久徳 浩太郎

Abstract By performing general relativistic hydrodynamics simulations with an approximate neutrino radiation transfer, the properties of ejecta in the dynamical and post-merger phases are investigated in the cases in which the remnant massive neutron star collapses into a black hole in ≲20 ms after the onset of the merger. The dynamical mass ejection is investigated in three-dimensional simulations. The post-merger mass ejection is investigated in two-dimensional axisymmetric simulations with viscosity using the three-dimensional post-merger systems as the initial conditions. We show that the typical neutron richness of the dynamical ejecta is higher for the merger of more asymmetric binaries; hence, heavier r-process nuclei are dominantly synthesized. The post-merger ejecta are shown to have only mild neutron richness, which results in the production of lighter r-process nuclei, irrespective of the binary mass ratios. Because of the larger disk mass, the post-merger ejecta mass is larger for more asymmetric binary mergers. Thus, the post-merger ejecta can compensate for the underproduced lighter r-process nuclei for asymmetric merger cases. As a result, by summing up both ejecta components, the solar residual r-process pattern is reproduced within the average deviation of a factor of three, irrespective of the binary mass ratio. Our result also indicates that the (about a factor of a few) light-to-heavy abundance scatter observed in r-process-enhanced stars can be attributed to variation in the binary mass ratio and total mass. Implications of our results associated with the mass distribution of compact neutron star binaries and the magnetar scenario of short gamma-ray bursts are discussed.

Abstract We review the current status of general relativistic studies for coalescences of black hole–neutron star binaries. First, high-precision computations of black hole–neutron star binaries in quasiequilibrium circular orbits are summarized, focusing on the quasiequilibrium sequences and the mass-shedding limit. Next, the current status of numerical-relativity simulations for the merger of black hole–neutron star binaries is described. We summarize our understanding for the merger process, tidal disruption and its criterion, properties of the merger remnant and ejected material, gravitational waveforms, and gravitational-wave spectra. We also discuss expected electromagnetic counterparts to black hole–neutron star coalescences.

We systematically perform numerical-relativity simulations for low-mass black hole-neutron star mergers for the models with seven mass ratios Q = M-BH/M-NS ranging from 1.5 to 4.4, and three neutron-star equations of state, focusing on the properties of matter remaining outside the black hole and ejected dynamically during the merger. We pay particular attention to the dependence on the mass ratio of the binaries. It is found that the rest mass remaining outside the apparent horizon after the merger depends only weakly on the mass ratio for the models with low mass ratios. It is also clarified that the rest mass of the ejecta has a peak at Q similar to 3, and decreases steeply as the mass ratio decreases for the low mass-ratio case. We present a novel analysis method for the behavior of matter during the merger, focusing on the matter distribution in the phase space of specific energy and specific angular momentum. Then we model the matter distribution during and after the merger. Using the result of the analysis, we discuss the properties of the ejecta.

We develop a method to compute low-eccentricity initial data of black hole-neutron star binaries in the puncture framework extending previous work on other types of compact binaries. In addition to adjusting the orbital angular velocity of the binary, the approaching velocity of a neutron star is incorporated by modifying the helical Killing vector used to derive equations of the hydrostationary equilibrium. The approaching velocity of the black hole is then induced by requiring the vanishing of the total linear momentum of the system, differently from the case of binary black holes in the puncture framework where the linear momentum of each black hole is specified explicitly. We successfully reduce the orbital eccentricity to less than or similar to 0.001 by modifying the parameters iteratively using simulations of approximate to 3 orbits both for nonprecessing and precessing configurations. We find that empirical formulas for binary black holes derived in the excision framework do not reduce the orbital eccentricity to approximate to 0.001 for black hole-neutron star binaries in the puncture framework, although they work for binary neutron stars.

Long-term viscous neutrino-radiation hydrodynamics simulations in full general relativity are performed for a massive disk surrounding spinning stellar-mass black holes with mass M-BH = 4, 6, and 10 M-circle dot and initial dimensionless spin chi approximate to 0.8. The initial disk is chosen to have mass M-disk approximate to 0.1 or 3 M-circle dot as plausible models of the remnants for the merger of black hole-neutron star binaries or the stellar core collapse from a rapidly rotating progenitor, respectively. For M-disk approximate to 0.1 M-circle dot with the outer disk edge initially located at r(out) similar to 200 km, we find that 15%-20% of M-disk is ejected and the average electron fraction of the ejecta is < Y-e > = 0.30-0.35 as found in the previous study. For M-disk approximate to 3 M-circle dot, we find that approximately 10%-20% of M-disk is ejected for r(out) approximate to 200-1000 km. In addition, the average electron fraction of the ejecta can be enhanced to < Y-e > greater than or similar to 0.4 because the electron fraction is increased significantly during the long-term viscous expansion of the disk with high neutrino luminosity until the mass ejection sets in. Our results suggest that not heavy r-process elements but light trans-iron elements would be synthesized in the matter ejected from a massive torus surrounding stellar-mass black holes. We also find that the outcomes of the viscous evolution for the high-mass disk case is composed of a rapidly spinning black hole surrounded by a torus with a narrow funnel, which appears to be suitable for generating gamma-ray bursts.

