Constraints on compact binary merger evolution from spin-orbit misalignment in gravitational-wave observations

The identification of the first confirmed neutron star-black hole (NS-BH) binary mergers by the LIGO, Virgo, and KAGRA collaboration provides the opportunity to investigate the properties of the early sample of confirmed and candidate events. Here, we focus primarily on the tilt angle of the BH's spin relative to the orbital angular momentum vector of the binary, and the implications for the physical processes that determine this tilt. The posterior tilt distributions of GW200115 and the candidate events GW190426_152155 and GW190917_114630 peak at significantly anti-aligned orientations (though display wide distributions). Producing these tilts through isolated binary evolution would require stronger natal kicks than are typically considered (and preferentially polar kicks would be ruled out), and/or an additional source of tilt such as stable mass transfer. The early sample of NS-BH events are less massive than expected for classical formation channels, and may provide evidence for efficient mass transfer that results in the merger of more massive NS-BH binaries before their evolution to the compact phase is complete. We predict that future gravitational-wave detections of NS-BH events will continue to display total binary masses of approximate to 7 M-circle dot and mass ratios of q similar to 3 if this interpretation is correct. Conversely, the high mass of the candidate GW191219_163120 suggests a dynamical capture origin. Large tilts in a significant fraction of merging NS-BH systems would weaken the prospects for electromagnetic detection. However, EM observations, including non-detections, can significantly tighten the constraints on spin and mass ratio.

Automated summary

The confirmation of the first neutron star-black hole (NS-BH) binary mergers by the LIGO, Virgo, and KAGRA collaboration allows for further investigation into these events. The tilt angle of the black hole's spin relative to the binary's orbital angular momentum vector is discussed, along with the implications for the physical processes that determine this tilt. The early sample of NS-BH events suggests efficient mass transfer and the possibility of more massive NS-BH binaries merging before completing their evolution. Future gravitational-wave detections are predicted to continue displaying similar total binary masses and mass ratios. However, a high mass candidate event may imply a dynamical capture origin, and large tilts in merging NS-BH systems would hinder electromagnetic detection prospects.