Establishing the Nonprimordial Origin of Black Hole–Neutron Star Mergers

Abstract

Primordial black holes (PBHs) from the early universe constitute attractive dark matter candidates. First detections of black hole–neutron star (BH–NS) candidate gravitational wave events by the LIGO/Virgo collaboration, GW200105 and GW200115, already prompted speculations about nonastrophysical origin. We analyze, for the first time, the total volumetric merger rates of PBH–NS binaries formed via two-body gravitational scattering, finding them to be subdominant to the astrophysical BH–NS rates. In contrast to binary black holes, a significant fraction of which can be of primordial origin, either formed in dark matter halos or in the early universe, PBH–NS rates cannot be significantly enhanced by contributions preceding star formation. Our findings imply that the identified BH–NS events are of astrophysical origin, even when PBH–PBH events significantly contribute to the gravitational wave observations.