We reanalyze gravitational waves from binary-neutron-star mergers GW170817 and GW190425 using a numerical-relativity (NR) calibrated waveform model, the TF2+_KyotoTidal model, which includes nonlinear tidal terms. For GW170817, by imposing a uniform prior on the binary tidal deformability (Lambda) over tilde, the symmetric 90% credible interval of (Lambda) over tilde is estimated to be 481(-359)(+436) and 402(-279)(+465) for the case of f(max) = 1000 and 2048 Hz, respectively, where f(max) is the maximum frequency in the analysis. We also reanalyze the event with other waveform models: two post-Newtonian waveform models (TF2_PNTidal and TF2+_PNTidal), the TF2+_NRTidal model that is another NR calibrated waveform model, and its upgrade, the TF2+_NRTidalv2 model. While estimates of parameters other than (Lambda) over tilde are broadly consistent among various waveform models, our results indicate that estimates of (Lambda) over tilde depend on waveform models. However, the difference is smaller than the statistical error. For GW190425, we can only obtain little information on the binary tidal deformability. The systematic difference among the NR calibrated waveform models will become significant to measure (Lambda) over tilde as the number of detectors and events increase and sensitivities of detectors are improved.

We study the postmerger mass ejection of low-mass binary neutron stars (NSs) with the system mass of 2.5Mand subsequent nucleosynthesis by performing general-relativistic, neutrino-radiation viscous-hydrodynamics simulations in axial symmetry. We find that the merger remnants are long-lived massive NSs surviving more than several seconds, irrespective of the nuclear equations of state (EOSs) adopted. The ejecta masses of our fiducial models are similar to 0.06-0.1M(depending on the EOS), being similar to 30% of the initial disk masses (similar to 0.15-0.3M). Postprocessing nucleosynthesis calculations indicate that the ejecta is composed mainly of lightr-process nuclei with small amounts of lanthanides (mass fraction similar to 0.002-0.004) and heavier species due to the modest average electron fraction (similar to 0.32-0.34) for a reasonable value of the viscous coefficient. Such abundance distributions are compatible with those in weakr-process stars such as HD 122563 but not with the solarr-process-like abundance patterns found in all measuredr-process-enhanced metal-poor stars. Therefore, low-mass binary NS mergers should be rare. If such low-mass NS mergers occur, their electromagnetic counterparts, kilonovae, will be characterized by an early bright blue emission because of the large ejecta mass as well as the small lanthanide fraction. We also show, however, that if the effective turbulent viscosity is very high, the electron fraction of the ejecta could be low enough that the solarr-process-like abundance pattern is reproduced and the lanthanide fraction becomes so high that the kilonova would be characterized by early bright blue and late bright red emissions.

We report results of numerical relativity simulations for 26 new nonspinning binary neutron star systems with 6 grid resolutions using an adaptive mesh refinement numerical relativity code SACRA-MPI. The finest grid spacing is approximate to 64-85 m, depending on the systems. First, we derive long-term high-precision inspiral gravitational waveforms and show that the accumulated gravitational-wave phase error due to the finite grid resolution is less than 0.5 rad during more than 200 rad phase evolution irrespective of the systems. We also find that the gravitational-wave phase error for a binary system with a tabulated equation of state (EOS) is comparable to that for a piecewise polytropic EOS. Then we validate the SACRA inspiral gravitational waveform template, which will be used to extract tidal deformability from gravitational wave observation, and find that accuracy of our waveform modeling is less than or similar to 0.1 rad in the gravitational-wave phase and <= 20% in the gravitational-wave amplitude up to the gravitational-wave frequency 1000 Hz. Finally, we calibrate the proposed universal relations between a postmerger gravitational wave signal and tidal deformability/ neutron star radius in the literature and show that they suffer from systematics and many relations proposed as universal are not very universal. Improved fitting formulas are also proposed.

New viscous neutrino-radiation hydrodynamics simulations are performed for accretion disks surrounding a spinning black hole with low mass 3 M-circle dot and dimensionless spin 0.8 or 0.6 in full general relativity, aiming at modeling the evolution of a merger remnant of massive binary neutron stars or low-mass black hole-neutron star binaries. We reconfirm the following results found by previous studies of other groups: 15%-30% of the disk mass is ejected from the system with the average velocity of similar to 5%-10% of the speed of light for the plausible profile of the disk as merger remnants. In addition, we find that for the not extremely high viscous coefficient case, the neutron richness of the ejecta does not become very high, because weak interaction processes enhance the electron fraction during the viscous expansion of the disk before the onset of the mass ejection, resulting in the suppression of the lanthanide synthesis. For high-mass disks, the viscous expansion timescale is increased by a longer-term neutrino emission, and hence, the electron fraction of the ejecta becomes even higher. We also confirm that the mass distribution of the electron fraction depends strongly on the magnitude of the given viscous coefficient. This demonstrates that a first-principle magnetohydrodynamics simulation is necessary for black hole-disk systems with sufficient grid resolution and with sufficiently long timescale (longer than seconds) to clarify the nucleosynthesis and electromagnetic signals from them.

