PHYSICAL REVIEW D 102, 043023 (2020) Aligned-spin neutron-star–black-hole waveform model based on the effective-one-body approach and numerical-relativity simulations Andrew Matas ,1 Tim Dietrich,2,3 Alessandra Buonanno,1,4 Tanja Hinderer,5,6 Michael Pürrer ,1 Francois Foucart,7 Michael Boyle,8 Matthew D. Duez,9 Lawrence E. Kidder,8 Harald P. Pfeiffer,1 and Mark A. Scheel10 1Max Planck Institute for Gravitational Physics (Albert Einstein Institute), D-14476 Potsdam, Germany 2Nikhef, Science Park 105, 1098 XG Amsterdam, Netherlands 3Institute for Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24/25, 14776 Potsdam, Germany 4Department of Physics, University of Maryland, College Park, Maryland 20742, USA 5GRAPPA, Anton Pannekoek Institute for Astronomy and Institute of High-Energy Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands 6Delta Institute for Theoretical Physics, Science Park 904, 1090 GL Amsterdam, Netherlands 7Department of Physics and Astronomy, University of New Hampshire, 9 Library Way, Durham, New Hampshire 03824, USA 8Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, New York 14853, USA 9Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, USA 10TAPIR, Walter Burke Institute for Theoretical Physics, MC 350-17, California Institute of Technology, Pasadena, California 91125, USA (Received 26 April 2020; accepted 6 August 2020; published 27 August 2020) After the discovery of gravitational waves from binary black holes (BBHs) and binary neutron stars (BNSs) with the LIGO and Virgo detectors, neutron-star black holes (NSBHs) are the natural next class of binary systems to be observed. In this work, we develop a waveform model for aligned-spin NSBHs combining a BBH baseline waveform (available in the effective-one-body approach) with a phenomeno- logical description of tidal effects (extracted from numerical-relativity simulations) and correcting the amplitude during the late inspiral, merger and ringdown to account for the NS tidal disruption. In particular, we calibrate the amplitude corrections using NSBH waveforms obtained with the numerical-relativity spectral Einstein code (SpEC) and the SACRA code. The model was calibrated using simulations with NS masses in the range 1.2–1.4 M⊙, tidal deformabilities up to 4200 (for a 1.2 M⊙ NS), and dimensionless BH spin magnitude up to 0.9. Based on the simulations used and on checking that sensible waveforms are produced, we recommend our model to be employed with a NS mass in the range 1–3 M⊙, tidal deformability 0–5000, and (dimensionless) BH spin magnitude up to 0.9. We also validate our model against two new, highly accurate NSBH waveforms with BH spin 0.9 and mass ratios 3 and 4, characterized by tidal disruption, produced with SpEC, and find very good agreement. Furthermore, we compute the unfaithfulness between waveforms from NSBH, BBH, and BNS systems, finding that it will be challenging for the Advanced LIGO-Virgo detector network at design sensitivity to distinguish different source classes. We perform a Bayesian parameter-estimation analysis on a synthetic numerical-relativity signal in zero noise to study parameter biases. Finally, we reanalyze GW170817, with the hypothesis that it is a NSBH. We do not find evidence to distinguish the BNS and NSBH hypotheses; however, the posterior for the mass ratio is shifted to less equal masses under the NSBH hypothesis. DOI: 10.1103/PhysRevD.102.043023 I. INTRODUCTION In their first two observing runs (O1 and O2), Advanced LIGO [1] and Advanced Virgo [2] have observed gravi- Published by the American Physical Society under the terms of tational waves (GWs) from ten binary black holes (BBHs) the Creative Commons Attribution 4.0 International license. and one binary neutron star (BNS), GW170817 [3]. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, Recently, in the third observing run (O3), a second and DOI. Open access publication funded by the Max Planck BNS, GW190425, was discovered [4]. Other groups have Society. reported additional GW observations analyzing the public 2470-0010=2020=102(4)=043023(19) 043023-1 Published by the American Physical Society ANDREW MATAS et al. PHYS. REV. D 102, 043023 (2020) data from the first two runs [5–7]. Neutron-star black holes systems. Tidal corrections [23–25,47,48] have been incor- (NSBHs) may be the next source class to be discovered. porated in the effective-one-body (EOB) formalism [49–51] Given the lack of a detection in O1 and O2, the rate of in Refs. [52–57]. References [58,59] developed a flexible NSBHs is uncertain. However, based on estimates from technique that starts from a point-mass BBH baseline þ19 Ref. [8], the expected number of NSBH detections is 0−0 waveform and applies tidal-phase