Binaries Wandering Around Supermassive Black Holes Due To Gravito-electromagnetism

TL;DR

This paper introduces a new method to simulate the dynamics of binary black holes near supermassive black holes, revealing novel effects like radial oscillation and azimuthal drift, which are important for gravitational wave detection.

Contribution

The study develops a perturbative gravito-electromagnetic approach in a free-fall frame to simulate BBH-SMBH systems, capturing effects missed by previous methods.

Findings

Detected radial oscillations of BBHs near SMBHs.

Observed azimuthal drift of BBHs in the gravitational field.

Provided new insights into EMRI evolution and detection.

Abstract

Extreme-mass-ratio inspirals (EMRIs) are important sources for space-borne gravitational-wave (GW) detectors. Such a source normally consists of a stellar-mass black hole (BH) and a Kerr supermassive BH (SMBH), but recent astrophysical models predict that the small body could also be a stellar-mass binary BH (BBH). A BBH reaching several gravitational radii of a SMBH will induce rich observable signatures in the waveform, but the current numerical tools are insufficient to simulate such a triple system while capturing the essential relativistic effects. Here we solve the problem by studying the dynamics in a frame freely falling alongside the BBH. Since the BBH is normally non-relativistic and much smaller than the curvature radius of the Kerr background, the evolution in the free-fall frame reduces to essentially Newtonian dynamics, except for a perturbative gravito-electromagnetic (GEM) force induced by the curved background. We use this method to study the BBHs on near-circular orbits around a SMBH and track their evolution down to a distance of $2-3$ gravitational radii from the SMBH. Our simulations reveal a series of dynamical effects which are not shown in the previous studies using conventional methods. The most notable one is a radial oscillation and azimuthal drift of the BBH relative to the SMBH. These results provide new insight into the evolution and detection of the EMRIs containing BBHs.