Constraining the neutron star–black hole merger rate

Current template-based gravitational-wave searches for compact binary mergers neglect the general relativistic phenomenon of spin-induced orbital precession. Owing to their asymmetric masses, gravitational waves from neutron star–black hole (NSBH) binaries are prime candidates for displaying strong imprints of spin precession. Current searches may therefore miss a significant fraction of the astrophysical population, and the detected NSBH population may be significantly suppressed or biased. Here we report the most sensitive search for NSBH binaries to date by including spin precession for the first time. We analyze data from the entirety of the third LIGO-Virgo-KAGRA gravitational-wave observing run and show that when accounting for spin precession, our search is up to 100% more sensitive than the search techniques currently adopted by the LIGO-Virgo-KAGRA collaboration (for systems with strong precessional effects). This allows us to more tightly constrain the rate of NSBH mergers in the local Universe. When focusing on a potentially precessing subpopulation of NSBH mergers, the lack of observed candidates allows us to place an upper limit on the merger rate of 𝑅 = 79  Gpc−3 yr−1 with 90% confidence. We then show that if there is no preferred direction of component spin, the overall rate of NSBH mergers is on average 16% smaller than previously believed. Finally, we report four new subthreshold NSBH candidates, all with strong imprints of spin precession, but note that these are most likely to be of terrestrial origin.