Uncovering a hidden black hole binary from secular eccentricity variations of a tertiary star

TL;DR

This paper identifies a new dynamical effect where a tertiary star's eccentricity oscillates due to an apsidal precession resonance with a black hole binary, offering a potential observational signature for hidden BHBs.

Contribution

It introduces the concept of eccentricity growth via apsidal precession resonance in star-BHB systems, applicable even in non-coplanar configurations, and discusses its detectability.

Findings

Eccentricity oscillations can be triggered by apsidal precession resonance.

The effect is detectable by Gaia and relevant for LISA gravitational wave sources.

Provides a new method to identify hidden stellar-mass black hole binaries.

Abstract

We study the dynamics of a solar-type star orbiting around a black hole binary (BHB) in a nearly coplanar system. We present a novel effect that can prompt a growth and significant oscillations of the eccentricity of the stellar orbit when the system encounters an "apsidal precession resonance", where the apsidal precession rate of the outer stellar orbit matches that of the inner BHB. The eccentricity excitation requires the inner binary to have a non-zero eccentricity and unequal masses, and can be created even in non-coplanar triples. We show that the secular variability of the stellar orbit's apocenter, induced by the changing eccentricity, could be potentially detectable by \textit{Gaia}. Detection is favorable for BHBs emitting gravitational waves in the frequency band of the Laser Interferometer Space Antenna (LISA), hence providing a distinctive, multi-messenger probe on the existence of stellar-mass BHBs in the Milky Way.