arXiv subjects

Masaru Shibata

Publications and source records attributed to Masaru Shibata.

At least 19 recordsLinked to original sources

arXiv2026

Gravitational waves from core collapse of rotating very-massive stars: 3D numerical relativity computation

We numerically study the collapse of rotating, very-massive stellar cores with masses of $\approx 200$, $300$, $500$, and $1100M_\odot$ into black holes using both axisymmetric and three-dimensional (3D) numerical relativity. Our results indicate that when the dimensionless spin of the resulting black hole exceeds 0.8, a massive disk consistently forms around it. These massive, compact disks, carrying more than about 10\% of the black hole's mass, are prone to non-axisymmetric deformations that trigger gravitational-wave bursts with frequencies around 10-50 Hz. Such waves could be detected by the Einstein Telescope and Cosmic Explorer, even from sources a few Gpc away. We also summarize the gravitational-wave signals from axisymmetric collapse, which tend to have lower amplitudes and higher frequencies than those caused by non-axisymmetric instabilities.

gr-qc↗

arXiv2026

ANRe-M1: a GPU-accelerated numerical relativity code with multi-energy M1 neutrino transport

We present ANRe-M1 (Accelerated Numerical Relativity code with multi-energy M1 neutrino transport), a performance-portable, GPU-accelerated code for multidimensional core-collapse supernova simulations in full general relativity. ANRe-M1 builds on our original CPU-based Fortran framework and is implemented in C++ using Kokkos. It retains the underlying general-relativistic neutrino-radiation-hydrodynamics algorithms while redesigning the data layout, parallel decomposition, and memory-access patterns for accelerator-resident execution. The spacetime sector employs the BSSN--Z4c formulation and is coupled to relativistic hydrodynamics and energy-dependent M1 neutrino transport. We validate the implementation through a suite of hydrodynamic, radiation-transport, and dynamical-spacetime tests, together with comparisons to results from an earlier code-comparison study. We further demonstrate its applicability with a representative three-dimensional stellar-collapse computation including multi-energy neutrino transport. For the benchmark and computing systems considered here, ANRe-M1 runs approximately an order of magnitude faster on a cluster of AMD MI300A accelerated processing units than the Fortran version on a CPU cluster. When the allocation is increased from 2 to 128 MI300A APUs, the aggregate throughput rises by a factor of 39.7, corresponding to a weak-scaling efficiency of 62 per cent relative to the two-APU case. These results establish ANRe-M1 as an efficient and portable framework for long-term, multidimensional supernova simulations on current and forthcoming accelerator-based supercomputers.

astro-ph.HE↗

arXiv2026

A first-principles binary neutron star merger model of GW170817, GRB170817A, and AT2017gfo

The multimessenger observation of the binary neutron star merger event GW170817, associated with its electromagnetic counterparts GRB170817A and AT2017gfo, marked a milestone in astrophysics, yet its unified physical explanation remains elusive. We conduct an end-to-end simulation based on a first-principles general-relativistic magnetohydrodynamics neutrino-radiation transfer merger simulation, followed by nucleosynthesis calculations and photon radiative transfer to generate kilonova light curves. We show that the large-scale dynamo simultaneously produces a relativistic jet with an isotropic-equivalent luminosity of $\sim 10^{51}~{\rm erg~s^{-1}}$ and $\approx 0.08M_\odot$ of neutron-rich ejecta, reproducing the GRB170817A afterglow and the AT2017gfo kilonova light curves. Our results establish a unified first-principles framework for interpreting binary neutron star mergers across gravitational wave, gamma-ray burst, and kilonova observations.

astro-ph.HE↗

arXiv2026

Nucleosynthesis in the Accretion-induced Collapse of magnetised, Rotating White Dwarfs

