From Wide Triples to Ultracompact X-ray Binaries: Multimessenger Signatures of Dynamically Formed Black Hole–White Dwarf Systems in the LISA Band

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 or red giant 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 ∼3–27 (∼1–5) detectable UCXBs in the Milky Way (Andromeda galaxy), including ∼1 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 ∼10³ 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.