We find that the Hanford and Livingston detectors of Advanced LIGO derive a distinct posterior probability distribution of binary tidal deformability (Lambda) over tilde of the first binary-neutron-star merger GW170817. By analyzing public data of GW170817 with a nested-sampling engine and the default TaylorF2 waveform provided by the LALInference package, the probability distribution of the binary tidal deformability derived by the LIGO-Virgo detector network turns out to be determined dominantly by the Hanford detector. Specifically, by imposing the flat prior on tidal deformability of individual stars, symmetric 90% credible intervals of (Lambda) over tilde are estimated to be 527(-345)(+619) with the Hanford detector, 927(-619)(+522) with the Livingston detector, and 455(-281)(+668) with the LIGO-Virgo detector network. Furthermore, the distribution derived by the Livingston detector changes irregularly when we vary the maximum frequency of the data used in the analysis. This feature is not observed for the Hanford detector. While they are all consistent, the discrepancy and irregular behavior suggest that an in-depth study of noise properties might improve our understanding of GW170817 and future events.

We revisit the lower bound on binary tidal deformability (Lambda) over tilde 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 (Lambda) over tilde less than or similar to 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 deformability of neutron stars with a typical mass of approximate to 1.4 M-circle dot. If the maximum mass is so large that the binary does not collapse into a black hole immediately after merger, the mass of the ejecta can be sufficiently large irrespective of the binary tidal deformability. We present models of binary mergers with (Lambda) over tilde down to 242 that satisfy the requirement on the mass of the ejecta from the luminosity of AT 2017gfo. We further find that the luminosity of AT 2017gfo could be explained by models that do not experience bounce after merger. We conclude that the luminosity of AT 2017gfo is not very useful for constraining the binary tidal deformability. Accurate estimation of the mass ratio will be necessary to establish a lower bound using electromagnetic counterparts in the future. We also caution that merger simulations that employ a limited class of tabulated equations of state could be severely biased due to the lack of generality.

We identify various contributors of systematic effects in the measurement of the neutron star (NS) tidal deformability and quantify their magnitude for several types of neutron star-black hole (NSBH) binaries. Gravitational waves from NSBH mergers contain information about the components' masses and spins as well as the NS equation of state. Extracting this information requires comparison of the signal in noisy detector data with theoretical templates derived from some combination of post-Newtonian (PN) approximants, effective one-body (EOB) models, and numerical relativity (NR) simulations. The accuracy of these templates is limited by errors in the NR simulations, by the approximate nature of the PN/EOB waveforms, and by the hybridization procedure used to combine them. In this paper, we estimate the impact of these errors by constructing and comparing a set of PN-NR hybrid waveforms, for the first time with NR waveforms from two different codes, namely, SPEC and SACRA, for such systems. We then attempt to recover the parameters of the binary using two non-precessing template approximants. As expected, these errors have negligible effect on detectability. Mass and spin estimates are moderately affected by systematic errors for near equal-mass binaries, while the recovered masses can be inaccurate at higher mass ratios. Large uncertainties are also found in the tidal deformability., due to differences in PN base models used in hybridization, numerical relativity NR errors, and inherent limitations of the hybridization method. We find that systematic errors are too large for tidal effects to be accurately characterized for any realistic NS equation of state model. We conclude that NSBH waveform models must be significantly improved if they are to be useful for the extraction of NS equation of state information or even for distinguishing NSBH systems from binary black holes.

© 2018 American Physical Society. We investigate the effect of inhomogeneity on primordial black hole formation in the matter dominated era. In the gravitational collapse of an inhomogeneous density distribution, a black hole forms if the apparent horizon prevents information of the central region of the configuration from leaking. Since information cannot propagate faster than the speed of light, we identify the threshold of the black hole formation by considering the finite speed for propagation of information. We show that the production probability βinhom(σ) of primordial black holes, where σ is the density fluctuation at horizon entry, is significantly enhanced from that derived in previous work in which the speed of propagation was effectively regarded as infinite. For σ1, we obtain βinhom≃3.70σ3/2, which is larger by about an order of magnitude than the probability derived in earlier work by assuming instantaneous propagation of information.

