Kenta Kiuchi

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 ${10}^{13}\text{ }\text{ }\mathrm{G}$ is amplified at least by a factor of $\ensuremath{\approx}{10}^{3}$. We also explore th

This article reports on the results of novel, extremely high-resolution simulations of the general relativistic magnetohydrodynamics (MHD) of neutron star mergers, focussing on angular momentum transport due to the MHD turbulence. The authors show that the Kelvin-Helmholtz instability at merger amplifies the magnetic energy to $\ensuremath{\sim}1%$ of the thermal energy.

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

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 fi

General relativistic simulations for the merger of binary neutron stars are performed as an extension of a previous work [M. Shibata and K. Taniguchi, Phys. Rev. D 73, 064027 (2006).]. We prepare binary neutron stars with a large initial orbital separation and employ the moving-puncture formulation, which enables one to follow merger and ringdown phases for a long time, even after black hole formation. For modeling inspiraling neutron stars, which should be composed of cold neutron stars, the Ak

Abstract We revisit the lower bound on binary tidal deformability 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 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

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 $\ensuremath{\approx}63--86$ 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 $\ensuremath{\gtrsim}210\text{ }\text{ }\mathrm{rad}$ in

Black hole (BH)-torus systems are promising candidates for the central engine of γ-ray bursts (GRBs), and also possible outcomes of the collapse of supermassive stars to supermassive black holes (SMBHs). By three-dimensional general relativistic numerical simulations, we show that an m = 1 nonaxisymmetric instability grows for a wide range of self-gravitating tori orbiting BHs. The resulting nonaxisymmetric structure persists for a time scale much longer than the dynamical one, becoming a strong

Abstract The merger of two neutron stars launches a relativistic jet, which must be driven by a strong large-scale magnetic field. However, the magnetohydrodynamical mechanism required to build up this magnetic field remains uncertain. By performing an ab initio super-high-resolution neutrino-radiation magnetohydrodynamics merger simulation in full general relativity, we show that the αΩ dynamo mechanism, driven by the magnetorotational instability, builds up the large-scale magnetic field insid

We investigate the effects of the purely toroidal magnetic field on the equilibrium structures of the relativistic stars. The basic equations for obtaining equilibrium solutions of relativistic rotating stars containing purely toroidal magnetic fields are derived for the first time. To solve these basic equations numerically, we extend the Cook-Shapiro-Teukolsky scheme for calculating relativistic rotating stars containing no magnetic field to incorporate the effects of the purely toroidal magne

We elucidate the feature of gravitational waves (GWs) from a binary-neutron-star merger collapsing to a black hole by general relativistic simulation. We show that GW spectrum imprints the coalescence dynamics, formation process of disk, equation of state for neutron stars, total masses, and mass ratio. A formation mechanism of the central engine of short-gamma-ray bursts, which are likely to be composed of a black hole and surrounding disk, therefore could be constrained by GW observation.

We report results of numerical relativity simulations for 26 new nonspinning binary neutron star systems with 6 grid resolutions using an adaptive mesh refinement numerical relativity code SACRA-MPI. The finest grid spacing is 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 irresp

We perform a general-relativistic neutrino-radiation magnetohydrodynamic simulation of a one second-long binary neutron star merger on the Japanese supercomputer Fugaku using about 85 million CPU hours with 20 736 CPUs. We consider an asymmetric binary neutron star merger with masses of 1.2M_{⊙} and 1.5M_{⊙} and a "soft" equation of state SFHo. It results in a short-lived remnant with the lifetime of ≈0.017 s, and subsequent massive torus formation with the mass of ≈0.05M_{⊙} after the remnant c

We construct a new three-dimensional general relativistic magnetohydrodynamics code, in which a fixed mesh refinement technique is implemented. To ensure the divergence-free condition as well as the magnetic flux conservation, we employ the method by Balsara [J. Comp. Physiol. 174, 614 (2001); J. Comp. Phys. 228, 5040 (2009)]. Using this new code, we evolve differentially rotating magnetized neutron stars, and find that a magnetically driven outflow is launched from the star exhibiting a kink in

Numerical simulations for the merger of binary neutron stars are performed in full general relativity incorporating both nucleonic and hyperonic finite-temperature equations of state (EOS) and neutrino cooling. It is found that for the nucleonic and hyperonic EOS, a hyper massive neutron star (HMNS) with a long lifetime $(t_{\rm life}\gtrsim 10 {\rm ms})$ is the outcome for the total mass $\approx 2.7 M_\odot$. For the total mass $\approx 3 M_\odot$, a long-lived (short-lived with $t_{\rm life}\