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 non-zero eccentricity or large BH spin-orbit misalignment. This is in tension with the leading formation scenarios to date. Residual eccentricity rules out formation through isolated binary star evolution, while NS natal kicks and the unequal masses of NSBHs inhibit efficient pairing in dense stellar environments. Here, 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. Modelling their evolution from the ZAMS, we investigate NSBH mergers caused by gravitational perturbations from a tertiary companion. We show that the formation of the NS decisively impacts the triple stability, preferentially leaving behind surviving NSBHs in compact triple architectures. The rich three-body dynamics of compact, unequal-mass triples enables mergers across a wide range of orbital parameters without requiring fine-tuned highly inclined tertiary orbits and provides a natural explanation for an abundance of residual eccentricity and spin-orbit misalignment. We infer a total NSBH merger rate of $R\sim1-23\,\rm Gpc^{-3}\,yr^{-1}$, with more than a few 10% exhibiting eccentricity $e_{20}>0.1$ or large spin-orbit misalignment $\cosθ_{\rm BH}<0$, consistent with current observations. Tertiary-driven NSBH mergers closely track the cosmic star formation rate due to their short delay times, include a substantial fraction of burst-like highly eccentric systems ($e_{20} > 0.9$), and almost universally retain eccentricities $e_{20}>10^{-3}$, potentially 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 NSBH mergers.