precession period and the inclination of the magnetic axis, as well as a possible disc wind, can influence the evolution behaviors of the RM and dispersion measure (DM) of FRB emission. As the foremost consequence, the RM variation can be significantly altered on timescales longer than the orbital period, producing super-orbital evolution and complex patterns. Applying this model to FRB 20220529, we find that its RM evolution could be reproduced with a precession period of 182 days and an inclination angle of approximately $19^{\circ}$, while the other binary parameters are fixed at their typical values. Meanwhile, the absence of significant variation of the DM argues against the presence of a dense equatorial disc around the companion star, which would be constrained by future long-term observations.

Figures (6)

• Figure 1: Schematic diagram of the magnetic axis precession and magnetic field environment of a massive star, with the pulsar and associated bow shock also shown.
• Figure 2: Modulation of long-term RM evolution by precession periods of 0.4, 0.9, 2.7 and 5.3 times of orbital period, along with different magnetic axis inclinations of $30^{\circ}$, $70^{\circ}$. The surface magnetic field strength of the companion is assumed to be 0.03 G. In each subfigure, the light gray lines represent the RM evolution without accounting for precession.
• Figure 3: The normalized power spectra for four different precession periods obtained from LSP analysis of the theoretical RM evolution. The black dashed line is the orbital period. The precession period and beat period are exhibited by dash-dotted and dotted lines, respectively. The peaks on the far right are the LCM periods, which depicted by gray dashed lines.
• Figure 4: The DM (left) or RM (right) evolution in the presence of a disc stellar wind, which is assumed to precess in synchronization with the magnetic axis of the companion. The solid and dashed lines represent the contribution of the isotropic and disc wind components, respectively, while the dash-dotted line gives their sum.
• Figure 5: Top: The RM evolution of FRB 20220529 in comparison with the binary models with (solid) and without (dashed) the precession effect. The observed RM data are taken from the data reported by Liang et al.Liang_2025. Bottom: Corner plot showing the posterior distributions of the model parameters.