Stellar Dynamics of Extreme-Mass-Ratio Inspirals

TL;DR

This study uses direct N-body simulations to analyze the dynamics of stars around massive black holes, revealing how relativistic effects influence star capture rates and gravitational wave signals for space-based detectors.

Contribution

It provides an essentially exact post-Newtonian N-body simulation approach to study stellar dynamics near black holes, highlighting the impact of relativistic precession on capture rates.

Findings

Relativistic precession significantly reduces star capture rates.

Penetration of the Schwarzschild barrier occurs via two mechanisms.

Derived formula predicts suppression of captures below a certain orbital size.

Abstract

Inspiral of compact stellar remnants into massive black holes (MBHs) is accompanied by the emission of gravitational waves at frequencies that are potentially detectable by space-based interferometers. Event rates computed from statistical (Fokker-Planck, Monte-Carlo) approaches span a wide range due to uncertaintities about the rate coefficients. Here we present results from direct integration of the post-Newtonian N-body equations of motion descrbing dense clusters of compact stars around Schwarzschild MBHs. These simulations embody an essentially exact (at the post-Newtonian level) treatment of the interplay between stellar dynamical relaxation, relativistic precession, and gravitational-wave energy loss. The rate of capture of stars by the MBH is found to be greatly reduced by relativistic precession, which limits the ability of torques from the stellar potential to change orbital angular momenta. Penetration of this "Schwarzschild barrier" does occasionally occur, resulting in capture of stars onto orbits that gradually inspiral due to gravitational wave emission; we discuss two mechanisms for barrier penetration and find evidence for both in the simulations. We derive an approximate formula for the capture rate, which predicts that captures would be strongly disfavored from orbits with semi-major axes below a certain value; this prediction, as well as the predicted rate, are verified in the N-body integrations. We discuss the implications of our results for the detection of extreme-mass-ratio inspirals from galactic nuclei with a range of physical properties.