Masaru Shibata

We present the SimulAtor for Compact objects in Relativistic Astrophysics with Kokkos ( SACRA-K ), a performance-portable numerical relativity (NR) 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 (BNS) configurations. Across these tests, the waveform discrepancies are well below both the scatter among independent NR codes and the resolution dependent variation within a given code, and remain at or below the distinguishability threshold of current gravitational-wave (GW) detectors; the π symmetry is preserved exactly at the bitwise level; and the GW phase of the BNS 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.

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 _⊙ , depending on the initial conditions of the host clusters. Notably, the VMS mass increases by up to a factor of 3 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 × 10 ⁻² M _⊙ 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 ≳100 M _⊙ .

We perform fully general-relativistic hydrodynamics simulations of binary neutron star mergers over 100 ms postmerger to investigate the dynamics of remnant massive neutron stars (NSs). Our focus is mainly on the analysis of convective stability and mode characteristics of the massive NSs. We derive stability criteria for hot, differentially rotating relativistic stars that account for both buoyant and rotational restoring forces, and apply them for the first time to the postmerger massive NSs. Our results show no evidence of large-scale convective instability, as both angle-averaged specific entropy and specific angular momentum increase outward within the massive NSs. Rotational effects significantly enhance stability for local regions that would be otherwise unstable by the Schwarzschild criterion. Additionally, our mode analysis of matter fields and gravitational waves reveals no excitation of observable inertial modes after the damping of quadrupolar f -modes in the massive NSs, contrasting with previous studies. As in many previous works, we observe the excitation of an m = 1 one-armed mode. However, we also find that the growth of the m = 1 mode amplitude after the merger may correlate strongly with the violation of linear momentum conservation, indicating that we cannot reject the possibility that the excitation of the one-armed mode has a numerical origin.

We derive models of rotating very massive stellar cores with mass ≈10 ² –10 ⁴ M _⊙ that are marginally stable to the pair-unstable collapse, assuming that the core is isentropic and composed primarily of oxygen. It is shown that cores with mass ≲10 ³ M _⊙ 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 massive disk formation. By using the result of our previous study, which showed that the massive disk is unstable to the nonaxisymmetric 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 the Einstein Telescope. The detection of such gravitational waves will provide us with important information about the formation process of intermediate mass black holes.

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 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 ∼ 10 − 3 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.

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 ∼10⁻³ M _⊙ to 0.1 M _⊙, 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 ≲10 days, 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.