An Enhanced Formation Channel for Galactic Dual-Line Gravitational-Wave Sources: von Zeipel-Lidov-Kozai Effect in Triples Involving Sgr A*

TL;DR

This paper proposes that the von Zeipel-Lidov-Kozai effect in hierarchical triples involving Sgr A* can significantly increase the formation rate of dual-line gravitational wave sources in the Galactic Center, enhancing their detectability.

Contribution

It introduces a new formation channel via the ZLK effect that boosts dual-line GW source counts by a factor of 5-10, making such sources more observable.

Findings

ZLK oscillations modulate GW emission from binaries and NS spins.

Dual-line source count increases by a factor of 5-10 due to ZLK effect.

Expected to observe about one dual-line source in 4 years.

Abstract

The dense Galactic Center environment is expected to host compact binary inspirals detectable by future space-borne gravitational wave (GW) observatories (e.g., LISA, TianQin, Taiji) in the millihertz band. Aided by information from these facilities, next-generation ground-based GW detectors (e.g., Cosmic Explorer, Einstein Telescope) can potentially capture gravitational radiation in the hectohertz band from rapidly spinning neutron star (NS) components in such binaries. These Galactic Center systems are thus anticipated to act as dual-line (i.e., low-frequency inspiral and high-frequency spin) GW sources. However, the formation channels of these systems remain largely unexplored. In this \textit{Letter}, we propose that the von Zeipel-Lidov-Kozai (ZLK) effect can enhance the formation of dual-line GW sources in hierarchical triples involving the Galactic supermassive black hole, Sgr A*. We show that ZLK-driven oscillations in the eccentricity and inclination of the inner binary can modulate the GW emission from both the binary inspiral and the individual NS spins. This effect boosts the expected dual-line source count by a factor of $\sim 5-10$, from rare to $\mathcal{O}(1)$ in 4 years, making dual-line observations substantially more probable. Our results demonstrate that the ZLK effect may provide an important formation channel for Galactic dual-line GW sources.