arXiv subjects

Kenta Kiuchi

Publications and source records attributed to Kenta Kiuchi.

At least 55 records · Page 3Linked to original sources

arXiv2021

Evolution of bare quark stars in full general relativity: Single star case

We introduce our approaches, in particular the modifications of the primitive recovery procedure, to handle bare quark stars in numerical relativity simulations. Reliability and convergence of our implementation are demonstrated by evolving two triaxially rotating quark star models with different mass as well as a differentially rotating quark star model which has sufficiently large kinetic energy to be dynamically unstable. These simulations allow us to verify that our method is capable of resolving the evolution of the discontinuous surface of quark stars and possible mass ejection from them. The evolution of the triaxial deformation and the properties of the gravitational-wave emission from triaxially rotating quark stars have been also studied, together with the mass ejection of the differentially rotating case. It is found that supramassive quark stars are not likely to be ideal sources of continuous gravitational wave as the star recovers axisymmetry much faster than models with smaller mass and gravitational-wave amplitude decays rapidly in a timescale of $10\,$ms, although the instantaneous amplitude from more massive models is larger. As with the differentially rotating case, our result confirms that quark stars could experience non-axisymmetric instabilities similar to the neutron star case but with quite small degree of differential rotation, which is expected according to previous initial data studies.

gr-qc↗

arXiv2021

Alternative possibility of GW190521: Gravitational waves from high-mass black hole-disk systems

We evolve high-mass disks of mass $15$-$50M_\odot$ orbiting a $50M_\odot$ spinning black hole in the framework of numerical relativity. Such high-mass systems could be an outcome during the collapse of rapidly-rotating very-massive stars. The massive disks are dynamically unstable to the so-called one-armed spiral-shape deformation with the maximum fractional density-perturbation of $δρ/ρ\gtrsim 0.1$, and hence, high-amplitude gravitational waves are emitted. The waveforms are characterized by an initial high-amplitude burst with the frequency of $\sim 40$-$50$ Hz and the maximum amplitude of $(1$-$10)\times 10^{-22}$ at the hypothetical distance of 100 Mpc and by a subsequent low-amplitude quasi-periodic oscillation. We illustrate that the waveforms in our models with a wide range of the disk mass resemble that of GW190521. We also point out that gravitational waves from rapidly-rotating very-massive stars can be the source for 3rd-generation gravitational-wave detectors for exploring the formation process of rapidly-spinning high-mass black holes of mass $\sim 50$-$100M_\odot$ in an early universe.

astro-ph.HE↗

arXiv2021

Reducing orbital eccentricity in initial data of black hole--neutron star binaries in the puncture framework

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 <~0.001 by modifying the parameters iteratively using simulations of ~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 ~0.001 for black hole--neutron star binaries in the puncture framework, although they work for binary neutron stars.

gr-qc↗

arXiv2020

FRB 181112 as a Rapidly-Rotating Massive Neutron Star just after a Binary Neutron Star Merger?: Implications for Future Constraints on Neutron Star Equations of State

The light curve of the fast radio burst (FRB) 181112 is resolved into four successive pulses, and the time interval ($\sim0.8$ ms) between the first and third pulses coincides with that between the second and fourth pulses, which can be interpreted as a neutron star (NS) spinning at a period of about $0.8$ ms. Although this period is shorter than the most rapidly rotating pulsar currently known ($1.4$ ms), it is typical for a simulated massive NS formed immediately after the coalescence of binary neutron stars (BNS). Therefore, a BNS merger is a good candidate for the origin of this FRB if the periodicity is real. We discuss the future implications that can be obtained if such a periodicity is detected from FRBs simultaneously with gravitational waves (GW). The remnant spin period $P_{\rm rem}$ inferred from the FRB observation is unique information which is not readily obtained by current GW observations at the post-merger phase. If combined with the mass of the merger remnant $M_{\rm rem}$ inferred from GW data, it would set a new constraint on the equation of state of nuclear matter. Furthermore, the post-merger quantity $P_{\rm rem}/M_{\rm rem}$, or the tidal deformability of the merger remnant, is closely related to the binary tidal deformability parameter $Λ$ of NSs before they merge, and a joint FRB-GW observation will establish a new limit on $Λ$. Thus, if $Λ$ is also well measured by GW data, a comparison between these two will provide further insights into the nature of nuclear matter and BNS mergers.

