profile2

Research activities of our group

(04/04/2017 last update: under construction)

Research fields: Theoretical Astrophysics, General Relativistic Astrophysics
Numerical relativity, Gravitational waves, Black hole formation, Neutron-star merger, Magnetohydrodynamics, Radiation hydrodynamics, High-energy phenomena associated with black holes and neutron stars like gamma-ray bursts.

Numerical relativity

Predicting gravitational waveforms

LIGO

VIRGO

KAGRA(LCGT)

Gravitational waveforms of BH-NS binaries by numerical relativity

Gravitational waveforms of NS-NS binaries by numerical relativity

Merger of binary neutron stars

NS + NS --> BH

NS + NS --> HMNS

LIGO

VIRGO

KAGRA(LCGT)

Animations by Sekiguchi

Animations by Kiuchi

Animations by Hotoke

Merger of black hole-neutron star binaries

Gravitational collapse to a black hole

Massive stars are evolved through a series of nuclear burning and eventually, in their center, iron core is formed. If the mass of iron core (and surrounding layers) is sufficiently large, a black hole will be formed after the stellar core collapse. Massive stellar-size black holes recently discovered in binary black holes observed by advanced LIGO are likely to be formed in such process. However, the formation process of such black holes is not well understood yet.

Recent observations have shown that large galaxies usually have a supermassive black hole (SMBH) of million to 10 billion solar mass in their center. However, the formation process of SMBH is still unknown: Actually clarifying the formation process of SMBH has been a long standing problem in astrophysics.

We are challenging these problems in the framework of numerical relativity. Specifically, we prepare a variety of plausible progenitor stars and perform a variety of numerical-relativity simulations aiming at clarifying the black-hole formation process. We then explore the possible signals of the black hole formation like gravitational waves, neutrinos, and electromagentic waves, which may be detected in the future observations leading to clarifying the formation process of black holes.

UN100-density

POPIII-density

Instability of rotating neutron stars

Instability of differentially rotating star

Magnetohydrodynamics simulation in GR

Collapse of Hyper Massive NS: Magnetic field line and Density

Simulation by Kenta Kiuchi　(Animation by 4D2U of NAOJ)

highest-level resolution simulation

Viscous hydrodynamics

evolution of differentially rotating system

evolution of differentially rotating system (wide)

evolution of massive torus surrounding a black hole

Higher-dimensional black holes

Collision to merge
Collision to scattering

Binary neutron stars in scalar-tenros theory

We have not had any evidence that general relativity is violated. However, the tests of general relativity have been performed in relatively weakly gravitational environment like in the solar system and binary neutron stars of large orbital separation. If gravitational waves from neutron-star merger are detected, we will have a great opportunity for testing general relativity in a strongly gravitational environment. For this test, it is important to expect what happens in alternative theory of gravity in the strongly selfgravitating systems. As one effort, we performed simulations for the merger of binary neutron stars in a scalar-tensor theory of gravity.

Animations for binary neutron star mergers for a scalar-tensor theory of gravity that admits spontaneous scalarization (e.g, Shibata et al., PRD 89, 084005, 2014)

Evolution of scalar field
Evolution of density

Evolution of scalar field
Evolution of density