We propose a long-term strategy for detecting thermal neutrinos from the remnant of binary neutron-star mergers with a future M-ton water-Cherenkov detector such as Hyper-Kamiokande. Monitoring 2500 mergers within 200 Mpc, we may be able to detect a single neutrino with a human time-scale operation of ≈80 Mtyears for the merger rate of 1 Mpc-3 Myr-1, which is slightly lower than the median value derived by the LIGO-Virgo Collaboration with GW170817. Although the number of neutrino events is minimal, contamination from other sources of neutrinos can be reduced efficiently to ≈0.03 by analyzing only ≈1 s after each merger identified with gravitational-wave detectors if gadolinium is dissolved in the water. The contamination may be reduced further to ≈0.01 if we allow the increase of waiting time by a factor of ≈1.7. The detection of even a single neutrino can pin down the energy scale of thermal neutrino emission from binary neutron-star mergers and could strongly support or disfavor formation of remnant massive neutron stars. Because the dispersion relation of gravitational waves is now securely constrained to that of massless particles with a corresponding limit on the graviton mass of 10-22 eV/c2 by binary black-hole mergers, the time delay of a neutrino from gravitational waves can be used to put an upper limit of O(10) meV/c2 on the absolute neutrino mass in the lightest eigenstate. Large neutrino detectors will enhance the detectability, and, in particular, 5 Mt Deep-TITAND and 10 Mt MICA planned in the future will allow us to detect thermal neutrinos every ≈16 and 8 years, respectively, increasing the significance.

Following the detection of the GW170817 signal and its associated electromagnetic emissions, we discuss the prospects of the local Hubble parameter measurement using double neutron stars (DNSs). The kilonova emissions of GW170817, AT 2017gfo, are genuinely unique in terms of the rapid evolution of colour and magnitude, and we expect that, for a good fraction ≳ 50 per cent of the DNS events within ~ 200 Mpc, we could identify their host galaxies, using their kilonovae. At present, the estimated DNS merger rate (1.5-1.2 +3.2) × 10-6 Mpc-3 yr-1 has a large uncertainty. But, if it is at the high end, we could measure the local Hubble parameter HL with the level of ΔHL/HL ~ 0.049 (1σ level), after the third observational run (O3). This accuracy is four times better than that obtained from GW170817 alone, and we will be able to examine the Hubble tension at 1.8σ level.

We develop a model for frequency-domain gravitational waveforms from inspiraling binary neutron stars. Our waveform model is calibrated by comparison with hybrid waveforms constructed from our latest high-precision numerical-relativity waveforms and the SEOBNRv2T waveforms in the frequency range of 10-1000 Hz. We show that the phase difference between our waveform model and the hybrid waveforms is always smaller than 0.1 rad for the binary tidal deformability Λ in the range 300Λ1900 and for a mass ratio between 0.73 and 1. We show that, for 10-1000 Hz, the distinguishability for the signal-to-noise ratio 50 and the mismatch between our waveform model and the hybrid waveforms are always smaller than 0.25 and 1.1×10-5, respectively. The systematic error of our waveform model in the measurement of Λ is always smaller than 20 with respect to the hybrid waveforms for 300Λ1900. The statistical error in the measurement of binary parameters is computed employing our waveform model, and we obtain results consistent with the previous studies. We show that the systematic error of our waveform model is always smaller than 20% (typically smaller than 10%) of the statistical error for events with a signal-to-noise ratio of 50.

We study the merger of black hole-neutron star binaries by fully general-relativistic neutrino-radiation-hydrodynamics simulations throughout the coalescence, particularly focusing on the role of neutrino irradiation in dynamical mass ejection. Neutrino transport is incorporated by an approximate transfer scheme based on the truncated moment formalism. While we fix the mass ratio of the black hole to the neutron star to be 4 and the dimensionless spin parameter of the black hole to be 0.75, the equations of state for finite-temperature neutron-star matter are varied. The hot accretion disk formed after tidal disruption of the neutron star emits a copious amount of neutrinos with the peak total luminosity ∼1-3×1053 erg s-1 via thermal pair production and subsequent electron/positron captures on free nucleons. Nevertheless, the neutrino irradiation does not modify significantly the electron fraction of the dynamical ejecta from the neutrinoless β-equilibrium value at zero temperature of initial neutron stars. The mass of the wind component driven by neutrinos from the remnant disk is negligible compared to the very neutron-rich dynamical component, throughout our simulations performed until a few tens milliseconds after the onset of merger, for the models considered in this study. These facts suggest that the ejecta from black hole-neutron star binaries are very neutron rich and are expected to accommodate strong r-process nucleosynthesis, unless magnetic or viscous processes contribute substantially to the mass ejection from the disk. We also find that the peak neutrino luminosity does not necessarily increase as the disk mass increases, because tidal disruption of a compact neutron star can result in a remnant disk with a small mass but high temperature.

We present a new method for extracting the instantaneous orbital axis only from gravitational wave strains of precessing binary systems observed from a particular observer direction. This method enables us to reconstruct the coprecessing frame waveforms only from observed strains for the ideal case with the high signal-to-noise ratio. Specifically, we do not presuppose any theoretical model of the precession dynamics and coprecessing waveforms in our method. We test and measure the accuracy of our method using the numerical relativity simulation data of precessing binary black holes taken from the SXS Catalog. We show that the direction of the orbital axis is extracted within ≈0.07 rad error from gravitational waves emitted during the inspiral phase. The coprecessing waveforms are also reconstructed with high accuracy the mismatch (assuming white noise) between them and the original coprecessing waveforms is typically a few times 10-3 including the merger-ringdown phase, and can be improved by an order of magnitude focusing only on the inspiral waveform. In this method, the coprecessing frame waveforms are not only the purely technical tools for understanding the complex nature of precessing waveforms but also direct observables.

