Black Hole - Neutron Star Binary Mergers: The Impact of Stellar Compactness

TL;DR

This study uses numerical simulations to analyze how the compactness of neutron stars influences gravitational wave signals and merger outcomes in black hole-neutron star binaries, revealing that higher compactness makes the system resemble black hole binaries.

Contribution

It extends previous work by examining the effects of neutron star compactness on gravitational wave signatures and merger dynamics in black hole-neutron star systems.

Findings

Higher compactness increases similarity to black hole binaries in gravitational wave signals.

Disruption of the neutron star reduces kicks and slightly alters the final black hole spin.

Low compactness leads to complex post-merger gravitational wave signals without clear quasi-normal ringing.

Abstract

Recent gravitational wave observations include possible detections of black hole - neutron star binary mergers. As with binary black hole mergers, numerical simulations help characterize the sources. For binary systems with neutron star components, the simulations help to predict the imprint of tidal deformations and disruptions on the gravitational wave signals. In a previous study, we investigated how the mass of the black hole has an impact on the disruption of the neutron star and, as a consequence, on the shape of the gravitational waves emitted. We extend these results to study the effects of varying the compactness of the neutron star. We consider neutron star compactness in the 0.123 to 0.2 range for binaries with mass ratios of 3 and 5. As the compactness and the mass ratio increase, the binary system behaves during the late inspiral and merger more like a black hole binary. For the case with the highest mass ratio and most compact neutron star, the gravitational waves emitted, in terms of mismatches, are almost indistinguishable from those by a binary black hole. The disruption of the star significantly suppresses the kicks on the final black hole. The disruption also affects, although not dramatically, the spin of the final black hole. Lastly, for neutron stars with low compactness, the quasi-normal ringing of the black hole after the merger does not show a clean quasi-normal ringing because of the late accretion of debris from the neutron star.