2023ApJ...956L...2L

Recent observations discovered that some repeating fast radio bursts (FRBs) show a large value and complex variations of Faraday rotation measures (RMs). The binary systems containing a supermassive black hole and a neutron star can be used to explain such RM variations. Meanwhile, such systems produce low-frequency gravitational-wave (GW) signals, which are one of the primary interests of three proposed space-based GW detectors: the Laser Interferometer Space Antenna (LISA), Tianqin, and Taiji. These signals are known as extreme mass-ratio inspirals (EMRIs). Therefore, FRBs can serve as candidates of electromagnetic counterparts for EMRI signals. In this Letter, we study the EMRI signals in this binary system, which can be detected up to z ∼ 0.04 by LISA and Tianqin for the most optimistic case. Assuming the cosmic comb model for FRB production, the total event rate can be as high as ∼1 Gpc^–3 yr^–1. EMRI signals associated with FRBs can be used to reveal the progenitor of FRBs. It is also a new type of standard siren, which can be used as an independent cosmological probe.