Gravitational-wave observation together with a large number of electromagnetic observations shows that the source of the latest gravitational-wave event, GW170817, detected primarily by advanced LIGO, is the merger of a binary neutron star. We attempt to interpret this observational event based on our results of numerical-relativity simulations performed so far, paying particular attention to the optical and infrared observations. We finally reach a conclusion that this event is described consistently by the presence of a long-lived hypermassive or supramassive neutron star as the merger remnant because (i) significant contamination by lanthanide elements along our line of sight to this source can be avoided by the strong neutrino irradiation from it and (ii) it could play a crucial role in producing an ejecta component of appreciable mass with fast motion in the postmerger phase. We also point out that (I) the neutron-star equation of state has to be sufficiently stiff (i.e., the maximum mass of cold spherical neutron stars, M-max, has to be appreciably higher than 2 M-circle dot) in order for a long-lived massive neutron star to be formed as the merger remnant for the binary systems of GW170817, for which the initial total mass is greater than or similar to 2.73 M-circle dot, and (II) the absence of optical counterparts associated with relativistic ejecta suggests a not-extremely-high value of M-max approximately as 2.15-2.25 M-circle dot.

Extending our previous studies, we perform high-resolution simulations of inspiraling binary neutron stars in numerical relativity. We thoroughly carry through a convergence study in our currently available computational resources with the smallest grid spacing of approximate to 63-86 meter for the neutron-star radius 10.9-13.7 km. The estimated total error in the gravitational-wave phase is of order 0.1 rad for the total phase of greater than or similar to 210 rad in the last similar to 15-16 inspiral orbits. We then compare the waveforms (without resolution extrapolation) with those calculated by the latest effective-one-body formalism (tidal SEOBv2 model referred to as TEOB model). We find that for any of our models of binary neutron stars, the waveforms calculated by the TEOB formalism agree with the numerical-relativity waveforms up to approximate to 3 ms before the peak of the gravitational-wave amplitude is reached: For this late inspiral stage, the total phase error is less than or similar to 0.1 rad. Although the gravitational waveforms have an inspiral-type feature for the last similar to 3 ms, this stage cannot be well reproduced by the current TEOB formalism, in particular, for neutron stars with large tidal deformability (i.e., lager radius). The reason for this is described.

We propose that stellar-mass binary black holes like GW150914 will become a tool to explore the local Universe within similar to 100 Mpc in the era of the Laser Interferometer Space Antenna (LISA). High calibration accuracy and annual motion of LISA could enable us to localize up to approximate to 60 binaries more accurately than the error volume of similar to 100 Mpc(3) without electromagnetic counterparts under moderately optimistic assumptions. This accuracy will give us a fair chance to determine the host object solely by gravitational waves. By combining the luminosity distance extracted from gravitational waves with the cosmological redshift determined from the host, the local value of the Hubble parameter will be determined up to a few % without relying on the empirically constructed distance ladder. Gravitational-wave cosmography would pave the way for resolution of the disputed Hubble tension, where the local and global measurements disagree in the value of the Hubble parameter at 3.4 sigma level, which amounts to approximate to 9%.

We discuss the gravitational wave (GW) emission and the orbital evolution of a hierarchical triple system composed of an inner binary black hole (BBH) and an outer tertiary. Depending on the kick velocity at the merger, the merged BBH could tidally disrupt the tertiary. Even though the fraction of BBH mergers accompanied by such disruptions is expected to be much smaller than unity, the existence of a tertiary and its basic parameters (e.g., semimajor axis, projected mass) can be examined for more than 10(3) BBHs with the follow-on missions to the space GW detector LISA. This allows us to efficiently prescreen the targets for the follow-up searches for the tidal disruption events (TDEs). The TDE probability would be significantly higher for triple systems with aligned orbital-and spin-angular momenta, compared with random configurations.

We study prospects for detecting extragalactic binary black holes similar to GW150914 by evolved Laser Interferometer Space Antenna (eLISA). We find that the majority of detected binary black holes will not merge within reasonable observation periods of eLISA in any configuration. While long-arm detectors are highly desired for promoting multiband gravitational wave astronomy by increasing the detections of merging binaries, the number of total detections can be increased also by improving the acceleration noise. A monochromatic approximation works well to derive semiquantitative features of observational prospects for non-merging binaries with clearly indicating the parameter dependence. Our estimate also suggests that the number of galaxies in the error volume is so small that the host galaxy may be determined uniquely with high confidence.

