Captured are circularized: A relativistic treatment of extreme mass ratio inspirals crossing accretion disks

TL;DR

This paper presents a relativistic analysis of how stellar-mass objects in extreme mass-ratio inspirals are influenced by accretion disks around supermassive black holes, revealing disk alignment effects, eccentricity behavior, and capture rates.

Contribution

It provides a fully relativistic study of EMRIs crossing accretion disks, highlighting the disk's role in orbital alignment, eccentricity evolution, and the limited capture efficiency.

Findings

Disks tend to align the orbit of the small body regardless of initial inclination.

Captured orbits tend to have low eccentricity, with temporary growth at high inclinations.

Only a small fraction of stellar-mass black holes are captured within typical disk lifetimes.

Abstract

A small body orbiting around an accreting massive object and periodically crossing its accretion disk is a common configuration in astrophysics. In this work, we investigate the secular evolution of extreme mass-ratio inspirals (EMRIs), in which a stellar-mass object (SMO), e.g., a star or a stellar-mass black hole (sBH), collides with the accretion disk of a central supermassive black hole (SMBH), within a fully relativistic framework. We find (1) the disk always tends to align the SMO no matter what the initial orbital inclination $\iota$ relative to the disk is, (2) the final orbital eccentricity of the SMO captured by the disk is always low though the orbital eccentricity may temporarily grow when the orbital inclination $\iota$ is large and the SMO is an sBH, and (3) via collisions with the accretion disk only, only a small fraction of sBHs that are initially close to the SMBH and close to the disk can be captured by the disk within typical disk lifetime of active galactic nuclei. Two-body scatterings between SMOs in the nuclear stellar cluster play an essential role in randomly kicking sBHs towards the disk and boosting the capture rate.