arXiv subjects

Koutarou Kyutoku

Publications and source records attributed to Koutarou Kyutoku.

At least 37 records · Page 2Linked to original sources

arXiv2020

Properties of the remnant disk and the dynamical ejecta produced in low-mass black hole-neutron star mergers

We systematically perform numerical-relativity simulations for low-mass black hole-neutron star mergers for the models with seven mass ratios $Q=M_{\rm BH}/M_{\rm NS}$ ranging from 1.5 to 4.4, and three neutron-star equations of state, focusing on 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 $Q \sim 3$, 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.

astro-ph.HE↗

arXiv2020

Viscous evolution of a massive disk surrounding stellar-mass black holes in full general relativity

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 $M_{\rm BH}=4$, $6$, and $10M_\odot$ and initial dimensionless spin $χ\approx 0.8$. The initial disk is chosen to have mass $M_{\rm disk}\approx 0.1$ or $3M_\odot$ 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 $M_{\rm disk} \approx 0.1M_\odot$ with the outer disk edge initially located at $r_{\rm out} \sim 200$ km, we find that $15$%-$20$% of $M_{\rm disk}$ is ejected and the average electron fraction of the ejecta is $\langle Y_e \rangle = 0.30$-$0.35$ as found in the previous study. For $M_{\rm disk} \approx 3M_\odot$, we find that $\approx 10$%-$20$% of $M_{\rm disk}$ is ejected for $r_{\rm out}\approx 200$-$1000$ km. In addition, $\langle Y_e \rangle$ of the ejecta can be enhanced to be $\gtrsim 0.4$ 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.

astro-ph.HE↗

arXiv2020

Sub-radian-accuracy gravitational waves from coalescing binary neutron stars II: Systematic study on the equation of state, binary mass, and mass ratio

We report results of numerical relativity simulations for {\it new} 26 non-spinning binary neutron star systems with 6 grid resolutions using an adaptive mesh refinement numerical re\ lativity code {\tt SACRA-MPI}. The finest grid spacing is $\approx 64$--$85$ 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 $\lesssim 0.1$ rad in the gravitational-wave phase and $\lesssim 20 \%$ in the gravitational-wave amplitude up to the gravitational-wave frequency $1000$ Hz.\ Finally, we calibrate the proposed universal relations between a post-merger gravitational wave signal and tidal deformability/neutron star radius in the literature and show that th\ ey suffer from systematics and many relations proposed as universal are not very universal. Improved fitting formulae are also proposed.

astro-ph.HE↗

arXiv2020

On the possibility of GW190425 being a black hole--neutron star binary merger

We argue that the kilonova/macronova associated with the gravitational-wave event GW190425 could have been bright enough to be detected if it was caused by the merger of a low-mass black hole and a neutron star. Although tidal disruption occurs for such a low-mass black hole as is generally expected, the masses of the dynamical ejecta are limited to <~10^{-3}M_sun, which is consistent with previous work in the literature. The remnant disk could be as massive as 0.05--0.1M_sun, and the disk outflow of ~0.01--0.03M_sun is likely to be driven by viscous or magnetohydrodynamic effects. The disk outflow may not be neutron-rich enough to synthesize an abundance of lanthanide elements, even in the absence of strong neutrino emitter, if the ejection is driven on the viscous time scale of >~0.3s. If this is the case, the opacity of the disk outflow is kept moderate, and a kilonova/macronova at the distance of GW190425 reaches a detectable brightness of 20--21mag at 1day after merger for most viewing angles. If some disk activity ejects the mass within ~0.1s, instead, lanthanide-rich outflows would be launched and the detection of emission becomes challenging. Future possible detections of kilonovae/macronovae from GW190425-like systems will disfavor the prompt collapse of binary neutron stars and a non-disruptive low-mass black hole--neutron star binary associated with a small neutron-star radius, whose mass ejection is negligible. The host-galaxy distance will constrain the viewing angle and deliver further information about the mass ejection.

astro-ph.HE↗

arXiv2020

Mass ejection from disks surrounding a low-mass black hole: Viscous neutrino-radiation hydrodynamics simulation in full general relativity

New viscous neutrino-radiation hydrodynamics simulations are performed for accretion disks surrounding a spinning black hole with low mass $3M_\odot$ 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 $\sim $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.