We reach the. robust conclusion that, by combining the observed cosmic rays of r-process elements with the fact that the velocity of the neutron-star-merger ejecta is much higher than that of the supernova ejecta, either (1) the reverse shock in the neutron-star-merger ejecta is a very inefficient accelerator that converts. less than 0.003% of the ejecta kinetic energy to the cosmic-ray energy. or (2) the neutron star merger is not the origin of the Galactic r-process elements. We also find that the acceleration efficiency should be less than 0.1% for the reverse shock of the supernova ejecta with observed cosmic rays lighter than the iron.

Black hole-neutron star (BH-NS) mergers are among the most promising gravitational-wave sources for ground-based detectors, and gravitational waves from BH-NS mergers are expected to be detected in the next few years. The simultaneous detection of electromagnetic counterparts with gravitational waves would provide rich information about merger events. Among the possible electromagnetic counterparts from BH-NS mergers is the so-called kilonova/macronova, emission powered by the decay of radioactive r-process nuclei, which is one of the best targets for follow-up observations. We derive fitting formulas for the mass and the velocity of ejecta from a generic BH-NS merger based on recently performed numerical-relativity simulations. We combine these fitting formulas with a new semi-analytic model for a BH-NS kilonova/macronova lightcurve, which reproduces the results of radiation-transfer simulations. Specifically, the semi-analytic model reproduces the results of each band magnitude obtained by the previous radiation-transfer simulations within similar to 1 mag. By using this semi-analytic model we found that, at 400 Mpc, the kilonova/macronova is as bright as 22-24 mag for cases with a small chirp mass and a high black hole spin, and &gt; 28 mag for a large chirp mass and a low black hole spin. We also apply our model to GRB 130603B as an illustration, and show that a BH-NS merger with a rapidly spinning black hole and a large neutron star radius is favored.

We perform neutrino radiation-hydrodynamics simulations for the merger of asymmetric binary neutron stars in numerical relativity. Neutron stars are modeled by soft and moderately stiff finite-temperature equations of state (EOS). We find that the properties of the dynamical ejecta such as the total mass, neutron richness profile, and specific entropy profile depend on the mass ratio of the binary systems for a given EOS in a unique manner. For a soft EOS (SFHo), the total ejecta mass depends weakly on the mass ratio, but the average of electron number per baryon (Y-e) and specific entropy (s) of the ejecta decreases significantly with the increase of the degree of mass asymmetry. For a stiff EOS (DD2), with the increase of the mass asymmetry degree, the total ejecta mass significantly increases while the average of Y-e and s moderately decreases. We find again that only for the SFHo, the total ejecta mass exceeds 0.01M(circle dot) irrespective of the mass ratio chosen in this paper. The ejecta have a variety of electron number per baryon with an average approximately between Y-e similar to 0.2 and similar to 0.3 irrespective of the EOS employed, which is well suited for the production of the rapid neutron capture process heavy elements (second and third peaks), although its averaged value decreases with the increase of the degree of mass asymmetry.

Extracting the unique information on ultradense nuclear matter from the gravitational waves emitted by merging neutron-star binaries requires robust theoretical models of the signal. We develop a novel effective-one-body waveform model that includes, for the first time, dynamic (instead of only adiabatic) tides of the neutron star as well as the merger signal for neutron-star-black-hole binaries. We demonstrate the importance of the dynamic tides by comparing our model against new numerical-relativity simulations of nonspinning neutron-star-black-hole binaries spanning more than 24 gravitational-wave cycles, and to other existing numerical simulations for double neutron-star systems. Furthermore, we derive an effective description that makes explicit the dependence of matter effects on two key parameters: tidal deformability and fundamental oscillation frequency.

Combining new gravitational waveforms derived by long-term (14 to 16 orbit) numerical-relativity simulations with waveforms by an effective-one-body (EOB) formalism for coalescing binary neutron stars, we construct hybrid waveforms and estimate the measurability for the dimensionless tidal deformability of the neutron stars, Lambda by advanced gravitational-wave detectors. We focus on the equal-mass case with the total mass 2.7M(circle dot). We find that for an event at a hypothetical effective distance of D-eff = 200 Mpc, the distinguishable difference in the dimensionless tidal deformability will be approximate to 100, 400, and 800 at 1 sigma, 2 sigma, and 3 sigma levels, respectively, for Advanced LIGO. If the true equation of state is stiff and the typical neutron-star radius is R greater than or similar to 13 km, our analysis suggests that the radius will be constrained within approximate to 1 km at 2 sigma level for an event at D-eff = 200 Mpc. On the other hand, if the true equation of state is soft and the typical neutron-star radius is R less than or similar to 12 km, it will be difficult to narrow down the equation of state among many soft ones, although it is still possible to discriminate the true one from stiff equations of state with R greater than or similar to 13 km. We also find that gravitational waves from binary neutron stars will be distinguished from those from spinless binary black holes at more than 2 sigma level for an event at D-eff = 200 Mpc. The validity of the EOB formalism, Taylor-T4, and Taylor-F2 approximants as the inspiral waveform model is also examined.

