An important physical phenomenon that manifests itself during the inspiral of two orbiting compact objects is the tidal deformation of each under the gravitational influence of its companion. In the case of binary neutron star mergers, this tidal deformation and the associated Love numbers have been used to probe properties of dense matter and the nuclear equation of state. Non-spinning black holes on the other hand have a vanishing (field) tidal Love number in General Relativity. This pertains to the deformation of the asymptotic gravitational field. In certain cases, especially in the late stages of the inspiral phase when the black holes get close to each other, the source multipole moments might be more relevant in probing their properties and the No-Hair conjecture; contrastingly, these Love numbers do not vanish. In this paper, we track the source multipole moments in simulations of several binary black hole mergers and calculate these Love numbers. We present evidence that, at least for modest mass ratios, the behavior of the source multipole moments is universal.
Measurements of the properties of binary neutron star systems from gravitational-wave observations require accurate theoretical models for such signals. However, current models are incomplete, as they do not take into account all of the physics of these systems: some neglect possible tidal effects, others neglect spin-induced orbital precession, and no existing model includes the post-merger regime consistently. In this work, we explore the importance of two physical ingredients: tidal interactions during the inspiral and the imprint of the post-merger stage. We use complete inspiral--merger--…
Gravitational-wave astronomy with coalescing binary neutron star sources requires the availability of gravitational waveforms with tidal effects accurate up to merger. This article presents an improved version of \TEOBResum, a nonspinning effective-one-body (EOB) waveform model with enhanced analytical information in its tidal sector. The tidal potential governing the conservative dynamics employs resummed expressions based on post-Newtonian (PN) and gravitational self-force (GSF) information. In particular, we compute a GSF-resummed expression for the leading-order octupolar gravitoelectric t…
We discuss the properties of stellar mass black hole (BH) mergers induced by tidal encounters with a massive BH at galactic centres or potentially in dense star clusters. The tidal disruption of stellar binaries by a massive BH is known to produce hypervelocity stars. However, such a tidal encounter does not always lead to the break-up of binaries. Since surviving binaries tend to become hard and eccentric, this process can produce BH mergers in principle. For initially circular binaries, we show that the gravitational wave (GW) merger times become shorter by a factor of more than $10^{2}$ ($1…
One of the main targets for ground-based gravitational wave (GW) detectors such as Advanced LIGO (Laser Interferometer Gravitational wave Observatory) and Virgo is coalescences of neutron star (NS) binaries. Even though a NS's macroscopic properties such as mass and radius have been obtained from electro-magnetic wave observations, its internal structure has been studied mainly by using theoretical approaches. However, with the advent of Advanced LIGO and Virgo, the tidal deformability of a NS, which depends on the internal structure of the NS, has been recently obtained from GW observations. …
Inter-University Centre for Astronomy and Astrophysics（大学间天文学与天体物理中心）；International Centre for Theoretical Sciences, Tata Institute of Fundamental Research（塔塔基础研究所国际理论科学中心）；Gran Sasso Science Institute (GSSI)（格兰萨索科学研究所）；INFN, Laboratori Nazionali del Gran Sasso（意大利国家核物理研究所格兰萨索国家实验室）；Department of Physics and Astronomy, Washington State University（华盛顿州立大学物理与天文系）；School of Physics and Astronomy and Institute for Gravitational Wave Astronomy, University of Birmingham（伯明翰大学物理与天文学院及引力波天文研究所）
The increase in the sensitivity of gravitational wave interferometers will bring additional detections of binary black hole and double neutron star mergers. It will also very likely add many merger events of black hole - neutron star binaries. Distinguishing mixed binaries from binary black holes mergers for high mass ratios could be challenging because in this situation the neutron star coalesces with the black hole without experiencing significant disruption. To investigate the transition of mixed binary mergers into those behaving more like binary black hole coalescences, we present results…
Motivated by events in which black holes can lose their environment due to tidal interactions in a binary system, we develop a waveform model in which the tidal deformability interpolates between a finite value (dressed black hole) at relatively low frequency and a zero value (naked black hole) at high frequency. We then apply this model to the example case of a black hole dressed with an ultralight scalar field and investigate the detectability of the tidal Love number with the Einstein Telescope. We show that the parameters of the tidal deformability model could be measured with high accurac…
In this second part of a two-part paper, we discuss numerical simulations of a head-on merger of two non-spinning black holes. We resolve the fate of the original two apparent horizons by showing that after intersecting, their world tubes "turn around" and continue backwards in time. Using the method presented in the first paper to locate these surfaces, we resolve several such world tubes evolving and connecting through various bifurcations and annihilations. This also draws a consistent picture of the full merger in terms of apparent horizons, or more generally, marginally outer trapped surf…
