Orbital Eccentricity and Spin–Orbit Misalignment Are Evidence that Neutron Star–Black Hole Mergers Form through Triple Star Evolution

Abstract There is growing evidence that a substantial fraction of the neutron star–black holes (NSBHs) detected through gravitational waves merge with nonzero eccentricity or large black hole spin–orbit misalignment. This is in tension with leading formation scenarios to date. Residual eccentricity rules out formation from isolated binary stars, while neutron star (NS) natal kicks and unequal masses of NSBHs inhibit efficient pairing in dense stellar environments. We report that all observed properties—NSBH merger rate, eccentricity, and spin–orbit misalignment—are explained by the high prevalence of massive stellar triples in the field. Modeling their evolution from the zero-age main sequence, we investigate NSBH mergers caused by gravitational perturbations from a tertiary companion. The NS formation decisively impacts the triple stability, preferentially leaving behind surviving NSBHs in compact triple architectures with mild hierarchies. The rich three-body dynamics of compact, unequal-mass triples enables mergers across a wide range of orbital parameters and provides a natural explanation for an abundance of residual eccentricity and spin–orbit misalignment. We infer a total NSBH merger rate of R NSBH ∼ 1 –23 Gpc −3 yr −1 (within uncertainties on NS kicks) with a few 10% exhibiting residual eccentricity e 20 > 0.1 or large spin–orbit misalignment cos θ BH 0 , consistent with current observations. The mergers closely track the cosmic star formation rate due to short delay times (∼10–100 Myr), include a substantial fraction of burst-like highly eccentric systems ( e 20 > 0.9), and almost universally retain eccentricities e 20 > 10 −3 detectable by next-generation detectors. If evidence for eccentric and misaligned events solidifies, our results suggest that triple dynamics is the dominant formation channel of gravitational-wave events from NSBH mergers.