2025MNRAS.536L..19C

We investigate the effects of prior selection on the inferred mass and spin parameters of the neutron star-black hole merger GW230529_181500. Specifically, we explore models motivated by astrophysical considerations, including massive binary and pulsar evolution. We examine mass and spin distributions of neutron stars constrained by radio pulsar observations, alongside black hole spin observations from previous gravitational-wave detections. We show that the inferred mass distribution highly depends upon the spin prior. Specifically, under the most restrictive, binary stellar evolution models, we obtain narrower distributions of masses with a black hole mass of $4.3^{+0.1}_{-0.1}\ {\rm M}_{\odot }$ and neutron star mass of $1.3^{+0.03}_{-0.03}\ {\rm M}_{\odot }$ where, somewhat surprisingly, it is the prior on component spins that has the greatest impact on the inferred mass distributions. Re-weighting using neutron star mass and spin priors from observations of radio pulsars, with black hole spins from observations of gravitational waves, yields the black hole and the neutron star masses to be $3.8^{+0.5}_{-0.6}$ and $1.4^{+0.2}_{-0.1} \ \mathrm{ M}_\odot$, respectively. The sequence of compact object formation - whether the neutron star or the black hole formed first - cannot be determined at the observed signal-to-noise ratio. However, there is no evidence that the black hole was tidally spun up.