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Register a new userInitial data for black hole-neutron star binaries, with rotating stars
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The coalescence of a neutron star with a black hole is a primary science target of ground-based gravitational wave detectors. Constraining or measuring the neutron star spin directly from gravitational wave observations requires knowledge of the dependence of the emission properties of these systems on the neutron star spin. This paper lays foundations for this task, by developing a numerical method to construct initial data for black hole--neutron star binaries with arbitrary spin on the neutron star. We demonstrate the robustness of the code by constructing initial-data sets in large regions of the parameter space. In addition to varying the neutron star spin-magnitude and spin-direction, we also explore neutron star compactness, mass-ratio, black hole spin, and black hole spin-direction. Specifically, we are able to construct initial data sets with neutron stars spinning near centrifugal break-up, and with black hole spins as large as $S_{rm BH}/M_{rm BH}^2=0.99$.
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Gravitational waves from binary neutron star (BNS) and black hole/neutron star (BHNS) inspirals are primary sources for detection by the Advanced Laser Interferometer Gravitational-Wave Observatory. The tidal forces acting on the neutron stars induce changes in the phase evolution of the gravitational waveform, and these changes can be used to constrain the nuclear equation of state. Current methods of generating BNS and BHNS waveforms rely on either computationally challenging full 3D hydrodynamical simulations or approximate analytic solutions. We introduce a new method for computing inspiral waveforms for BNS/BHNS systems by adding the post-Newtonian (PN) tidal effects to full numerical simulations of binary black holes (BBHs), effectively replacing the nontidal terms in the PN expansion with BBH results. Comparing a waveform generated with this method against a full hydrodynamical simulation of a BNS inspiral yields a phase difference of $<1$ radian over $sim 15$ orbits. The numerical phase accuracy required of BNS simulations to measure the accuracy of the method we present here is estimated as a function of the tidal deformability parameter ${lambda}$.
Binary neutron stars in circular orbits can be modeled as helically symmetric, i.e., stationary in a rotating frame. This symmetry gives rise to a first integral of the Euler equation, often employed for constructing equilibrium solutions via iteration. For eccentric orbits, however, the lack of helical symmetry has prevented the use of this method, and the numerical relativity community has often resorted to constructing initial data by superimposing boosted spherical stars without solving the Euler equation. The spuriously excited neutron star oscillations seen in evolutions of such data arise because such configurations lack the appropriate tidal deformations and are stationary in a linearly comoving---rather than rotating---frame. We consider eccentric configurations at apoapsis that are instantaneously stationary in a rotating frame. We extend the notion of helical symmetry to eccentric orbits, by approximating the elliptical orbit of each companion as instantaneously circular, using the ellipses inscribed circle. The two inscribed helical symmetry vectors give rise to approximate instantaneous first integrals of the Euler equation throughout each companion. We use these integrals as the basis of a self-consistent iteration of the Einstein constraints to construct conformal thin-sandwich initial data for eccentric binaries. We find that the spurious stellar oscillations are reduced by at least an order of magnitude, compared with those found in evolutions of superposed initial data. The tidally induced oscillations, however, are physical and qualitatively similar to earlier evolutions. Finally, we show how to incorporate radial velocity due to radiation reaction in our inscribed helical symmetry vectors, which would allow one to obtain truly non-eccentric initial data when our eccentricity parameter $e$ is set to zero.
The first direct detection of neutron-star-black-hole binaries will likely be made with gravitational-wave observatories. Advanced LIGO and Advanced Virgo will be able to observe neutron-star-black-hole mergers at a maximum distance of 900Mpc. To acheive this sensitivity, gravitational-wave searches will rely on using a bank of filter waveforms that accurately model the expected gravitational-wave signal. The angular momentum of the black hole is expected to be comparable to the orbital angular momentum. This angular momentum will affect the dynamics of the inspiralling system and alter the phase evolution of the emitted gravitational-wave signal. In addition, if the black holes angular momentum is not aligned with the orbital angular momentum it will cause the orbital plane of the system to precess. In this work we demonstrate that if the effect of the black holes angular momentum is neglected in the waveform models used in gravitational-wave searches, the detection rate of $(10+1.4)M_{odot}$ neutron-star--black-hole systems would be reduced by $33 - 37%$. The error in this measurement is due to uncertainty in the Post-Newtonian approximations that are used to model the gravitational-wave signal of neutron-star-black-hole inspiralling binaries. We describe a new method for creating a bank of filter waveforms where the black hole has non-zero angular momentum, but is aligned with the orbital angular momentum. With this bank we find that the detection rate of $(10+1.4)M_{odot}$ neutron-star-black-hole systems would be reduced by $26-33%$. Systems that will not be detected are ones where the precession of the orbital plane causes the gravitational-wave signal to match poorly with non-precessing filter waveforms. We identify the regions of parameter space where such systems occur and suggest methods for searching for highly precessing neutron-star-black-hole binaries.
Observations of gravitational waves and their electromagnetic counterparts may soon uncover the existence of coalescing compact binary systems formed by a stellar-mass black hole and a neutron star. These mergers result in a remnant black hole, possibly surrounded by an accretion disk. The mass and spin of the remnant black hole depend on the properties of the coalescing binary. We construct a map from the binary components to the remnant black hole using a sample of numerical-relativity simulations of different mass ratios $q$, (anti-)aligned dimensionless spins of the black hole $a_{rm BH}$, and several neutron star equations of state. Given the binary total mass, the mass and spin of the remnant black hole can therefore be determined from the three parameters $(q,a_{rm BH},Lambda)$, where $Lambda$ is the tidal deformability of the neutron star. Our models also incorporate the binary black hole and test-mass limit cases and we discuss a simple extension for generic black hole spins. We combine the remnant characterization with recent population synthesis simulations for various metallicities of the progenitor stars that generated the binary system. We predict that black-hole-neutron-star mergers produce a population of remnant black holes with masses distributed around $7M_odot$ and $9M_odot$. For isotropic spin distributions, nonmassive accretion disks are favoured: no bright electromagnetic counterparts are expected in such mergers.
Gravitational waves radiated by the coalescence of compact-object binaries containing a neutron star and a black hole are one of the most interesting sources for the ground-based gravitational-wave observatories Advanced LIGO and Advanced Virgo. Advanced LIGO will be sensitive to the inspiral of a $1.4, M_odot$ neutron star into a $10,M_odot$ black hole to a maximum distance of $sim 900$ Mpc. Achieving this sensitivity and extracting the physics imprinted in observed signals requires accurate modeling of the binary to construct template waveforms. In a NSBH binary, the black hole may have significant angular momentum (spin), which affects the phase evolution of the emitted gravitational waves. We investigate the ability of post-Newtonian (PN) templates to model the gravitational waves emitted during the inspiral phase of NSBH binaries. We restrict the black holes spin to be aligned with the orbital angular momentum and compare several approximants. We examine restricted amplitude waveforms that are accurate to 3.5PN order in the orbital dynamics and complete to 2.5PN order in the spin dynamics. We also consider PN waveforms with the recently derived 3.5PN spin-orbit and 3PN spin-orbit tail corrections. We compare these approximants to the effective-one-body model. For all these models, large disagreements start at low to moderate black hole spins, particularly for binaries where the spin is anti-aligned with the orbital angular momentum. We show that this divergence begins in the early inspiral at $v sim 0.2$ for $chi_{BH} sim 0.4$. PN spin corrections beyond those currently known will be required for optimal detection searches and to measure the parameters of neutron star--black hole binaries. While this complicates searches, the strong dependence of the gravitational-wave signal on the spin dynamics will make it possible to extract significant astrophysical information.
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