Science Opportunities of Wet Extreme Mass-Ratio Inspirals

TL;DR

Wet EMRIs, involving stellar-mass black holes inspiraling into SMBHs within AGN disks, offer unique gravitational and electromagnetic signals, enabling precise SMBH measurements, accretion physics insights, and cosmological applications with space-based GW detectors.

Contribution

This paper highlights the distinct properties and scientific opportunities of wet EMRIs, emphasizing their multi-messenger signals and potential for advancing black hole and cosmology research.

Findings

Wet EMRIs produce transient electromagnetic signals like QPEs.

GW signals from wet EMRIs enable SMBH parameter measurements with high precision.

Wet EMRIs can serve as standard sirens for cosmological measurements.

Abstract

Wet extreme mass-ratio inspirals (wet EMRIs), which arise from stellar-mass black holes inspiral into supermassive black holes (SMBHs) within the gas-rich environments of Active Galactic Nuclei (AGN), are primary sources of gravitational waves (GWs) for space-borne detectors like LISA, TianQin, and Taiji. Unlike "dry EMRIs", which form through gravitational scattering in nuclear star clusters, wet EMRIs are naturally accompanied by interactions with accretion disks, offering rich multi-messenger science opportunities. They are distinct in generating transient electromagnetic (EM) signals, such as quasi-periodic eruptions (QPEs), which serve as valuable probes of accretion disk physics and SMBH environments. Their GW signals provide an unprecedented precision of the order of $O(10^{-4}\sim 10^{-6})$ in measuring SMBH mass and spin, enabling the calibration of traditional EM techniques and offering insights into jet formation models. Additionally, wet EMRIs serve as bright and dark sirens for cosmology, facilitating percent-level precision measurements of Hubble parameter through AGN host identification or statistical association. These systems hold immense potential for advancing our understanding of black hole dynamics, accretion physics, and cosmology.