Synchronizing the EMRIs and IMRIs in AGN accretion disks

TL;DR

This paper explores how EMRIs and IMRIs can form and evolve together in AGN disks, showing that their migration can be synchronized until strong gravitational wave emission causes them to separate, potentially allowing LISA to detect both.

Contribution

It introduces a model of synchronized migration of EMRIs and IMRIs in AGN disks, revealing their potential co-occurrence as gravitational-wave sources detectable by LISA.

Findings

sBHs can be trapped by IMBHs in AGN disks

migration of EMRIs and IMRIs can be synchronized

pair breaks up due to strong GW radiation near Schwarzschild radii

Abstract

Extreme-mass-ratio inspirals (EMRIs) and intermediate-mass-ratio inspirals (IMRIs) are important gravitational-wave (GW) sources for the Laser Interferometer Space Antenna (LISA). So far, their formation and evolution are considered to be independent, but recent theories suggest that stellar-mass black holes (sBHs) and intermediate-mass black hole (IMBHs) can coexist in the accretion disk of an active galactic nucleus (AGN), which indicates that EMRIs and IMRIs may form in the same place. Motivated by the fact that a gas giant migrating in a protoplanetary disk could trap planetesimals close to its orbit, we study in this paper a similar interaction between a gap-opening IMBH in an AGN disk and the sBHs surrounding it. We analyse the torques imposed on the sBHs by the disk as well as by the IMBH, and show that the sBHs can be trapped by the IMBH if they are inside the orbit of the IMBH. Then we implement the torques in our numerical simulations to study the migration of an outer IMBH and an inner sBH, both embedded in an AGN disk. We find that their migration is synchronized until they reach a distance of about ten Schwarzschild radii from the central supermassive black hole, where the pair breaks up due to strong GW radiation. This result indicates that LISA may detect an EMRI and an IMRI within several years from the same AGN. Such a GW source will bring rich information about the formation and evolution of sBHs and IMBHs in AGNs.