Accretion-induced collapse (AIC) of a rotating oxygen-neon-magnesium white dwarf (WD) is an alternative channel of neutron-star formation, in which a mass-accreting star near the Chandrasekhar mass collapses instead of being disrupted thermonuclearly. If the progenitor WD is sufficiently magnetised, it can drive magnetorotational outflows analogous to those of magnetorotational supernovae and may serve as a site for rapid neutron-capture ($r$-process) nucleosynthesis. We post-process tracer particles from five three-dimensional general-relativistic neutrino-(magneto)hydrodynamic AIC simulations, spanning four initial rotation rates, with the WinNet nuclear reaction network. Ejecta mass, the fraction of matter reaching nuclear statistical equilibrium, and the mass ejected beyond the iron group all increase monotonically with rotation rate, with magnetic fields amplifying these trends. The slowest-rotating model synthesises essentially only iron-group material, while the most rapidly rotating and magnetised model reaches the second $r$-process abundance peak at mass number $A \approx 130$, with a trace, non-robust signal up to $A \approx 244$. The corresponding $^{56}$Ni mass rises from $0.007$ to $0.014 M_\odot$, lower than representative yields for core-collapse supernovae and hypernovae by factors of five to more than forty, implying a correspondingly faint radioactively powered component. Contrary to the picture established for magnetorotationally driven supernovae, the heaviest ejecta do not track the polar outflow: they instead reside in equatorial-to-mid-latitude, moderate-entropy lobes, while the polar column is itself a local minimum in some heavy elements. We conclude that rapidly rotating, magnetised AIC events could contribute to the Galactic yields of trans-iron and weak $r$-process nuclei, although none of our models produces a robust third $r$-process peak.

astro-ph.HE↗

arXiv2026

Revisiting the Growth Rate of the Relativistic Tearing Instability: The Role of the Non-ideal MHD Structure

Magnetic reconnection in magnetically dominated pair plasmas is a key process in high-energy astrophysical systems. We revisit the relativistic tearing instability in a Harris current sheet and derive an improved analytical expression for its linear growth rate and the most unstable wavenumber. The key modification is the treatment of the vector potential perturbation in the non-ideal magnetohydrodynamic (MHD) region. Instead of the conventional constant-A approximation, we use an extrapolated-A approximation, in which the ideal-MHD solution is linearly extrapolated into the non-ideal region. Comparison with two-dimensional particle-in-cell simulations shows that the revised theory improves the prediction of the most unstable wavenumber. The improvement is most pronounced at low particle drift velocities, where the particle gyroradius is smaller than the current-sheet thickness and the fastest-growing mode shifts to longer wavelength. The resulting analytical expressions provide an updated benchmark for magnetically dominated reconnection and its applications to high-energy astrophysical plasmas, including gamma-ray bursts and fast radio bursts.

physics.plasm-ph↗

arXiv2026

Explosions from Rotating Very Massive Star Collapses to Black Holes: Effects of Nuclear Burning

We investigate the collapse of rotating very massive and supermassive stellar cores using numerical relativity simulations including an alpha-chain nuclear reaction network and neutrino cooling. Our main survey focuses on newly constructed models with initial core masses of $2 \times {10}^{3}-5\times 10^4M_\odot$. The collapse is triggered either by pair instability in lower-mass cores or by general-relativistic instability in higher-mass cores. We find that higher-mass cores undergo a nearly homologous collapse, whereas lower-mass cores show a more runaway-like collapse because neutrino cooling becomes more efficient at their higher densities and temperatures. Consequently, the black hole formed in lower-mass models initially contains a smaller fraction of the core mass, and disk formation occurs while a larger amount of matter remains outside the black hole. The lower compactness of pair-unstable cores also allows larger dimensionless angular momentum, favoring the formation of rapidly rotating black holes and massive disks. The disk bounce drives mass ejection with ejecta masses of order $10-10^3M_\odot$ and kinetic energies of order $10^{53}-10^{55}\,\mathrm{erg}$. Significant $^{56}$Ni production in the disk-bounce ejecta occurs only in the lowest-mass models. For selected models, we further follow the viscous evolution of the disk and find that viscosity enhances the ejecta mass and kinetic energy. In models with $\lesssim10^4M_\odot$, the viscosity-driven ejecta can originate from disk matter that has reached nuclear statistical equilibrium and can therefore become rich in $^{56}$Ni. These results suggest that rotating very massive star collapses can produce massive, energetic ejecta and, for sufficiently low core masses, substantial iron-group elements.