astro-ph.HE↗

arXiv2020

Reanalysis of the binary neutron star mergers GW170817 and GW190425 using numerical-relativity calibrated waveform models

We reanalyze gravitational waves from binary-neutron-star mergers GW170817 and GW190425 using a numerical-relativity (NR) calibrated waveform model, the TF2+_Kyoto model, which includes nonlinear tidal terms. For GW170817, by imposing a uniform prior on the binary tidal deformability $\tildeΛ$, the symmetric $90\%$ credible interval of $\tildeΛ$ is estimated to be $481^{+436}_{-359}$ and $402^{+465}_{-279}$ for the case of $f_\mathrm{max}=1000$ and $2048~\mathrm{Hz}$, respectively, where $f_\mathrm{max}$ 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 $\tildeΛ$ are broadly consistent among various waveform models, our results indicate that estimates of $\tildeΛ$ 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 $\tildeΛ$ as the number of detectors and events increase and sensitivities of detectors are improved.

gr-qc↗

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

Post-merger Mass Ejection of Low-mass Binary Neutron Stars

We study the post-merger mass ejection of low-mass binary neutron stars (NSs) with the system mass of $2.5\, M_\odot$, and 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 $\sim 0.06$-$0.1\, M_\odot$ (depending on the EOS), being $\sim 30\%$ of the initial disk masses ($\sim 0.15$-$0.3\, M_\odot$). Post-processing nucleosynthesis calculations indicate that the ejecta is composed mainly of light $r$-process nuclei with small amounts of lanthanides (mass fraction $\sim 0.002$-$0.004$) and heavier species due to the modest average electron fraction ($\sim 0.32$-$0.34$) for a reasonable value of the viscous coefficient. Such abundance distributions are incompatible with the solar $r$-process-like abundance patterns found in all measured $r$-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, or there is an efficient mass ejection working in the early post-merger phase, the electron fraction of the ejecta could be low enough that the solar $r$-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.

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

Jet Propagation in Neutron Star Mergers and GW170817

The gravitational wave event from the binary neutron star (BNS) merger GW170817 and the following multi-messenger observations present strong evidence for i) merger ejecta expanding with substantial velocities and ii) a relativistic jet which had to propagate through the merger ejecta. The ejecta's expansion velocity is not negligible for the jet head motion, which is a fundamental difference from the other systems like collapsars and active galactic nuclei. Here we present an analytic model of the jet propagation in an expanding medium. In particular, we notice a new term in the expression of the breakout time and velocity. In parallel, we perform a series of over a hundred 2D numerical simulations of jet propagation. The BNS merger ejecta is prepared based on numerical relativity simulations of a BNS merger with the highest-resolution to date. We show that our analytic results agree with numerical simulations over a wide parameter space. Then we apply our analytic model to GW170817, and obtain two solid constraints on: i) the central engine luminosity as $L_{iso,0} \sim 3\times10^{49}-2.5\times10^{52}$ erg s$^{-1}$, and on ii) the delay time between the merger and engine activation $t_0-t_m < 1.3$ s. The engine power implies that the apparently-faint \textit{short} gamma-ray burst (\textit{s}GRB) \textit{s}GRB 170817A is similar to typical \textit{s}GRBs if observed on-axis.

astro-ph.HE↗

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

Constraint on the maximum mass of neutron stars using GW170817 event

We revisit the constraint on the maximum mass of cold spherical neutron stars coming from the observational results of GW170817. We develop a new framework for the analysis by employing both energy and angular momentum conservation laws as well as solid results of latest numerical-relativity simulations and of neutron stars in equilibrium. The new analysis shows that the maximum mass of cold spherical neutron stars can be only weakly constrained as $M_{\rm max} \alt 2.3M_\odot$. Our present result illustrates that the merger remnant neutron star at the onset of collapse to a black hole is not necessarily rapidly rotating and shows that we have to take into account the angular momentum conservation law to impose the constraint on the maximum mass of neutron stars.

