arXiv subjects

Kenta Kiuchi

Publications and source records attributed to Kenta Kiuchi.

At least 73 records · Page 4Linked to original sources

arXiv2018

Synchrotron radiation from the fast tail of dynamical ejecta of neutron star mergers

We find, using high resolution numerical relativistic simulations, that the tail of the dynamical ejecta of neutron star mergers extends to mildly relativistic velocities faster than $0.7c$. The kinetic energy of this fast tail is $\sim 10^{47}$--$10^{49}$ erg, depending on the neutron star equation of state and on the binary masses. The synchrotron flare arising from the interaction of this fast tail with the surrounding ISM can power the observed non-thermal emission that followed GW170817, provided that the ISM density is $\sim 10^{-2}\,{\rm cm^{-3}}$, the two neutron stars had roughly equal masses and the neutron star equation of state is soft (small neutron star radii). One of the generic predictions of this scenario is that the cooling frequency crosses the X-ray band on a time scale of a few months to a year, leading to a cooling break in the X-ray light curve. If this dynamical ejecta scenario is correct, we expect that the synchrotron radio flare from the ejecta that have produced the macronova/kilonova emission will be observable on time scales of $10^3$ to $10^5$ days. Further multi-frequency observations will confirm or rule out this dynamical ejecta scenario.

astro-ph.HE↗

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

Repeating and Non-repeating Fast Radio Bursts from Binary Neutron Star Mergers

Most of fast radio bursts (FRB) do not show evidence for repetition, and such non-repeating FRBs may be produced at the time of a merger of binary neutron stars (BNS), provided that the BNS merger rate is close to the high end of the currently possible range. However, the merger environment is polluted by dynamical ejecta, which may prohibit the radio signal to propagate. We examine this by using a general-relativistic simulation of a BNS merger, and show that the ejecta appears about 1 ms after the rotation speed of the merged star becomes the maximum. Therefore there is a time window in which an FRB signal can reach outside, and the short duration of non-repeating FRBs can be explained by screening after ejecta formation. A fraction of BNS mergers may leave a rapidly rotating and stable neutron star, and such objects may be the origin of repeating FRBs like FRB 121102. We show that a merger remnant would appear as a repeating FRB in a time scale of about 1-10 yrs, and expected properties are consistent with the observations of FRB 121102. We construct an FRB rate evolution model including these two populations of repeating and non-repeating FRBs from BNS mergers, and show that the detection rate of repeating FRBs relative to non-repeating ones rapidly increases with improving search sensitivity. This may explain that the only repeating FRB 121102 was discovered by the most sensitive FRB search with Arecibo. Several predictions are made, including appearance of a repeating FRB 1-10 years after a BNS merger that is localized by gravitational wave and subsequent electromagnetic radiation.

astro-ph.HE↗

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

High-Energy Neutrino Emission from Short Gamma-Ray Bursts: Prospects for Coincident Detection with Gravitational Waves

We investigate current and future prospects for coincident detection of high-energy neutrinos and gravitational waves (GWs). Short gamma-ray bursts (SGRBs) are believed to originate from mergers of compact star binaries involving neutron stars. We estimate high-energy neutrino fluences from prompt emission, extended emission, X-ray flares, and plateau emission, and show that neutrino signals associated with the extended emission are the most promising. Assuming that the cosmic-ray loading factor is $\sim10$ and the Lorentz factor distribution is lognormal, we calculate the probability of neutrino detection from extended emission by current and future neutrino detectors, and find that the quasi-simultaneous detection of high-energy neutrinos, gamma rays, and GWs is possible with future instruments or even with current instruments for nearby SGRBs having extended emission. We also discuss stacking analyses that will also be useful with future experiments such as IceCube-Gen2.

astro-ph.HE↗

arXiv2017

Sub-radian-accuracy gravitational waveforms of coalescing binary neutron stars in numerical relativity

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 $\approx 63$--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 $\gtrsim 210$\,rad in the last $\sim 15$--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 $\approx 3$\,ms before the peak of the gravitational-wave amplitude is reached: For this late inspiral stage, the total phase error is $\lesssim 0.1$\,rad. Although the gravitational waveforms have an inspiral-type feature for the last $\sim 3$\,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.

astro-ph.HE↗

arXiv2017

Properties of Neutrino-driven Ejecta from the Remnant of Binary Neutron Star Merger : Purely Radiation Hydrodynamics Case

We performed general relativistic, long-term, axisymmetric neutrino radiation hydrodynamics simulations for the remnant formed after the binary neutron star merger, which consist of a massive neutron star and a torus surrounding it. As an initial condition, we employ the result derived in a three-dimensional, numerical relativity simulation for the binary neutron star merger. We investigate the properties of neutrino-driven ejecta. Due to the pair-annihilation heating, the dynamics of the neutrino-driven ejecta is significantly modified. The kinetic energy of the ejecta is about two times larger than that in the absence of the pair-annihilation heating. This suggests that the pair-annihilation heating plays an important role in the evolution of the merger remnants. The relativistic outflow, which is required for driving gamma-ray bursts, is not observed because the specific heating rate around the rotational axis is not sufficiently high due to the baryon loading caused by the neutrino-driven ejecta from the massive neutron star. We discuss the condition for launching the relativistic outflow and the nucleosynthesis in the ejecta.