astro-ph.HE↗

arXiv2026

SACRA-K: A Performance-Portable Numerical Relativity Code with Kokkos

We present SACRA-K (SimulAtor for Compact objects in Relativistic Astrophysics with Kokkos), a performance-portable numerical relativity code ported from the Fortran code SACRA-MPI to C++ with the Kokkos library. SACRA-K retains the physics and numerical methods of SACRA-MPI, namely a Baumgarte-Shapiro-Shibata-Nakamura (BSSN) spacetime evolution with Z4c constraint propagation and a box-in-box Berger-Oliger adaptive mesh refinement, together with the high resolution shock capturing scheme for the general relativistic hydrodynamics from NANASI, while gaining cross vendor portability. We validate the port against SACRA-MPI across binary black hole, black hole neutron star, and binary neutron star configurations. Across these tests, the waveform discrepancies are well below both the scatter among independent numerical relativity codes and the resolution dependent variation within a given code, and remain at or below the distinguishability threshold of current gravitational-wave detectors; the $\pi$ symmetry is preserved exactly at the bitwise level; and the gravitational wave phase of the binary neutron star merger exhibits second order convergence. In the smallest test configuration, SACRA-K runs about an order of magnitude faster on the NVIDIA A100 GPU cluster or the AMD MI300A APU cluster than the Fortran SACRA-MPI on the CPU cluster, and we measure its scaling up to 256 accelerator devices.

astro-ph.HE↗

arXiv2026

Subsolar-mass binary mergers of strange stars and neutron stars: gravitational waves and ejecta

We present the first numerical-relativity simulations of subsolar-mass binary strange star (SS) mergers and compare with binary neutron star (NS) mergers across equations of state, masses, and mass ratios. The self-bound nature of SSs makes them less deformed during the inspiral and keeps a sharp surface up to contact, driving strong shock heating and a large radial bounce that are far weaker in the NS. The more compact SS thus reaches a higher gravitational-wave cutoff frequency $f_\mathrm{cut}$ before contact but a lower post-merger peak frequency $f_2$. Within each class these frequencies follow quasi-universal relations with the tidal deformability, and their ratio $f_2/f_\mathrm{cut}$ cleanly separates the two classes. Both classes can eject $\sim10^{-2}\,M_\odot$ of material, neutron-rich for the NS and decompressed quark matter for the SS, a potential source of an electromagnetic counterpart whose observation could test the SS and NS hypotheses for subsolar-mass events.

astro-ph.HE↗

arXiv2026

A magnetar formation in binary neutron star merger

We conduct a global general relativistic neutrino-radiation-transfer magnetohydrodynamics simulation of a $1.35$-$1.35M_\odot$ binary neutron star with the unprecedented spatial resolution of $6.25$\,m on the Japanese supercomputer FUGAKU. The total consumed CPU time is $\approx 530$ million core hours. We initialize the binary neutron star's magnetic field to be $3.16\times 10^{12}$~G at maximum, which is compatible with the upper end of the observed binary pulsars. We demonstrate that the Kelvin-Helmholtz instability that emerges when the two neutron stars touch amplifies the magnetic field to an expected electromagnetic saturation energy of $\sim 10^{50}$~erg within $3$~ms after the merger. The spectral analysis indicates that the Kazantsev and Kolmogorov spectra are reproduced in the magnetic and kinetic power spectral densities, respectively. We also find that it induces stellar-scale magnetic field amplification by at least a factor of $316$. We conclude that a magnetar may form at least temporarily following neutron star mergers in a few ms.

astro-ph.HE↗

arXiv2026

Mass and Spin Growth of Very Massive Stars in Star Clusters Potentially Associated with Little Red Dots

