First direct comparison of non-disrupting neutron star-black hole and binary black hole merger simulations

TL;DR

This study compares neutron star-black hole and black hole-black hole mergers in full general relativity, finding they are remarkably similar in gravitational wave signals and post-merger characteristics when the neutron star is not tidally disrupted.

Contribution

It provides the first direct comparison showing that non-disrupting neutron star-black hole mergers are effectively indistinguishable from black hole-black hole mergers in gravitational wave data.

Findings

Gravitational wave signals are nearly identical for both merger types.

Post-merger black hole masses and spins agree within very small margins.

Neutron star presence is undetectable in gravitational waves for these configurations.

Abstract

We present the first direct comparison of numerical simulations of neutron star-black hole and black hole-black hole mergers in full general relativity. We focus on a configuration with non spinning objects and within the most likely range of mass ratio for neutron star-black hole systems (q=6). In this region of the parameter space, the neutron star is not tidally disrupted prior to merger, and we show that the two types of mergers appear remarkably similar. The effect of the presence of a neutron star on the gravitational wave signal is not only undetectable by the next generation of gravitational wave detectors, but also too small to be measured in the numerical simulations: even the plunge, merger and ringdown signals appear in perfect agreement for both types of binaries. The characteristics of the post-merger remnants are equally similar, with the masses of the final black holes agreeing within dM< 5 10^{-4}M_BH and their spins within da< 10^{-3}M_BH. The rate of periastron advance in the mixed binary agrees with previously published binary black hole results, and we use the inspiral waveforms to place constraints on the accuracy of our numerical simulations independent of algorithmic choices made for each type of binary. Overall, our results indicate that non-disrupting neutron star-black hole mergers are exceptionally well modeled by black hole-black hole mergers, and that given the absence of mass ejection, accretion disk formation, or differences in the gravitational wave signals, only electromagnetic precursors could prove the presence of a neutron star in low-spin systems of total mass ~10Msun, at least until the advent of gravitational wave detectors with a sensitivity comparable to that of the proposed Einstein Telescope.