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

TL;DR

This study addresses the puzzling long-term RM variations observed in FRB 20201124A by proposing a Be star–magnetar binary where a decretion disk around the Be star precesses. The authors formulate a physical model with a toroidal magnetic field and a density profile for the disk, and compute RM by integrating the electron density and magnetic field along the line of sight as the magnetar orbits and the disk precesses. They fit the RM data from three FAST epochs, obtaining a best-fit orbital period $P_{ m orb}=73$ days and a precession period $P_{ m prec}\, oughly 7.8 imes10^2$ days, along with constrained disk density, vertical structure, and viewing geometry. The results show that orbital motion plus disk precession can reproduce the observed RM pattern, including sign changes and amplitude modulation, and the framework is extensible to other FRBs with RM variations, offering a path to diagnose the local magneto-ionic environments of repeating FRBs.

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.

Figures (3)

• Figure 1: A schematic diagram of the binary system in Section \ref{['3']} (not to scale). Note that the darker regions in the disk have higher density. The magnetar continuously emits FRBs and orbits the Be star along an elliptical orbit. FRBs emitted by the magnetar at different positions along its orbit propagate along the LoS and reach the observer. The precession of the Be star disk leads to varying RMs among different FRBs.
• Figure 2: The RM variation of FRB 20201124A in three FAST observation episodes, i.e., $\Delta\rm{RM}_{E1}$ (blue dots), $\Delta\rm{RM}_{E2}$ (maroon dots) and $\Delta\rm{RM}_{E3}$ (dark green dots). The ranges of observed RM of E1, E2 and E3 are shown as the blue, maroon, and green shadow regions, respectively. The fitting of the model presented in Section \ref{['3']} is shown as the red curve, while the parameters of the fitting is shown in Table \ref{['tab:1']}.
• Figure 3: Three FRB sources with notable RM variation, each with model fitting RM curves of $P_{\rm{prec}}=10\times P_{\rm{orb}}$ (blue curve) and without precession (dotted red curve). The two curves in each subplot share a same set of parameters except $\theta$ and $\omega$, since these two parameters are both 0 for the no-precession case. Evolution of this two curves are shown for five orbital periods in each subplot. The observed $\Delta\rm{RM}$s is shown as the gray dots. The parameters of each FRB source are shown in Table \ref{['tab:2']}.