Merging Compact Binaries in Hierarchical Triple Systems: Resonant Excitation of Binary Eccentricity

TL;DR

This paper investigates how hierarchical triple systems can cause eccentricity excitation in compact binaries through apsidal precession resonance, affecting gravitational wave signals detectable by LISA.

Contribution

It introduces a new mechanism for eccentricity excitation in compact binaries within triple systems via apsidal precession resonance, with implications for gravitational wave observations.

Findings

Eccentricity can be resonantly excited when the precession rates match.

Outer companion's eccentricity and inclination influence excitation.

Resonance can occur in the LISA frequency band for certain parameters.

Abstract

We study the secular dynamics of compact binaries (consisting of white dwarfs, neutron stars or black holes) with tertiary companions in hierarchical triple systems. As the inner binary (with initially negligible eccentricity) undergoes orbital decay due to gravitational radiation, its eccentricity can be excited by gravitational forcing from the tertiary. This excitation occurs when the triple system passes through an "apsidal precession resonance", when the precession rate of the inner binary, driven by the gravitational perturbation of the external companion and general relativity, matches the precession rate of the outer binary. The eccentricity excitation requires the outer companion to be on an eccentric orbit, with the mutual inclination between the inner and outer orbits less than $\sim 40^\circ$. Gravitational wave (GW) signals from the inner binary can be significantly modified as the system evolves through the apsidal precession resonance. For some system parameters (e.g., a white dwarf binary with a brown dwarf tertiary), the resonance can happen when the binary emits GWs in the $10^{-4}-10^{-1}$~Hz range (the sensitivity band of LISA).