Takahashi, Masaaki, Koyama, Hiroko
The observational data from some black hole candidates suggest the importance of electromagnetic fields in the vicinity of a black hole. Highly magnetized disk accretion may play an importance rule,...
Dynamical response to supernova-induced gas removal in spiral galaxies with dark matter halo (2008)
Koyama, Hiroko, Nagashima, Masahiro, Kakehata, Takayuki, Yoshii, Yuzuru
We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies. A thin baryonic disc having the Kuzmin density...
A subcutaneous tumor of a patient with epidermodysplasia verruciformis was studied by the light microscopy, the electronmicroscopy and the immunofluorescent test. The tumor cells were histologically...
Taihei Yano, Hiroko Koyama, Thomas Buchert
We numerically investigate the long{time evolution of density perturbations after the rst appearance of caustics in an expanding cosmological model with one{dimensional `single{wave ' initial...
1/f fluctuations in spinning-particle motions around a Schwarzschild black hole (2007)
Koyama, Hiroko, Kiuchi, Kenta, Konishi, Tetsuro
We study the properties of chaos in the motions of a spinning test particle in Schwarzschild spacetime. We characterize the chaos using the power spectrum of the time series of z components of the...
Gravitational Wave Signals from Chaotic System: A Point Mass with A Disk (2007)
Kiuchi, Kenta, Koyama, Hiroko, Maeda, Kei-ichi
We study gravitational waves from a particle moving around a system of a point mass with a disk in Newtonian gravitational theory. A particle motion in this system can be chaotic when the...
1/f fluctuations in spinning-particle motions around Schwarzschild black hole (2007)
Koyama, Hiroko, Kiuchi, Kenta, Konishi, Tetsuro
We study the properties of chaos in the motions of a spinning test particle in Schwarzschild spacetime. We characterize the chaos using the power spectrum of the time series of $z$ components of the...
Clusters die hard: Time-correlated excitation in the Hamiltonian Mean Field model (2006)
Koyama, Hiroko, Konishi, Tetsuro, Ruffo, Stefano
The Hamiltonian Mean Field (HMF) model has a low-energy phase where $N$ particles are trapped inside a cluster. Here, we investigate some properties of the trapping/untrapping mechanism of a single...
Formation of fractal structure in many-body systems with attractive power-law potentials (2005)
Koyama, Hiroko, Konishi, Tetsuro
We study the formation of fractal structure in one-dimensional many-body systems with attractive power-law potentials. Numerical analysis shows that the range of the index of the power for which...
Construction and enlargement of traversable wormholes from Schwarzschild black holes (2004)
Koyama, Hiroko, Hayward, Sean A.
Analytic solutions are presented which describe the construction of a traversable wormhole from a Schwarzschild black hole, and the enlargement of such a wormhole, in Einstein gravity. The matter...
How to make a traversable wormhole from a Schwarzschild black hole (2004)
Hayward, Sean A., Koyama, Hiroko
The theoretical construction of a traversable wormhole from a Schwarzschild black hole is described, using analytic solutions in Einstein gravity. The matter model is pure phantom radiation (pure...
The connection between the two-point and the three-point correlation functions in the non-linear gravitational clustering regime is studied. Under a scaling hypothesis, we find that the three-point...
Universality in the distribution of caustics in the expanding Universe (2003)
Yano, Taihei, Koyama, Hiroko, Buchert, Thomas, Gouda, Naoteru
We numerically investigate the long--time evolution of density perturbations after the first appearance of caustics in an expanding cosmological model with one--dimensional `single--wave' initial...
Construction and enlargement of dilatonic wormholes by impulsive radiation (2002)
Koyama, Hiroko, Hayward, Sean A., Kim, Sung-Won
The dynamical behavior of traversable wormholes and black holes under impulsive radiation is studied in an exactly soluble dilaton gravity model. Simple solutions are presented where a traversable...
Slowly decaying tails of massive scalar fields in spherically symmetric spacetimes (2001)
Koyama, Hiroko, Tomimatsu, Akira
We study the dominant late-time behaviors of massive scalar fields in static and spherically symmetric spacetimes. Considering the field evolution in the far zone where the gravitational field is...
Asymptotic tails of massive scalar fields in Schwarzschild background (2001)
Koyama, Hiroko, Tomimatsu, Akira
We investigate the asymptotic tail behavior of massive scalar fields in Schwarzschild background. It is shown that the oscillatory tail of the scalar field has the decay rate of $t^{-5/6}$ at...
Asymptotic power-law tails of massive scalar fields in Reissner-Nordstr\"{o}m background (2000)
Koyama, Hiroko, Tomimatsu, Akira
We investigate dominant late-time tail behaviors of massive scalar fields in nearly extreme Reissner-Nordstr\"{o}m background. It is shown that the oscillatory tail of the scalar fields has the decay...
Koyama, Hiroko, Konishi, Tetsuro
Long-time behavior of spatial power-law correlation in one-dimensional self-gravitating system is numerically investigated. It is found that the power-law structure persists even after the system is...
Koyama, Hiroko, Konishi, Tetsuro
The process of formation of fractal structure in one-dimensional self-gravitating system is examined numerically. It is clarified that structures created in small spatial scale grow up to larger...
Emergence of power-law correlation in 1-dimensional self-gravitating system (2000)
Koyama, Hiroko, Konishi, Tetsuro
A new phase of temporal evolution of the one-dimensional self-gravitating system is numerically discovered. Fractal structure is dynamically created from non-fractal initial conditions. Implication...
Vacuum Polarization of Massive Scalar Fields on the Black Hole Horizon (2000)
Koyama, Hiroko, Nambu, Yasusada, Tomimatsu, Akira
Vacuum polarization of massive scalar fields in a thermal state at arbitrary temperature is studied near the horizon of a Reissner-Nordstr\"{o}m black hole. We derived an analytic form of $$...
Tomimatsu, Akira, Koyama, Hiroko
We study vacuum polarization of quantized massive scalar fields $\phi$ in equilibrium at black-hole temperature in Reissner-Nordstr\"{o}m background. By means of the Euclidean space Green's function...