We model an extreme mass ratio merger (EMR) as a point particle radially plunging into a large Schwarzschild black hole. We assume that the mass of the point particle, $μ$, is much smaller than the black hole mass M. Under this assumption we can employ the Zerilli formalism modified to include a source term which arises from the energy-momentum tensor of the small object. We solve the Zerilli equation by numerically evolving initial data. Then, we ray trace the null geodesics of the event horizon from after the merger backward in time to extract the geometry of the perturbed event horizon. Fur…
In a binary black hole merger, it is known that the inspiral portion of the waveform corresponds to two distinct horizons orbiting each other, and the merger and ringdown signals correspond to the final horizon being formed and settling down to equilibrium. However, we still lack a detailed understanding of the relation between the horizon geometry in these three regimes and the observed waveform. Here we show that the well known inspiral chirp waveform has a clear counterpart on black hole horizons, namely, the shear of the outgoing null rays at the horizon. We demonstrate that the shear beha…
Gravitational-wave sources can serve as standard sirens to probe cosmology by measuring their luminosity distance and redshift. Such standard sirens are also useful to probe theories beyond general relativity with a modified gravitational-wave propagation. Many of previous studies on the latter assume multi-messenger observations so that the luminosity distance can be measured with gravitational waves while the redshift is obtained by identifying sources' host galaxies from electromagnetic counterparts. Given that gravitational-wave events of binary neutron star coalescences with associated el…
In this work, we uncover new features in the evolution of the deformations of the dynamical horizon geometry in a binary black hole merger scenario using numerical relativity. First, in the inspiral phase, owing to the deformations, the dynamical horizons of the two black holes are found to steadily acquire multipole moments that vanish when the horizons are isolated. Out of these, the dominant moment is found to be the quadrupole moment. Second, we show that they encode detailed information about the dynamics of the binary black hole system. The dominant quadrupole multipole moment is particu…
We study the multimessenger signals from the merger of a black hole with a magnetized neutron star using resistive magnetohydrodynamics simulations coupled to full general relativity. We focus on a case with a 5:1 mass ratio, where only a small amount of the neutron star matter remains post-merger, but we nevertheless find that significant electromagnetic radiation can be powered by the interaction of the neutron star's magnetosphere with the black hole. In the lead-up to merger, strong twisting of magnetic field lines from the inspiral leads to plasmoid emission and results in a luminosity in…
We resolve the fate of the two original apparent horizons during the head-on merger of two non-spinning black holes. We show that following the appearance of the outer common horizon and subsequent inter-penetration of the original horizons, they continue to exist for a finite period of time before they are individually annihilated by unstable MOTSs. The inner common horizon vanishes in a similar, though independent, way. This completes the understanding of the analogue of the event horizon's "pair of pants" diagram for the apparent horizon. Our result is facilitated by a new method for locati…
The merger of a binary black hole gives birth to a highly distorted final black hole. The gravitational radiation emitted as this black hole relaxes presents us with the unique opportunity to probe extreme gravity and its connection with the dynamics of the black hole horizon. Using numerical relativity simulations, we demonstrate a connection between a concrete observable feature in the gravitational waves and geometrical features on the dynamical apparent horizon of the final black hole. Specifically, we show how the line-of-sight passage of a "cusp"-like defect on the horizon of the final b…
In classical numerical relativity, marginally outer trapped surfaces (MOTSs) are the main tool to locate and characterize black holes. For five decades it has been known that during a binary merger, a new outer horizon forms around the initial apparent horizons of the individual holes once they are sufficiently close together. However the ultimate fate of those initial horizons has remained a subject of speculation. Recent axisymmetric studies have shed new light on this process and this pair of papers essentially completes that line of research: we resolve the key features of the post-swallow…
We explore the correlation between the pre-merger tidal deformability and the post-merger remnant oscillations seen in numerical simulation of neutron star binaries, with the aim of understanding to what extent the physics support the existence of such a relation. We consider the impact of thermal and rotational effects and argue that the proposed relation does, indeed, make sense and provide simple arguments that help explain the result.