Using gravitational $N$-body simulations, we investigate the evolution of mass and spin for very massive stars (VMSs) in dense star clusters, which could subsequently evolve into Little Red Dots (LRDs). Our results show that VMS masses can reach $10^3$--$10^4\,M_\odot$, depending on the initial conditions of the host clusters. Notably, the VMS mass increases by up to a factor of three when accounting for the bloated state at the Hayashi track induced by stellar collisions, provided that this state is maintained at accretion rates exceeding $3 \times 10^{-2}\,M_\odot\,{\rm yr}^{-1}$. In all cases, the spin of the VMS, when normalized to the dimensionless black hole (BH) spin parameter, exceeds $10$, although the mass and spin of VMSs after the post-main sequence phase could be decreased by the stellar evolution process. We nonetheless demonstrate that VMSs formed in dense star clusters can be highly spinning. Such a rapidly spinning VMS is expected to collapse into an intermediate-mass BH surrounded by a massive accretion disk. This BH-disk system could trigger powerful explosions and emit burst gravitational waves, similar to those observed in GW190521 and GW231123, for which the remnant BH masses are estimated to be $\gtrsim 100\,M_\odot$.

astro-ph.HE↗

arXiv2026

A formation scenario of black hole-envelope systems --viscous hydrodynamics simulation in general relativity--

By performing a viscous hydrodynamics simulation in general relativity for super-Eddington accretion flows onto massive black holes of mass $M=10^5$--$10^7M_\odot$, we discuss a formation scenario for black hole-envelope systems. We consider the mass accretion rate of $a^3/G \approx 1.5 \times 10^{25} (a/10\,\mathrm{km\,s^{-1}})^3$\,g/s, comparable to the Eddington mass accretion rate of a $10^7M_\odot$ black hole, assuming that the gas temperature of the infalling matter is $\lesssim 10^4$\,K. Here, $a$ and $G$ denote the sound speed and gravitational constant. For the accretion flow, we set up a quasi-spherical Bondi-type flow in which radial inflow dominates over angular momentum in the distant region. It is found that (i) for low-mass black holes with $M \lesssim 10^6M_\odot$, a photon-trapped region forms in the inner region, and a significant viscous outflow driven near the polar region overcomes the ram pressure of the mass inflow, leading to an inflow-outflow structure; (ii) for massive black holes of $M \gtrsim 3 \times 10^6M_\odot$, the outflow is not launched, and a convective envelope around the black hole gradually develops; and (iii) irrespective of the black-hole mass, the mass accretion rate onto the black hole is of order 10\% of the Eddington accretion rate for reasonable values of the viscous coefficient. As the mass accretion rate onto the black holes is much lower than the mass growth rate of the envelope for low-mass black holes with $M\lesssim 10^6M_\odot$, the envelope mass is likely to increase until the total viscous heating rate exceeds the Eddington luminosity of the system, if the mass accretion rate is preserved to be high for $\gtrsim 10^8 (M/10^7M_\odot)$\,yrs.

astro-ph.HE↗

arXiv2026

Accretion-powered flares from black hole-disk collisions in galactic nuclei

Black hole impacts on accretion disks in galactic nuclei can power luminous transients, but predicting their observable signatures is challenging because the post-collision flow is highly time-dependent and inhomogeneous. We present a radiative post-processing framework for relativistic hydrodynamics simulations of black hole-disk collisions. Using physically motivated prescriptions for shock heating, optical depth via an eikonal solver, and photon escape fractions that account for advection trapping and diffusion, we predict light curves and spectral energy distributions over a range of disk densities and collision velocities. Our results indicate that the emission is dominated by the long-lived, highly super-Eddington accretion flow onto the secondary black hole, rather than by cooling of the unbound ejecta. In the parameter range explored, the luminosity can reach several times the Eddington luminosity of the secondary, and the emission is generically dominated by soft X-rays. We find that lower velocity collisions produce brighter flares, while the disk surface density mainly controls spectral evolution: low-density disks typically produce keV-peaked flares with weak spectral evolution, whereas high-density disks show softer early emission and late-time hardening. A depletion-time estimate calibrated to our results suggests characteristic durations of hours to days for intermediate-mass secondaries, and yields $t_{\rm flare} \propto P_{\rm QPE}$. We discuss implications for QPE-like transients and for the SMBH-binary candidate OJ 287.

astro-ph.HE↗

arXiv2026

The hydrodynamics of stratified ultra-relativistic outflows and the origin of GRB X-ray plateaus

