383 citations · 3.2k across the 50 of their papers we have counts for
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Unstable GRB photospheres and electron-positron annihilation lines
Kunihito Ioka, Kohta Murase, Kenji Toma +2
We propose an emission mechanism of prompt gamma-ray bursts (GRBs) that can reproduce the observed non-thermal spectra with high radiative efficiencies, >50%. Internal dissipation…
Low-Luminosity GRB 060218: A Collapsar Jet from a Neutron Star, Leaving a Magnetar as a Remnant?
Kenji Toma, Kunihito Ioka, Takanori Sakamoto +1
The gamma-ray burst (GRB) 060218 has ~ 10^5 times lower luminosity than typical long GRBs, and is associated with a supernova (SN). The radio afterglow displays no jet break, so th…
High Energy Neutrinos and Cosmic-Rays from Low-Luminosity Gamma-Ray Bursts?
Kohta Murase, Kunihito Ioka, Shigehiro Nagataki +1
The recently discovered gamma-ray burst (GRB) GRB 060218/SN 2006aj is classified as an X-ray Flash with very long duration driven possibly by a neutron star. Since GRB 060218 is ve…
Ep-Eiso Correlation in a Multiple Subjet Model of Gamma-Ray Bursts
Kenji Toma, Ryo Yamazaki, Takashi Nakamura
We perform Monte Carlo simulations to study Ep-Eiso correlation in the context of a multiple subjet model (or inhomogeneous jet model) for gamma-ray bursts (GRBs), X-ray--rich GRBs…
Possible Origin of Lognormal Distributions in Gamma-Ray Bursts
Kunihito Ioka, Takashi Nakamura
We show that if the intrinsic break energy of Gamma-Ray Bursts (GRBs) is determined by the product of more than three random variables the observed break energy distribution become…
Is dark energy the only solution to the apparent acceleration of the present universe?
Hideo Iguchi, Takashi Nakamura, Ken-ichi Nakao
Even for the observed luminosity distance D_L(z), which suggests the existence of dark energy, we show that an inhomogeneous dust universe solution without dark energy is possible…
Peak Luminosity-Spectral Lag Relation Caused by the Viewing Angle of the Collimated Gamma-Ray Bursts
Kunihito Ioka, Takashi Nakamura
We compute the kinematical dependence of the peak luminosity, the pulse width and the spectral lag of the peak luminosity on the viewing angle of a jet. For appropriate model…
Microlensing of collimated Gamma-Ray Burst afterglows
Kunihito Ioka, Takashi Nakamura
We investigate stellar microlensing of the collimated gamma-ray burst afterglows. A spherical afterglow appears on the sky as a superluminally expanding thin ring (``ring-like'' im…
Determining the Equation of State of the Expanding Universe Using a New Independent Variable
Takashi Nakamura, Takeshi Chiba
To determine the equation of state of the universe, we propose to use a new independent variable , where and are the present Hubble pa…
Equilibrium sequences of irrotational binary polytropic stars : The case of double polytropic stars
Keisuke Taniguchi, Takashi Nakamura
Solutions to equilibrium sequences of irrotational binary polytropic stars in Newtonian gravity are expanded in a power of , where R and are the orbital separation o…
Off-axis Emission from the Beamed Afterglow of Gamma Ray Bursts and a Possible Interpretation of Slowly Declining X-Ray Afterglow of GRB980425
Takashi Nakamura
A formula for off-axis emissions from the beamed afterglow of GRBs in a synchrotron model is derived. The formula is applied to an inhomogeneous circumstellar matter obeying n prop…
Density Distribution and Shape of Galactic Dark Halo Can be Determined by Low Frequency Gravitational Waves ?
Kunihito Ioka, Takahiro Tanaka, Takashi Nakamura
Under the assumption that the Milky Way's dark halo consists of primordial black hole MACHOs (PBHMACHOs), the mass density of the halo can be measured by the low frequency gravitat…
Determining the Equation of State of the Expanding Universe: Inverse Problem in Cosmology
Takashi Nakamura, Takeshi Chiba
Even if the luminosity distance as a function of redshift is obtained accurately using, for example, Type Ia supernovae, the equation of state of the Universe cannot be determined…
Low frequency gravitational waves from black hole MACHO binaries
Kunihito Ioka, Takahiro Tanaka, Takashi Nakamura
The intensity of low frequency gravitational waves from black hole MACHO binaries is studied. First we estimate the gravitational wave background produced by black hole MACHO binar…
The Luminosity Distance, the Equation of State, and the Geometry of the Universe
Takeshi Chiba, Takashi Nakamura
The second derivative of the luminosity distance with respect to the redshift is written in terms of the deceleration parameter . We point out that the third derivative contai…
Black Hole Binary Formation in the Expanding Universe --- Three Body Problem Approximation ---
Kunihito Ioka, Takeshi Chiba, Takahiro Tanaka +1
We study black hole MACHO binary formation through three-body interactions in the early universe at s. The probability distribution functions of the eccentricity and…
Observational tests of x-matter models
Takeshi Chiba, Naoshi Sugiyama, Takashi Nakamura
We study gravitational lensing statistics, matter power spectra and the angular power spectra of the cosmic microwave background (CMB) radiation in x-matter models. We adopt an equ…
Expected EAGLE event rate towards the Magellanic Clouds
Takashi Nakamura, Ryoichi Nishi
We propose to search for MACHOs by observing EAGLE (Extremely Amplified Gravitational LEnsing) events of a majority of dim stars. This search is independent of the usual one. For t…
Gravitational Waves from Coalescing Black Hole MACHO Binaries
Takashi Nakamura, Misao Sasaki, Takahiro Tanaka +1
If MACHOs are black holes of mass about 0.5 solar mass, they must have been formed in the early universe when the temperature was about 1 GeV. We estimate that in this case in our…
Cosmology with x-matter
Takeshi Chiba, Naoshi Sugiyama, Takashi Nakamura
Motivated by the possibility of where and are the Hubble parameter and the age of the universe, respectively, we investigate the cosmology including x-matt…
The Minimum Mass of the First Stars and the Anthropic Principle
Takashi Nakamura, Hideya Uehara, Takeshi Chiba
The lower limit of the mass of the first stars suggested recently may imply the formation of massive stars of mass greater than 8 solar mass irrespective of the details of the init…
Fragmentation of the Primordial Gas Clouds and the Lower Limit on the Mass of the First Stars
Hideya Uehara, Hajime Susa, Ryoichi Nishi +2
We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the time scale of collapse with that of fragmentation, we obtain the typical mass…
Volume Expansion Rate and The Age of The Universe
Takashi Nakamura, Ken-ichi Nakao, Takeshi Chiba +1
Under four assumptions such that 1) Einstein's theory of gravity is correct, 2) existence of foliation by geodesic slicing, 3) the trace of the extrinsic curvature, , is negativ…
The Microlensing and Halo Models of the Galaxy
Yukitoshi Kan-ya, Ryoichi Nishi, Takashi Nakamura
We investigated the dependence of the optical depth of the microlensing events on model parameters of the Galactic halo. We only consider Galactic mass models in which the rota…