Rotation Measure Variations and Reversals of Repeating Fast Radio Bursts in Massive Binary Systems

TL;DR

This paper models how stellar winds and discs in binary systems can cause complex Faraday rotation measure variations and reversals in repeating fast radio bursts, providing insights into their environments.

Contribution

It introduces a detailed model linking magnetic field configurations in stellar winds and discs to RM variations in repeating FRBs, especially applied to FRB 20180916B.

Findings

RM reversals depend on magnetic field geometry and inclination.

Stellar wind and disc clumps cause short-term RM fluctuations.

The model explains long-term RM trends in FRB 20180916B.

Abstract

Recent observations discovered that some repeating fast radio bursts (FRBs) show complicated variations and reversals of Faraday rotation measures (RMs), indicating that the sources of these FRBs are embedded in a dynamically magnetized environment. One possible scenario is that repeating FRBs are generated by pulsars in binary systems, especially containing a high-mass companion with strong stellar outflows. Here, we study the RM variations caused by stellar winds, and a possible stellar disc. If the magnetic field is radial in the stellar wind, RM will not reverse except if the magnetic axis inclination angle is close to $90 ^\circ$. For the toroidal magnetic field in the wind, RM will reverse at the super conjunction. For the case of the toroidal field in the disc, the RM variations may have a multimodal and multiple reversal profile because the radio signals travel through different components of the disc during periastron passage. We also apply this model to FRB 20180916B. By assuming its 16.35-day period is from a slowly rotating or freely precessing magnetar, we find that the secular RM variation can be explained by the periastron passage of a magnetar in a massive binary system. In addition, the clumps in the stellar wind and disc can cause short time-scale ($< 1$ day) variations or reversals of RM. Therefore, long-term monitoring of RM variations can reveal the environments of repeating FRBs.