We explore magnetic-field amplification due to the Kelvin-Helmholtz instability during binary neutron star mergers. By performing high-resolution general relativistic magnetohydrodynamics simulations with a resolution of 17.5 m for 4-5 ms after the onset of the merger on the Japanese supercomputer "K", we find that an initial magnetic field of moderate maximum strength 10(13) G is amplified at least by a factor of approximate to 10(3). We also explore the saturation of the magnetic-field energy and our result shows that it is likely to be greater than or similar to 4 x10(50) erg, which is greater than or similar to 0.1% of the bulk kinetic energy of the merging binary neutron stars.

Tidal disruption has a dramatic impact on the outcome of neutron star-black hole mergers. The phenomenology of these systems can be divided in three classes: nondisruptive, mildly disruptive, and disruptive. The cutoff frequency of the gravitational radiation produced during the merger (which is potentially measurable by interferometric detectors) is very different in each regime, and when the merger is disruptive it carries information on the neutron star equation of state. Here we use semianalytical tools to derive a formula for the critical binary mass ratio Q = M-BH/M-NS below which mergers are disruptive as a function of the stellar compactness C = M-NS/R-NS and the dimensionless black hole spin chi. We then employ a new gravitational waveform amplitude model, calibrated to 134 general relativistic numerical simulations of binaries with black hole spin (anti-) aligned with the orbital angular momentum, to obtain a fit to the gravitational-wave cutoff frequency in the disruptive regime as a function of C, Q, and chi. Our findings are important to build gravitational-wave template banks, to determine whether neutron star-black hole mergers can emit electromagnetic radiation (thus helping multimessenger searches), and to improve event rate calculations for these systems.

The gravitational radiation emitted during the merger of a black hole with a neutron star is rather similar to the radiation from the merger of two black holes when the neutron star is not tidally disrupted. When tidal disruption occurs, gravitational waveforms can be broadly classified in two groups, depending on the spatial extent of the disrupted material. Extending previous work by some of us, here we present a phenomenological model for the gravitational waveform amplitude in the frequency domain encompassing the three possible outcomes of the merger: no tidal disruption, and "mild" and "strong" tidal disruption. The model is calibrated to 134 general-relativistic numerical simulations of binaries where the black hole spin is either aligned or antialigned with the orbital angular momentum. All simulations were produced using the SACRA code and piecewise polytropic neutron star equations of state. The present model can be used to determine when black-hole binary waveforms are sufficient for gravitational-wave detection, to extract information on the equation of state from future gravitational-wave observations, to obtain more accurate estimates of black hole-neutron star merger event rates, and to determine the conditions under which these systems are plausible candidates as central engines of gamma-ray bursts and macronovae/kilonovae.

We report results of a high resolution numerical-relativity simulation for the merger of black hole-magnetized neutron star binaries on Japanese supercomputer "K." We focus on a binary that is subject to tidal disruption and subsequent formation of a massive accretion torus. We find the launch of thermally driven torus wind, subsequent formation of a funnel wall above the torus and a magnetosphere with collimated poloidal magnetic field, and high Blandford-Znajek luminosity. We show for the first time this picture in a self-consistent simulation. The turbulencelike motion induced by the nonaxisymmetric magnetorotational instability as well as the Kelvin-Helmholtz instability inside the accretion torus works as an agent to drive the mass accretion and converts the accretion energy to thermal energy, which results in the generation of a strong wind. By an in-depth resolution study, we reveal that high resolution is essential to draw such a picture. We also discuss the implication for the r-process nucleosynthesis, the radioactively powered transient emission, and short gamma ray bursts.

We investigate properties of material ejected dynamically in the merger of black hole-neutron star binaries by numerical-relativity simulations. We systematically study the dependence of ejecta properties on the mass ratio of the binary, spin of the black hole, and equation of state of the neutron-star matter. Dynamical mass ejection is driven primarily by tidal torque, and the ejecta is much more anisotropic than that from binary neutron star mergers. In particular, the dynamical ejecta is concentrated around the orbital plane with a half opening angle of 10 degrees-20 degrees and often sweeps out only a half of the plane. The ejecta mass can be as large as similar to 0.1M(circle dot), and the velocity is subrelativistic with similar to 0.2-0.3c for typical cases. The ratio of the ejecta mass to the bound mass (disk and fallback components) is larger, and the ejecta velocity is larger, for larger values of the binary mass ratio, i.e., for larger values of the black-hole mass. The remnant black hole-disk system receives a kick velocity of O(100) km s(-1) due to the ejecta linear momentum, and this easily dominates the kick velocity due to gravitational radiation. Structures of postmerger material, velocity distribution of the dynamical ejecta, fallback rates, and gravitational waves are also investigated. We also discuss the effect of ejecta anisotropy on electromagnetic counterparts, specifically a macronova/kilonova and synchrotron radio emission, developing analytic models.

