Accuracy and precision of gravitational-wave models of inspiraling neutron star-black hole binaries with spin: Comparison with matter-free numerical relativity in the low-frequency regime

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Abstract

Coalescing binaries of neutron stars and black holes are one of the most important sources of gravitational waves for the upcoming network of ground-based detectors. Detection and extraction of astrophysical information from gravitational-wave signals requires accurate waveform models. The effective-one-body and other phenomenological models interpolate between analytic results and numerical relativity simulations, that typically span $\mathcal{O}(10)$ orbits before coalescence. In this paper we study the faithfulness of these models for neutron star-black hole binaries. We investigate their accuracy using new numerical relativity (NR) simulations that span 36--88 orbits, with mass ratios $q$ and black hole spins ${\ensuremath{\chi}}_{\mathrm{BH}}$ of $(q,{\ensuremath{\chi}}_{\mathrm{BH}})=(7,\ifmmode\pm\else\textpm\fi{}0.4),(7,\ifmmode\pm\else\textpm\fi{}0.6)$, and $(5,\ensuremath{-}0.9)$. These simulations were performed treating the neutron star as a low-mass black hole, ignoring its matter effects. We find that (i) the recently published SEOBNRv1 and SEOBNRv2 models of the effective-one-body family disagree with each other (mismatches of a few percent) for black hole spins ${\ensuremath{\chi}}_{\mathrm{BH}}\ensuremath{\ge}0.5$ or ${\ensuremath{\chi}}_{\mathrm{BH}}\ensuremath{\le}\ensuremath{-}0.3$, with waveform mismatch accumulating during early inspiral; (ii) comparison with numerical waveforms indicates that this disagreement is due to phasing errors of SEOBNRv1, with SEOBNRv2 in good agreement with all of our simulations; (iii) phenomenological waveforms agree with SEOBNRv2 only for comparable-mass low-spin binaries, with overlaps below 0.7 elsewhere in the neutron star-black hole binary parameter space; (iv) comparison with numerical waveforms shows that most of this model's dephasing accumulates near the frequency interval where it switches to a phenomenological phasing prescription; and finally (v) both SEOBNR and post-Newtonian models are effectual for neutron star-black hole systems, but post-Newtonian waveforms will give a significant bias in parameter recovery. Our results suggest that future gravitational-wave detection searches and parameter estimation efforts would benefit from using SEOBNRv2 waveform templates when focused on neutron star-black hole systems with $q\ensuremath{\lesssim}7$ and ${\ensuremath{\chi}}_{\mathrm{BH}}\ensuremath{\approx}[\ensuremath{-}0.9,+0.6]$. For larger black hole spins and/or binary mass ratios, we recommend the models be further investigated as NR simulations in that region of the parameter space become available.

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