modifications by fitting þ91 in O3 and 1−1 in O4 [9], where the central value is the a Pad´e-resummed post-Newtonian (PN)-based ansatz median and the error bars give the 90% credible interval. As to the phasing extracted from numerical-relativity (NR) of this writing, in O3, the LIGO and Virgo Collaborations simulations (henceforth, we refer to this as the NRTidal have published seven circulars via the Gamma-ray approach). These corrections have been applied to BBH Coordinates Network (GCN) describing detection candi- baselines produced within the EOBNR framework [60] and dates for which the probability of the system being a NSBH within the inspiral-merger-ringdown phenomenological is larger than 1% and for which the candidate has not been (IMRPhenom) approach [61,62]. retracted [10–16]. Furthermore, GW data alone do not There have been several previous works constructing exclude the possibility that GW170817 is a NSBH [17–19], NSBH waveforms. An aligned-spin NSBH waveform and it has also been suggested that GW190425 could be a model was developed in Refs. [63,64], but it covered a NSBH [20,21]. Therefore it is timely to develop methods limited range of mass ratios. In Ref. [65], this waveform that can be used to study NSBHs in GW data. model was used in parameter and population studies in NSBH binaries exhibit a rich phenomenology that is conjunction with a former version of the EOBNR BBH imprinted on the gravitational waveform (for a review see baseline [66]. A NSBH model called PhenomNSBH, which Ref. [22]). First, as is the case for BNS systems, finite-size was constructed using a similar approach to modeling effects cause a dephasing of the waveform relative to a BBH NSBHs as the one discussed in this paper but developed with the same masses and spins [23–26]. Additionally, the within the IMRPhenom approach, was recently put forward amplitude of NSBH waveforms can be affected by tidal in Ref. [67]. This model uses the method of Ref. [68] to forces [27]. For unequal mass ratios and slowly spinning describe tidal disruption of the amplitude and uses the tidal- BHs, the amplitude of the waveform is well described by a phase corrections from Ref. [59]. BBH [28]. On the other hand, for near-equal mass ratios or In this work we develop a frequency-domain model for for highly spinning BHs, depending on the NS equation of the dominant, quadrupolar multipole of GWs emitted by state (EOS), the NS can undergo tidal disruption, in which aligned-spin NSBH systems. Together with the recent the star is ripped apart as it approaches the BH [27,29–32].If waveform model of Ref. [67], these are the first NSBH the disruption takes place before the NS crosses the inner- models covering a wide range of mass ratios and spin that most stable circular orbit, then the material ejected from the can be used to analyze GW data. In this paper, we refer to NS can form a disk around the BH [33–35]. If so, starting at a our model as SEOBNR_NSBH, which has already been characteristic (cutoff) frequency [27,36,37], the amplitude implemented in the LIGO algorithms library (LAL) [69].In of the waveform is strongly suppressed, and the ringdown Table I we provide a dictionary between the names we use stage is reduced or even effaced. The details of this process in this work and the name as implemented in LAL. The contain information about the NS EOS. Additionally, NSBH amplitude is based on an EOBNR BBH baseline model that mergers can be the progenitors of short gamma-ray bursts we refer to as SEOBNR_BBH [60]. We apply corrections [38–43], and the disk around the remnant BH and dynamical inspired by Pannarale et al. [68] to account for tidal ejecta can provide the engine for the kilonova signal [44,45], disruption. We have adapted the corrections of Ref. [68], like the ones observed for GW170817 [3,46]. originally developed for a former version of the In order to take advantage of this potentially rich source IMRPhenom BBH model [70], for use with EOBNR of information, it is crucial to have a fast and accurate waveforms [60], augmented with reduced-order modeling waveform model capturing effects due to relativistic matter, (ROM) [71,72] to enhance the speed. Differently from which can be used in analyzing GW data. Several Ref. [67], which uses the fit from Pannarale et al. [68], here approaches exist for describing finite-size effects in BNS we have performed a fit incorporating results from the new TABLE I. Dictionary relating the names we use in this paper for several waveforms from the SEOBNR family and the corresponding names of the waveforms implemented in the LIGO algorithms library (LAL). The second and third waveforms use tidal effects within the NRTidal approach. Name in this paper LAL name Ref.