The disk precession in a Be star-magnetar binary and its application to the rotation measure of FRB 20201124A

TL;DR

This paper models the rotation measure variations of FRB 20201124A as caused by a Be star-magnetar binary system, attributing changes to orbital motion and disk precession, providing insights into the local environment of repeating FRBs.

Contribution

It introduces a novel model linking RM variations to binary orbital motion and disk precession in a Be star-magnetar system, explaining observed long-term behaviors.

Findings

RM variation amplitude oscillates with a ~785-day period.

Disk precession influences the amplitude and pattern of RM variations.

The model explains the dynamic local environment of the FRB source.

Abstract

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 magneto-ionic 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 the most active one and exhibits substantial temporal variations of RM measured by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The physics behind this long-term behavior is poorly understood. Here we propose that, within the framework of the Be star-magnetar binary scenario, the observed variation of RM is attributed to a combination of orbital motion and the precession of the circumstellar disk of the Be star. While a ~785-day precession of the disk contributes to the observed decrease in the amplitude of the variation, our model predicts that the amplitude oscillates with this period.