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↗

arXiv2019

Nonthermal afterglow of the binary neutron star merger GW170817: a more natural modeling of electron energy distribution leads to a qualitatively different new solution

The observed nonthermal afterglow spectrum of the binary neutron star (BNS) merger GW170817 from radio to X-ray are consistent with synchrotron radiation by shock-accelerated electrons. However, previous afterglow modeling studies were based on a simplified assumption that the acceleration efficiency is extremely high, i.e. all electrons in the shock are accelerated as a nonthermal population. This affects the estimate of the minimum electron energy and hence $ν_m$, the peak frequency of the afterglow spectrum. Here we present Bayesian fitting to the observed data with a more natural electron energy distribution, in which the acceleration efficiency is a free parameter. Interestingly, the maximum likelihood solutions are found with radio flux below $ν_m$ in the early phase, in contrast to previous studies that found the radio frequency always above $ν_m$. Therefore the $ν_m$ passage through the radio band could have been clearly detected for GW170817, if sufficient low-frequency radio data had been taken in early time. In the new solutions, the lowest energy of electrons is found close to equipartition with the post shock protons, but only a small fraction ($<$10\%) of electrons are accelerated as nonthermal particles. The jet energy and interstellar medium density are increased by 1--2 orders of magnitude from the conventional modeling, though these are still consistent with other constraints. We encourage to take densely sampled low-frequency radio data in the early phase for future BNS merger events, which would potentially detect $ν_m$ passage and give a strong constraint on electron energy distribution and particle acceleration efficiency.

astro-ph.HE↗

arXiv2018

Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-Radiation Hydrodynamics Study

We perform long-term general relativistic neutrino radiation hydrodynamics simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a torus, which is a canonical remnant formed after the binary neutron star merger. We take into account the effects of viscosity, which is likely to arise in the merger remnant due to magnetohydrodynamical turbulence. As the initial condition, we employ the azimuthally averaged data of the MNS-torus system derived in a three-dimensional, numerical-relativity simulation for the binary neutron star merger. The viscous effect plays key roles for the remnant evolution and mass ejection from it in two phases of the evolution. In the first $t\lesssim10$ ms, a differential rotation state of the MNS is changed to a rigidly rotating state, and as a result, a sound wave, which subsequently becomes a shock wave, is formed in the vicinity of the MNS due to the variation of the quasi-equilibrium state of the MNS. The shock wave induces significant mass ejection of mass $\sim(0.5-2.0)\times 10^{-2}M_\odot$ for the alpha viscosity parameter of $0.01-0.04$. For the longer-term evolution with $\sim 0.1-10$ s, a significant fraction of the torus material is ejected. The ejecta mass is likely to be of order $10^{-2}M_\odot$, so that the total mass of the viscosity-driven ejecta could dominate that of the dynamical ejecta of mass $\lesssim 10^{-2}M_\odot$. The electron fraction, $Y_e$, of the ejecta is always high enough ($Y_e\gtrsim0.25$) that this post-merger ejecta is lanthanide-poor; hence, the opacity of the ejecta is likely to be $\sim 10-100$ times lower than that of the dynamical ejecta. This indicates that the electromagnetic signal from the ejecta would be rapidly evolving, bright, and blue if it is observed from a small viewing angle ($\lesssim 45^\circ$) for which the effect of the dynamical ejecta is minor.

astro-ph.HE↗

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↗