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.

The Merger Rate of Primordial Black Hole–Neutron Star Binaries in Ellipsoidal-collapse Dark Matter Halo Models

“…In Sasaki et al (2022), within the context of spherical-collapse dark matter halo models, the possibility that the BH components of BH-NS events may have a primordial origin is explored. Under such assumptions, the merger rate of PBH-NS binaries has been calculated and its results compared with the mergers estimated by the LIGO-Virgo detectors.…”

Compact Binary Merger Rate in Dark-Matter Spikes

“…where r * NS and ρ * NS are characteristic radius and density of NSs, respectively, which need to be determined. For the characteristic radius of NSs, an approximative value has been proposed as r * NS 0.1 r s (Sasaki et al 2022), which we use in our calculations.…”

“…Due to these circumstances, it seems imperative to fully understand how compact objects involved in these events are formed. In Sasaki et al (2022), taking into account sphericalcollapse dark matter halo models and the framework of the PBH scenario, the merger rate of BH-NS binaries has been calculated. Also, by comparing the results with the GW observations, it has been claimed that the BH components participating in such events can not have primordial origins.…”

Compact Binary Merger Rate in Dark-matter Spikes

“…Due to these circumstances, it seems imperative to fully understand how compact objects involved in these events are formed. In Sasaki et al (2022), taking into account spherical-collapse dark-matter halo models and the framework of the PBH scenario, the merger rate of BH-NS binaries has been calculated. Also, by comparing the results with the GW observations, it has been claimed that the BH components participating in such events cannot have primordial origins.…”