2026ApJ..1000L...7S

Fast radio bursts (FRBs) are bright, millisecond-duration radio bursts with poorly known origins. Most FRB sources are detected only once, while some are repeaters. Variation patterns observed in the rotation measure (RM) of some repeaters indicate that the local magnetoionic environments of these FRB sources are highly dynamic. It has been suggested that a Be star-magnetar binary system is a possible origin for such variation. FRB 20201124A is notable among these sources since it is a very active repeater and exhibits substantial temporal variations of RM as measured by the Five-hundred-meter Aperture Spherical radio Telescope. The physics behind this long-term behavior remains poorly understood. Here we propose that, within the Be star-magnetar binary scenario, the observed variation in RM is attributed to a combination of orbital motion and the precession of the circumstellar disk of the Be star. While a precession of the disk in ∼785 days contributes to the observed decrease in the amplitude of the variation, our model predicts that the amplitude oscillates with this period.