From Wide Triples to UCXBs: Multimessenger Signatures of Dynamically-formed Black Hole-White Dwarf Systems in the LISA Band

TL;DR

This paper explores how wide black hole-white dwarf binaries formed via the eccentric Kozai-Lidov mechanism can evolve into ultracompact X-ray binaries detectable through X-ray and gravitational wave observations, offering new insights into their formation.

Contribution

It demonstrates a natural formation pathway for UCXBs from wide BH-WD systems in triples, driven by EKL mechanism, gravitational waves, and WD tides, with predictions for detectable sources.

Findings

Predicts 3-27 UCXBs in the Milky Way, 1-5 detectable by LISA

Suggests up to 1000 UCXBs could form in the Galaxy

Identifies tertiary companions as evidence for formation pathway

Abstract

Ultracompact X-ray binaries (UCXBs) are a subclass of low-mass X-ray binaries characterized by tight orbits and degenerate donors, which pose significant challenges to our understanding of their formation. Recent discoveries of black hole (BH) candidates with main-sequence (MS) or red giant (RG) companions suggest that BH-white dwarf (BH-WD) binaries are common in the Galactic field. Motivated by these observations and the fact that most massive stars are born in triples, we show that wide BH-WD systems can naturally form UCXBs through the eccentric Kozai-Lidov (EKL) mechanism. Notably, EKL-driven eccentricity excitations combined with gravitational wave (GW) emission and WD dynamical tides can effectively shrink and circularize the orbit, leading to mass-transferring BH-WD binaries. These systems represent promising multimessenger sources in both X-ray and GW observations. Specifically, we predict that the wide triple channel can produce $\sim3-27$ ($\sim1-5$) detectable UCXBs in the Milky Way (Andromeda galaxy), including $\sim1$ system observable by the mHz GW detection of LISA. If the final WD mass can reach sufficiently small values, this channel could contribute up to $\sim 10^3$ UCXBs in the Galaxy. Furthermore, the identification of tertiary companions in observed UCXBs would provide direct evidence for this formation pathway and yield unique insights into their dynamical origins.