We systematically performed numerical-relativity simulations for black hole-neutron star (BH-NS) binary mergers with a variety of the BH spin orientation and nuclear-theory-based equations of state (EOS) of the NS. The initial misalignment angles of the BH spin measured from the direction of the orbital angular momentum are chosen in the range of i(tilt,0) approximate to 30 degrees - 90 degrees. We employed four models of nuclear-theory-based zero-temperature EOS for the NS in which the compactness of the NS is in the range of C = M-NS/R-NS = 0.138 - 0.180, where M-NS and R-NS are the mass and the radius of the NS, respectively. The mass ratio of the BH to the NS, Q = M-BH/M-NS, and the dimensionless spin parameter of the BH, chi, are chosen to be Q = 5 and chi = 0.75, together with M-NS = 1.35M(circle dot) so that the BH spin misalignment has a significant effect on tidal disruption of the NS. We obtain the following results: (i) The inclination angles of i(tilt,0) &lt; 70 degrees and i(tilt,0) &lt; 50 degrees are required for the formation of a remnant disk with its mass larger than 0.1M(circle dot) for the cases C = 0.140 and C = 0.160, respectively, while the disk mass is always smaller than 0.1M(circle dot) for C greater than or similar to 0.175. The ejecta with its mass larger than 0.01M(circle dot) is obtained for i(tilt,0) &lt; 85 degrees with C = 0.140, for i(tilt,0) &lt; 65 degrees with C = 0.160, and for i(tilt,0) &lt; 30 degrees with C = 0.175. (ii) The rotational axis of the dense part of the remnant disk with its rest-mass density larger than 10(9) g/cm(3) is approximately aligned with the remnant BH spin for i(tilt,0) approximate to 30 degrees. On the other hand, the disk axis is misaligned initially with similar to 30 degrees for i(tilt,0) approximate to 60 degrees, and the alignment with the remnant BH spin is achieved at similar to 50-60 ms after the onset of merger. The accretion time scale of the remnant disk is typically similar to 100 ms and depends only weakly on the misalignment angle and the EOS. (iii) The ejecta velocity is typically similar to 0.2-0.3c and depends only weakly on the misalignment angle and the EOS of the NS, while the morphology of the ejecta depends on its mass. (iv) The gravitational-wave spectra contains the information of the NS compactness in the cutoff frequency for i(tilt,0) less than or similar to 60 degrees.

We perform new long-term (15-16 orbits) simulations of coalescing binary neutron stars in numerical relativity using an updated Einstein equation solver, employing low-eccentricity initial data, and modeling the neutron stars by a piecewise polytropic equation of state. A convergence study shows that our new results converge more rapidly than the third order, and using the determined convergence order, we construct an extrapolated waveform for which the estimated total phase error should be less than one radian. We then compare the extrapolated waveforms with those calculated by the latest effective-one-body (EOB) formalism in which the so-called tidal deformability, higher post-Newtonian corrections, and gravitational self-force effects are taken into account. We show that for a binary of compact neutron stars with their radius 11.1 km, the waveform by the EOB formalism agrees quite well with the numerical waveform so that the total phase error is smaller than one radian for the total phase of similar to 200 radian up to the merger. By contrast, for a binary of less compact neutron stars with their radius 13.6 km, the EOB and numerical waveforms disagree with each other in the last few wave cycles, resulting in the total phase error of approximately three radian.

We perform radiation-hydrodynamics simulations of binary neutron-star mergers in numerical relativity on the Japanese "K" supercomputer, taking into account neutrino cooling and heating by an updated leakage-plus-transfer scheme for the first time. Neutron stars are modeled by three modern finite-temperature equations of state (EOS) developed by Hempel and his collaborators. We find that the properties of the dynamical ejecta of the merger such as total mass, average electron fraction, and thermal energy depend strongly on the EOS. Only for a soft EOS (the so-called SFHo), the ejecta mass exceeds 0.01M(circle dot). In this case, the distribution of the electron fraction of the ejecta becomes broad due to the shock heating during the merger. These properties are well-suited for the production of the solar-like r-process abundance. For the other stiff EOS (DD2 and TM1), for which a long-lived massive neutron star is formed after the merger, the ejecta mass is smaller than 0.01M(circle dot), although broad electron-fraction distributions are achieved by the positron capture and the neutrino heating.

We develop a method to compute low-eccentricity initial data of binary neutron stars required to perform realistic simulations in numerical relativity. The orbital eccentricity is controlled by adjusting the orbital angular velocity of a binary and incorporating an approaching relative velocity of the neutron stars. These modifications improve the solution primarily through the hydrostatic equilibrium equation for the binary initial data. The orbital angular velocity and approaching velocity of initial data are updated iteratively by performing time evolutions over similar to 3 orbits. We find that the eccentricity can be reduced by an order of magnitude compared to standard quasicircular initial data, specifically from similar to 0.01 to less than or similar to 0.001, by three successive iterations for equal-mass binaries leaving similar to 10 orbits before the merger.