The origin of the X-ray plateau phase observed in a large fraction of gamma-ray burst afterglows remains debated. We present a novel analytic framework for the hydrodynamics of ultra-relativistic, radially stratified outflows interacting with an external medium. By explicitly accounting for a continuous distribution of Lorentz factors within the ejecta, we derive analytic expressions describing the evolution of a long-lived, mildly relativistic reverse shock and determine its crossing time. Then, we compute the resulting synchrotron emission from both the forward and reverse shocks. The forward shock naturally produces a shallow, long-lasting X-ray decay consistent with the observed properties of X-ray plateaus, including the Dainotti relation, without requiring prolonged central-engine activity or an additional high-energy emission component. We further show that reproducing the observed plateau durations requires a broad distribution of ejecta Lorentz factors, extending down to $\gamma_\text{min}\sim70-100$, consistent with the ultra-relativistic outflow that powers the prompt $\gamma$-ray emission. The reverse shock generates a long-lived millimeter emission component that outshines the forward shock emission at these wavelengths. Both the plateau and reverse shock emission terminate smoothly once the slowest ejecta are processed, marking a transition to the standard Blandford-McKee self-similar evolution. Such stratified outflows are expected on physical grounds, as the ultra-relativistic ejecta responsible for the prompt $\gamma$-ray emission are unlikely to be launched with a single Lorentz factor. This model provides a unified picture in which the same outflow powers the prompt emission, the X-ray plateau, and the subsequent afterglow evolution.

astro-ph.HE↗

arXiv2025

Collapse of rotating very massive stellar cores leading to a black hole and a massive disk as a source of gravitational waves

We derive models of rotating very massive stellar cores with mass $\approx 10^2$--$10^4M_\odot$ which are marginally stable to the pair-unstable collapse, assuming that the core is isentropic and composed primarily of oxygen. It is shown that the cores with mass $\lesssim 10^3M_\odot$ can form a massive disk with the mass more than 10% of the core mass around the formed black hole if the core is rotating with more than 30% of the Keplerian limit. We also indicate that the formation of rapidly spinning massive black holes such as the black holes of GW231123 naturally accompanies the massive disk formation. By using the result of our previous study which showed that the massive disk is unstable to the non-axisymmetric deformation, we predict the amplitude and frequency of gravitational waves and show that the collapse of rotating very massive stellar cores can be a promising source of gravitational waves for Einstein Telescope. The detection of such gravitational waves will provide us with important information about a formation process of intermediate mass black holes.

astro-ph.HE↗

arXiv2025

Nonradial oscillations of stratified neutron stars with solid crusts: Mode characterization and tidal resonances in coalescing binaries

Dynamical tides of neutron stars in the late stages of binary inspirals provide a viable probe into dense matter through gravitational waves, and potentially trigger electromagnetic precursors. We model the tidal response as a set of driven harmonic oscillators, where the natural frequencies are given by the quasinormal modes of a nonrotating neutron star. These modes are calculated in general relativity by applying linear perturbation theory to stellar models that include a solid crust and compositional stratification. For the mode spectrum, we find that the canonical interface mode associated with the crust-core boundary vanishes in stratified neutron stars and is replaced by compositional gravity modes with mixed gravity-interfacial character, driven primarily by strong buoyancy in the outer core. We also find that fluid modes such as the core gravity mode and the fundamental mode can penetrate the crust, and we establish a criterion for such penetration. Regarding the tidal interaction, we find that transfer of binding energy to oscillations is dominated by the fundamental mode despite its frequency being too high to resonate with the tidal forcing. In general, we find that lower-frequency modes induce gravitational-wave phase shifts smaller than $\sim 10^{-3},\rm rad$ for the equation of state we consider. We discover that nonresonant fundamental and crustal shear modes can trigger crust breaking already near the first gravity-mode resonance, while gravity-mode resonance concentrates strain at the base of the crust and may marginally crack it. These results suggest that both resonant and nonresonant excitations can overstress the crust and may channel energy into the magnetosphere prior to merger, potentially powering electromagnetic precursors. Our work represents an important step toward realistic modeling of dynamical tides of neutron stars in multimessenger observations.

