Astrophysical Priors on the Properties of Observed Black Hole–Neutron Star Mergers

Abstract In this work, we analyze black hole-neutron star (BHNS) events using an astrophysically-motivated prior to derive updated, more tightly constrained estimates for component masses, mass ratios, and effective inspiral spin. For most BHNS systems, the mean primary mass shifts to higher values, with significant cases such as GW190426 and GW200115, where the primary mass is entirely excluded from the lower-mass gap. While the primary mass of GW230529 remains within the gap, its mean value shifts slightly upward. In contrast, the secondary mass generally shifts toward the higher end of the mass spectrum. These changes highlight the substantial influence of prior choices-particularly the spin prior-on the inferred mass distributions. Additionally, for all BHNS events, the effective inspiral spin is tightly concentrated around zero, reinforcing the expectation of nearly nonspinning components in these systems. We also discuss the classical common-envelope formation scenario, which is widely considered the dominant channel for BHNS formation, where the BH formed from the more massive progenitor star is typically expected to have negligible spin.