The Enhanced Population of Extreme Mass-Ratio Inspirals in the LISA Band from Supermassive Black Hole Binaries

TL;DR

This paper predicts a significantly higher rate of extreme mass-ratio inspirals (EMRIs) from supermassive black hole binaries in the LISA band, which could be used to study SMBH binary fractions.

Contribution

It demonstrates that SMBH binaries can produce thousands of EMRIs at redshift one, with hundreds detectable by LISA, and explores their impact on gravitational wave background.

Findings

LISA may detect a few hundred EMRIs with SNR>=8 during its mission.

Sub-threshold EMRIs contribute to a confusion noise an order of magnitude above LISA's sensitivity.

Detection of EMRIs can constrain SMBH binary fractions in the low-redshift universe.

Abstract

Extreme mass ratio inspirals (EMRIs) take place when a stellar-mass black hole (BH) merges with a supermassive black hole (SMBH). The gravitational wave emission from such an event is expected to be detectable by the future Laser Interferometer Space Antenna (LISA) and other mHz detectors. It was recently suggested that the EMRI rate in SMBH binary systems is orders of magnitude higher than the EMRI rate around a single SMBH with the same total mass. Here we show that this high rate can produce thousands of SMBH-BH sources at a redshift of unity. We predict that LISA may detect a few hundred of these EMRIs with signal-to-noise ratio above SNR>=8 within a four-year mission lifetime. The remaining sub-threshold sources will contribute to a large confusion noise, which is approximately an order of magnitude above LISA's sensitivity level. Finally, we suggest that the individually detectable systems, as well as the background noise from the sub-threshold EMRIs, can be used to constrain the SMBH binary fraction in the low-redshift Universe.