astro-ph.HE↗

arXiv2025

Confined Circumstellar Material as a Dust Formation Site in Type II Supernovae

We propose a model for dust formation in Type II supernovae (SNe) interacting with confined circumstellar material (CSM), motivated by recent time-domain surveys that have revealed a substantial fraction of SN progenitors to be surrounded by CSM ejected shortly before core-collapse. We simulate the pre-SN mass eruption and the resulting confined CSM using the open-source code CHIPS, and follow the subsequent evolution of the SN ejecta and its interaction with the CSM. We show that a cold dense shell (CDS) is formed at the radiative shock under a wide range of conditions and later undergoes rapid adiabatic cooling during free expansion, leading to efficient dust condensation. The resulting dust mass ranges from $\sim10^{-3}\,M_\odot$ to $0.1\,M_\odot$, depending on the mass and spatial extent of the CSM. We further calculate the infrared (IR) emission from the newly formed dust and find broad consistency with observations of SN~1998S. Notably, the IR light curve exhibits a rapid rise within $\lesssim10\,{\rm d}$, closely resembling that of kilonovae (KNe). This suggests that dust emission powered by confined CSM interaction may be also discovered in KN searches. Moreover, the high-density environment of the CDS may allow dust grains to grow to larger sizes, enhancing their survivability against destruction by reverse shocks propagating from the interstellar medium at later times.

astro-ph.SR↗

arXiv2025

Signatures of Exploding Supermassive PopIII Stars at High Redshift in JWST, EUCLID and Roman Space Telescope

Recently discovered supermassive black holes with masses of $\sim10^8\,M_\odot$ at redshifts $z\sim9$-$11$ in active galactic nuclei (AGN) pose severe challenges to our understanding of supermassive black hole formation. One proposed channel are rapidly accreting supermassive PopIII stars (SMSs) that form in large primordial gas halos and grow up to $<10^6\,M_\odot$. They eventually collapse due to the general relativistic instability and could lead to supernova-like explosions. This releases massive and energetic ejecta that then interact with the halo medium via an optically thick shock. We develop a semi-analytic model to compute the shock properties, bolometric luminosity, emission spectrum and photometry over time. The initial data is informed by stellar evolution and general relativistic SMS collapse simulations. We find that SMS explosion light curves reach a brightness $\sim10^{45\mathrm{-}47}\,\mathrm{erg/s}$ and last $10$-$200$ years in the source frame - up to $250$-$3000$ years with cosmic time dilation. This makes them quasi-persistent sources which vary indistinguishably to little red dots and AGN within $0.5$-$9\,(1+z)$ yrs. Bright SMS explosions are observable in long-wavelength JWST filters up to $z\leq20$ ($24$-$26$ mag) and pulsating SMSs up to $z\leq15$. EUCLID and the Roman space telescope (RST) can detect SMS explosions at $z<11$-$12$. Their deep fields could constrain the SMS rate down to $10^{-11}$Mpc$^{-3}$yr$^{-1}$, which is much deeper than JWST bounds. Based on cosmological simulations and observed star formation rates, we expect to image up to several hundred SMS explosions with EUCLID and dozens with RST deep fields.

astro-ph.HE↗

arXiv2025

Neutrino pair annihilation driven jets from black-hole torus systems

We perform axisymmetric general relativistic radiation-viscous hydrodynamics simulations of black hole (BH)-torus systems with full Boltzmann Monte-Carlo neutrino transport to investigate the role of neutrino-antineutrino pair annihilation in launching relativistic outflows. Our models span a wide range of BH spins, torus masses, and viscosity parameters. We find that the pair annihilation leads to the formation of relativistic fireballs in most cases, except for those with low black-hole spin and high viscosity. The isotropic-equivalent energies of these outflows reach $\lesssim 10^{51}\,{\rm erg}$ with durations $\lesssim 0.2\,{\rm s}$. While this is insufficient to explain the brightest short gamma-ray bursts (sGRBs), our results suggest that the pair annihilation may account for some low-luminosity sGRBs and GRB precursors. We also provide updated scaling relations for the pair annihilation energy deposition rate as a function of accretion rate, and discuss the sensitivity of outflow properties to numerical resolution and floor density.

astro-ph.HE↗