We numerically study the algebraic properties of the Weyl tensor through the merger of two non-spinning black holes (BHs). We are particularly interested in the conjecture that for such a vacuum spacetime, which is zeroth-order algebraically general, a geometric horizon (GH), on which the spacetime is algebraically special and which is identified by the vanishing of a complex scalar invariant (${\mathcal{D}}$), characterizes a smooth foliation independent surface (horizon) associated with the BH. In the first simulation we investigate the level-$0$ sets of $\text{Re}({\mathcal{D}})$ (since $\t…
We revisit the lower bound on binary tidal deformability tilde{Lambda} imposed by a luminous kilonova/macronova, AT 2017gfo, by numerical-relativity simulations of models that are consistent with gravitational waves from the binary neutron star merger GW170817. Contrary to the claim made in the literature, we find that binaries with tilde{Lambda}<~400 can explain the luminosity of AT 2017gfo, as long as moderate mass ejection from the remnant is assumed as had been done in previous work. The reason is that the maximum mass of a neutron star is not strongly correlated with the tidal deformabili…
Neutron star (NS) binaries formed dynamically may have significant eccentricities while emitting gravitational waves (GWs) in the LIGO/VIRGO band. We study tidal effects in such eccentric inspiralling NS binaries using a set of hybrid equations. The NS is modelled as a compressible ellipsoid, which can deform nonlinearly due to tidal forces, while the orbit evolution is treated with the post-Newtonian (PN) theory up to 2.5-PN order. We find that in general, the tidal interaction can accelerate the inspiral, and cause orbital frequency and phase shifts. For circular inspirals, our calculations …
Gravitational-wave signals from coalescing binary neutron stars can yield important information about the properties of nuclear-matter equation of state from the early part of the signal through tidal effects to the properties and oscillation frequencies of the merger product. We investigate a direct link between the properties of isolated neutron stars and their merger, by comparing the frequency of the fundamental oscillation mode ($f$-mode) of neutron stars with the gravitational-wave frequency associated with the merger of two neutron stars. We calculate the quadrupolar ($l=2$) $f$-mode os…
Dynamically formed compact object binaries may still be eccentric while in the LIGO/Virgo band. For a neutron star (NS) in an eccentric binary, the fundamental modes (f-modes) are excited at pericenter, transferring energy from the orbit to oscillations in the NS. We model this system by coupling the evolution of the NS f-modes to the orbital evolution of the binary as it circularizes and moves toward coalescence. NS f-mode excitation generally speeds up the orbital decay and advances the phase of the gravitational wave signal from the system. We calculate how this effect changes the timing of…
Resonant excitations of $f$-modes in binary neutron star coalescences influence the gravitational waves (GWs) emission in both quasicircular and highly eccentric mergers and can deliver information on the star interior. Most models of resonant tides are built using approximate, perturbative approaches and thus require to be carefully validated against numerical relativity (NR) simulations in the high-frequency regime. We perform detailed comparisons between a set of high-resolution NR simulations and the state of the art effective one body (EOB) model ${\tt TEOBResumS}$ with various tidal pote…
Gravitational-wave background from supermassive black holes without merger trees: Tidally regulated mergers and direct inference from the NANOGrav 15 yr data
GRAPPA Institute, University of Amsterdam（阿姆斯特丹大学 GRAPPA 研究所）；Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), University of Tokyo（东京大学 宇宙物理数学 Kavli 研究所）
We construct closed-form gravitational waveforms (GWs) with tidal effects for the coalescence and merger of binary neutron stars. The method relies on a new set of eccentricity-reduced and high-resolution numerical relativity (NR) simulations and is composed of three steps. First, tidal contributions to the GW phase are extracted from the time-domain NR data. Second, those contributions are employed to fix high-order coefficients in an effective and resummed post-Newtonian expression. Third, frequency-domain tidal approximants are built using the stationary phase approximation. Our tidal appro…
We present new numerical relativity results of neutron star mergers with chirp mass $1.188M_\odot$ and mass ratios $q=1.67$ and $q=1.8$ using finite-temperature equations of state (EOS), approximate neutrino transport and a subgrid model for magnetohydrodynamics-induced turbulent viscosity. The EOS are compatible with nuclear and astrophysical constraints and include a new microphysical model derived from ab-initio calculations based on the Brueckner-Hartree-Fock approach. We report for the first time evidence for accretion-induced prompt collapse in high-mass-ratio mergers, in which the tidal…
Modern tidal interaction models for rapid binary population synthesis: II. Binary black hole formation, mergers, and spins
Johns Hopkins University（约翰斯·霍普金斯大学）；Monash University（莫纳什大学）；OzGrav: Australian Research Council Centre of Excellence for Gravitational Wave Discovery（引力波发现澳大利亚卓越中心（OzGrav））；California Institute of Technology（加州理工学院）