astro-ph.HE↗

arXiv2019

r-Process enrichment in the Galactic halo characterized by nucleosynthesis variation in the ejecta of coalescing neutron star binaries

A large star-to-star variation in the abundances of r-process elements, as seen in the [Eu/Fe] ratio for Galactic halo stars, is a prominent feature that is distinguishable from other heavy elements. It is, in part, caused by the presence of highly r-process enriched stars, classified as r-II stars ([Eu/Fe]>+1). In parallel, halo stars show that the ratio of a light r-process element (Y) to Eu is tightly correlated with [Eu/Fe], giving the lowest [Y/Eu] ratio that levels off at r-II stars. On the other hand, recent hydrodynamical simulations of coalescing double neutron stars (cNSNSs) have suggested that r-process sites may be separated into two classes providing different electron-fraction distributions: tidally-driven dynamical ejecta and (dynamical or postmerger) non-tidal ejecta. Here, we show that a widely spanning feature of [Eu/Fe] can be reproduced by models that consider the different masses of tidally-driven dynamical ejecta from both cNSNSs and coalescing black hole/neutron star binaries (cBHNSs). In addition, the observed [Y/Eu] trend is explained by the combined nucleosynthesis in two kinds of ejecta with varying mass asymmetry in double NS systems. Our scenario suggests that massive tidally-driven dynamical ejecta accompanied by massive non-tidal part from cNSNSs or cBHNSs could alone accommodate r-II abundances, including an actinide boost in some cases. The event rate for cNSNSs estimated from our study agrees with the latest result of ~1000 (90% confidence interval of 110-3840) Gpc$^{-3}$yr$^{-1}$ by gravitational-wave detection, and a few events per Gpc$^3$ per year of cBHNSs associated with r-process production are predicted to emerge.

astro-ph.GA↗

arXiv2019

Discrepancy in tidal deformability of GW170817 between the Advanced LIGO twin detectors

We find that the Hanford and Livingston detectors of Advanced LIGO derive a distinct posterior probability distribution of binary tidal deformability tilde{Lambda} 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 tilde{Lambda} are estimated to be 527^{+619}_{-345} with the Hanford detector, 927^{+522}_{-619} with the Livingston detector, and 455^{+668}_{-281} 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.

astro-ph.HE↗

arXiv2019

Revisiting the lower bound on tidal deformability derived by AT 2017gfo

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 deformability of neutron stars with a typical mass of ~1.4M_sun. 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 tilde{Lambda} 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.

astro-ph.HE↗

arXiv2018

How to detect the shortest-period binary pulsars in the era of LISA

We discuss a multimessenger strategy to detect radio pulses from Galactic binary neutron stars in a very tight orbit with the period shorter than 10 min. On one hand, all-sky surveys by radio instruments are inefficient for detecting faint pulsars in very tight binaries due partly to the rarity of targets and primarily to the need of correction for severe Doppler smearing. On the other hand, the Laser Interferometer Space Antenna (LISA) will detect these binaries with a very large signal-to-noise ratio and determine the orbital frequency, binary parameters, and sky location to high accuracy. The information provided by LISA will reduce the number of required pointings by two to six orders of magnitude and that of required trials for the corrections by about nine orders of magnitude, increasing the chance of discovering radio pulsars. For making full use of this strategy, it is desirable to operate high-sensitivity radio instruments such as Square Kilometer Array Phase 2 simultaneously with LISA.

astro-ph.HE↗

arXiv2018

Effect of Inhomogeneity on Primordial Black Hole Formation in the Matter Dominated Era

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 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 $β_{inhom}(σ)$ of primordial black holes, where $σ$ is 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 $σ\ll 1$, we obtain $β_{inhom}\simeq 3.70 σ^{3/2}$, which is larger by about an order of magnitude than the probability derived in earlier work by assuming instantaneous propagation of information.

astro-ph.CO↗

arXiv2018

Systematic effects from black hole-neutron star waveform model uncertainties on the neutron star equation of state

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 %analytic fits to 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. 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.

gr-qc↗

arXiv2018

Global simulations of strongly magnetized remnant massive neutron stars formed in binary neutron star mergers