astro-ph.HE↗

arXiv2017

Gravitational waves from remnant massive neutron stars of binary neutron star merger: Viscous hydrodynamics effects

Employing a simplified version of the Israel-Stewart formalism of general-relativistic shear-viscous hydrodynamics, we explore the evolution of a remnant massive neutron star of binary neutron star merger and pay special attention to the resulting gravitational waveforms. We find that for the plausible values of the so-called viscous alpha parameter of the order $10^{-2}$, the degree of the differential rotation in the remnant massive neutron star is significantly reduced in the viscous timescale, $\alt 5$\,ms. Associated with this, the degree of non-axisymmetric deformation is also reduced quickly, and as a consequence, the amplitude of quasi-periodic gravitational waves emitted also decays in the viscous timescale. Our results indicate that for modeling the evolution of the merger remnants of binary neutron stars, we would have to take into account magnetohydrodynamics effects, which in nature could provide the viscous effects.

astro-ph.HE↗

arXiv2017

General relativistic viscous hydrodynamics of differentially rotating neutron stars

Employing a simplified version of the Israel-Stewart formalism for general-relativistic shear-viscous hydrodynamics, we perform axisymmetric general-relativistic simulations for a rotating neutron star surrounded by a massive torus, which can be formed from differentially rotating stars. We show that with our choice of a shear-viscous hydrodynamics formalism, the simulations can be stably performed for a long time scale. We also demonstrate that with a possibly high shear-viscous coefficient, not only viscous angular momentum transport works but also an outflow could be driven from a hot envelope around the neutron star for a time scale $\gtrsim 100\,$ms with the ejecta mass $\gtrsim 10^{-2}M_\odot$ which is comparable to the typical mass for dynamical ejecta of binary neutron star mergers. This suggests that massive neutron stars surrounded by a massive torus, which are typical outcomes formed after the merger of binary neutron stars, could be the dominant source for providing neutron-rich ejecta, if the effective shear viscosity is sufficiently high, i.e., if the viscous $α$ parameter is $\gtrsim 10^{-2}$. The present numerical result indicates the importance of a future high-resolution magnetohydrodynamics simulation that is the unique approach to clarify the viscous effect in the merger remnants of binary neutron stars by the first-principle manner.

astro-ph.HE↗

arXiv2016

Dynamical mass ejection from the merger of asymmetric binary neutron stars: Radiation-hydrodynamics study in general relativity

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 the 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 the 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 soft EOS (SFHo), the total ejecta mass exceeds $0.01M_\odot$ irrespective of the mass ratio chosen in this paper. The ejecta have a variety of electron number per baryon with its average approximately between $Y_e \sim 0.2$ and $\sim 0.3$ irrespective of the EOS employed, which is well-suited for the production of the r-process heavy elements (second and third peaks), although its averaged value decreases with the increase of the degree of mass asymmetry.

astro-ph.HE↗

arXiv2016

Multi-Messenger Tests for Fast-Spinning Newborn Pulsars Embedded in Stripped-Envelope Supernovae

Fast-spinning strongly magnetized newborn neutron stars, including nascent magnetars, are popularly implemented as the engine of luminous stellar explosions. Here, we consider the scenario that they power various stripped-envelope supernovae, not only super-luminous supernovae Ic but also broad-line supernovae Ibc and possibly some ordinary supernovae Ibc. This scenario is also motivated by the hypothesis that Galactic magnetars largely originate from fast-spinning neutron stars as remnants of stripped-envelope supernovae. By consistently modeling the energy injection from magnetized wind and Ni decay, we show that proto-neutron stars with >~ 10 ms rotation and B_dip >~ 5 x 10^14 G can be harbored in ordinary supernovae Ibc. On the other hand, millisecond proto-neuton stars can solely power broad-line supernovae Ibc if they are born with poloidal magnetic field of B_dip >~ 5 x 10^14 G, and superluminous supernovae Ic with B_dip >~ 10^13 G. Then, we study how multi-messenger emission can be used to discriminate such pulsar-driven supernova models from other competitive scenarios. First, high-energy x-ray and gamma-ray emission from embryonic pulsar wind nebulae is a promising smoking gun of the underlying newborn pulsar wind. Follow-up observations of stripped-envelope supernovae using NuSTAR ~ 50-100 days after the explosion is strongly encouraged for nearby objects. We also discuss possible effects of gravitational-waves on the spin-down of proto-neutron stars. If millisecond proto-neutron stars with B_dip <~ a few x 10^13 G emit gravitational waves through e.g., non-axisymmetric rotation deformed by the inner toroidal fields of B_t >~ 10^16 G, the gravitational wave signal can be detectable from ordinary supernova Ibc in the Virgo cluster by Advanced LIGO, Advanced Virgo, and KAGRA.

