Establishing the Non-Primordial Origin of Black Hole-Neutron Star Mergers

TL;DR

This paper demonstrates that the observed black hole-neutron star mergers are predominantly of astrophysical origin, with primordial black holes unlikely to contribute significantly to these events, based on merger rate analysis.

Contribution

It provides the first detailed analysis showing that primordial black hole-neutron star merger rates are too low to account for observed events, distinguishing their origin from astrophysical sources.

Findings

PBH-NS merger rates are subdominant to astrophysical rates.

PBH-NS rates cannot be significantly enhanced before star formation.

Observed BH-NS events are of astrophysical origin.

Abstract

Primordial black holes (PBHs) from the early Universe constitute an attractive dark matter candidate. 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 non-astrophysical 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 GW observations.