We perform a general-relativistic magnetohydrodynamics simulation for $\approx 30$ ms after merger of a binary neutron star to a remnant massive neutron star (RMNS) with a high spatial resolution of the finest grid resolution $12.5$ m. First, we estimate that the Kelvin-Helmholtz instability at merger could amplify the magnetic-field energy up to $\sim 1\%$ of the thermal energy. Second, we find that the magnetorotational instability in the RMNS envelope and torus with $ρ< 10^{13}~{\rm g~cm^{-3}}$ sustains magneto-turbulent state and the effective viscous parameter in these regions is likely to converge to $\approx 0.01$--$0.02$ with respect to the grid resolution. Third, the current grid resolution is not still fine enough to sustain magneto-turbulent state in the RMNS with $ρ\ge 10^{13}~{\rm g~cm^{-3}}$.

astro-ph.HE↗

arXiv2018

Detectability of thermal neutrinos from binary-neutron-star mergers and implication to neutrino physics

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 Mt years 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 are now securely constrained to that of massless particles with a corresponding limit on the graviton mass of <~10^{-22} eV/c^2 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/c^2 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.

astro-ph.HE↗

arXiv2018

Extracting the orbital axis from gravitational waves of precessing binary systems

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 co-precessing frame waveforms only from observed quantities for the ideal case that the signal-to-noise ratio is high enough to analyze the waveforms directly. Specifically, we do not assume knowledge of the time evolution of the instantaneous orbital axis and the co-precessing waveforms before analyzing the data 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 $\approx0.02~{\rm rad}$ error from gravitational waves emitted during the inspiral phase. The co-precessing waveforms are also reconstructed with high accuracy; the mismatch (assuming white noise) between them and the original co-precessing 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 co-precessing frame waveforms are not only the purely technical tools for understanding the complex nature of precessing waveforms but also direct observables.

gr-qc↗

arXiv2018

Frequency-domain gravitational waveform models for inspiraling binary neutron stars

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 $10$--$1000\,{\rm Hz}$. We show that the phase difference between our waveform model and the hybrid waveforms is always smaller than $0.1\, {\rm rad}$ for the binary tidal deformability, ${\tilde Λ}$, in the range $300\lesssim{\tilde Λ}\lesssim1900$ and for the mass ratio between 0.73 and 1. We show that, for $10$--$1000\,{\rm Hz}$, the distinguishability for the signal-to-noise ratio $\lesssim50$ and the mismatch between our waveform model and the hybrid waveforms are always smaller than 0.25 and $1.1\times10^{-5}$, respectively. The systematic error of our waveform model in the measurement of ${\tilde Λ}$ is always smaller than $20$ with respect to the hybrid waveforms for $300\lesssim{\tilde Λ}\lesssim1900$. 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 the signal-to-noise ratio of $50$.

gr-qc↗

arXiv2018

Neutrino transport in black hole-neutron star binaries: neutrino emission and dynamical mass ejection

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--3x10^53 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 beta-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.

astro-ph.HE↗

arXiv2017

Modeling GW170817 based on numerical relativity and its implications

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 infra-red 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 to produce an ejecta component of appreciable mass with fast motion in the post-merger 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 2M_sun in order that a long-lived massive neutron star can be formed as the merger remnant for the binary systems of GW170817, for which the initial total mass is >~ 2.73M_sun and (II) no detection of relativistic optical counterpart suggests a not-extremely high value of M_max approximately as 2.15-2.25M_sun.

astro-ph.HE↗

arXiv2017

Prospects of the local Hubble parameter measurement using gravitational waves from double neutron stars

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 are genuinely unique in terms of the rapid evolution of color and magnitude and we expect that, for a good fraction $\gtrsim 50\%$ of the DNS events within $\sim 200$Mpc, we could identify their host galaxies, using their kilonovae. At present, the estimated DNS merger rate $(1.5^{+3.2}_{-1.2})\times 10^{-6} {\rm Mpc^{-3} yr^{-1}}$ has a large uncertainty. But, if it is at the high end, we could measure the local Hubble parameter $H_L$ with the level of $ΔH_L/H_L\sim 0.042$ ($1σ$ 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 $2.1σ$ level.

astro-ph.CO↗