astro-ph.HE↗

arXiv2015

Efficient magnetic-field amplification due to the Kelvin-Helmholtz instability in binary neutron star mergers

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 $\approx 10^3$. We also explore the saturation of the magnetic-field energy and our result shows that it is likely to be $\gtrsim 4 \times 10^{50}$ erg, which is $\gtrsim 0.1\%$ of the bulk kinetic energy of the merging binary neutron stars.

astro-ph.HE↗

arXiv2015

High-resolution magnetohydrodynamics simulation of black hole-neutron star merger: Mass ejection and short gamma-ray burst

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 turbulence-like motion induced by the non-axisymmetric 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.

astro-ph.HE↗

arXiv2015

Gamma-Ray and Hard X-Ray Emission from Pulsar-Aided Supernovae as a Probe of Particle Acceleration in Embryonic Pulsar Wind Nebulae

It has been suggested that some classes of luminous supernovae (SNe) and gamma-ray bursts (GRBs) are driven by newborn magnetars. Fast-rotating proto-neutron stars have also been of interest as potential sources of gravitational waves (GWs). We show that for a range of rotation periods and magnetic fields, hard X rays and GeV gamma rays provide us with a promising probe of pulsar-aided SNe. It is observationally known that young pulsar wind nebulae (PWNe) in the Milky Way are very efficient lepton accelerators. We argue that, if embryonic PWNe satisfy similar conditions at early stages of SNe (in ~1-10 months after the explosion), external inverse-Compton emission via upscatterings of SN photons is naturally expected in the GeV range as well as broadband synchrotron emission. To fully take into account the Klein-Nishina effect and two-photon annihilation process that are important at early times, we perform detailed calculations including electromagnetic cascades. Our results suggest that hard X-ray telescopes such as NuSTAR can observe such early PWN emission by followup observations in months-to-years. GeV gamma rays may also be detected by Fermi for nearby SNe, which serve as counterparts of these GW sources. Detecting the signals will give us an interesting probe of particle acceleration at early times of PWNe, as well as clues to driving mechanisms of luminous SNe and GRBs. Since the Bethe-Heitler cross section is lower than the Thomson cross section, gamma rays would allow us to study subphotospheric dissipation. We encourage searches for high-energy emission from nearby SNe, especially Type Ibc SNe including super-luminous objects.

astro-ph.HE↗

arXiv2015

The dynamical mass ejection from binary neutron star mergers: Radiation-hydrodynamics study in general relativity

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 $0.01M_{\odot}$. 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 $0.01M_{\odot}$, although broad electron-fraction distributions are achieved by the positron capture and the neutrino heating.

astro-ph.HE↗

arXiv2014

High resolution numerical-relativity simulations for the merger of binary magnetized neutron stars

We perform high-resolution magnetohydrodynamics simulations of binary neutron star mergers in numerical relativity on the Japanese supercomputer K. The neutron stars and merger remnants are covered by a grid spacing of 70\,m, which yields the highest-resolution results among those derived so far. By an in-depth resolution study, we clarify several amplification mechanisms of magnetic fields during the binary neutron star merger for the first time. First, the Kelvin-Helmholtz instability developed in the shear layer at the onset of the merger significantly amplifies the magnetic fields. A hypermassive neutron star (HMNS) formed after the merger is then subject to the nonaxisymmetric magnetorotational instability, which amplifies the magnetic field in the HMNS. These two amplification mechanisms cannot be found with insufficient-resolution runs. We also show that the HMNS eventually collapses to a black hole surrounded by an accretion torus which is strongly magnetized at birth.

astro-ph.HE↗

arXiv2014

Production of all the r-process nuclides in the dynamical ejecta of neutron star mergers

Recent studies suggest that binary neutron star (NS-NS) mergers robustly produce the heavy r-process nuclei above the atomic mass number A ~ 130 because of their ejecta consisting of almost pure neutrons (electron fraction of Y_e < 0.1). However, little production of the lighter r-process nuclei (A = 90-120) conflicts with the spectroscopic results of r-process-enhanced Galactic halo stars. We present, for the first time, the result of nucleosynthesis calculations based on the fully general-relativistic simulation of a NS-NS merger with approximate neutrino transport. It is found that the bulk of the dynamical ejecta are appreciably shock-heated and neutrino-processed, resulting in a wide range of Y_e (= 0.09-0.45). The mass-averaged abundance distribution of calculated nucleosynthesis yields is in reasonable agreement with the full-mass range (A = 90-240) of the solar r-process curve. This implies, if our model is representative of such events, that the dynamical ejecta of NS-NS mergers can be the origin of the Galactic r-process nuclei. Our result also shows that the radioactive heating after ~ 1 day from the merging, giving rise to r-process-powered transient emission, is dominated by the beta-decays of several species close to stability with precisely measured half-lives. This implies that the total radioactive heating rate for such an event can be well constrained within about a factor of two if the ejected material has a solar-like r-process pattern.

astro-ph.SR↗