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Takashi Nakamura

Publications and source records attributed to Takashi Nakamura.

At least 127 records · Page 7Linked to original sources
arXiv2003

GRB980425 in the Off-Axis Jet Model of the Standard GRBs

Using a simple off-axis jet model of GRBs, we can reproduce the observed unusual properties of the prompt emission of GRB980425, such as the extremely low isotropic equivalent gamma-ray energy, the low peak energy, the high fluence ratio, and the long spectral lag when the jet with the standard energy of ~10^{51} ergs and the opening half-angle of Δθ=~10-30 degree is seen from the off-axis viewing angle ~Δθ+10/γ, where γis a Lorentz factor of the jet. For our adopted fiducial parameters, if the jet that caused GRB 980425 is viewed from the on-axis direction, the intrinsic peak energy Ep(1+z) is ~2.0-4.0 MeV, which corresponds to those of GRB990123 and GRB021004. Our model might be able to explain the other unusual properties of this event. We also discuss the connection of GRB980425 in our model with the X-ray flash, and the origin of a class of GRBs with small E_γsuch as GRB030329, GRB980329, GRB981226, and so on.

astro-ph↗
arXiv2003

Near-Infrared Colors of Gamma-Ray Burst Afterglows and Cosmic Reionization History

Using the near-infrared (NIR) observations of the afterglows of the high redshift ($5 \la z \la 25$) gamma-ray bursts (GRBs) which will be detected by the {\it Swift} satellite, we discuss a way to study the cosmic reionization history. In principle the details of the cosmic reionization history are well imprinted in the NIR spectra of the GRB afterglows. However the spectroscopy with a space telescope is required to obtain such an information for a very high redshift ($z\ga 15$) unless the neutral fraction of the high-$z$ universe is less than $10^{-6}$. The broad-band photometry has the higher sensitivity than that of the spectroscopy, so that the NIR photometric follow-up of the GRB afterglows is very promising to examine the cosmic reionization history. A few minutes exposure with a 8-m class ground-based telescope of the afterglow of the high-$z$ GRBs will reveal how many times the reionization occurred in the universe.

astro-ph↗
arXiv2003

An Off-Axis Jet Model For GRB980425 and Low Energy Gamma-Ray Bursts

Using a simple off-axis jet model of GRBs, we can reproduce the observed unusual properties of the prompt emission of GRB980425, such as the extremely low isotropic equivalent gamma-ray energy, the low peak energy, the high fluence ratio, and the long spectral lag when the jet with the standard energy of ~10^{51}ergs and the opening half-angle of Δθ=~10-30 degree is seen from the off-axis viewing angle ~Δθ+10/γ, where γis a Lorentz factor of the jet. For our adopted fiducial parameters, if the jet that caused GRB980425 is viewed from the on-axis direction, the intrinsic peak energy Ep(1+z) is ~2.0-4.0 MeV, which corresponds to those of GRB990123 and GRB021004. We also discuss the connection of GRB980425 in our model with the X-ray flash, and the origin of a class of GRBs with small E_gamma.

astro-ph↗
arXiv2003

Deci hertz Laser Interferometer can determine the position of the Coalescing Binary Neutron Stars within an arc minute a week before the final merging event to Black Hole

It may be possible to construct a laser interferometer gravitational wave antenna in space with $h_{rms}\sim 10^{-23}$ at $ f\sim 0.1{\rm Hz}$ in $\sim 2020$. This deci hertz antenna may be called DECIGO/BBO,which stand for DECi hertz Interferometer Gravitational wave Observatory and Big Bang Observer, respectively. The analysis of 1-10 years observational data of the coalescing binary neutron stars or black holes at the distance of $\sim$ 300Mpc will give us the spatial position within $\sim$ an arc minute and the time of the coalescence within $\sim 0.1$ sec beforehand. With the knowledge of the accurate position and the time of final merging event, the follow up simultaneous observation using high frequency ($ f\sim 100{\rm Hz}$) gravitational wave antennae as well as electro-magnetic wave antennae from the radio frequency to the ultra high energy gamma ray will reveal the physics in the enigmatic event of the coalescence and the formation of the black hole.

astro-ph↗
arXiv2003

Small localized black holes in a braneworld: Formulation and numerical method

No realistic black holes localized on a 3-brane in the Randall-Sundrum infinite braneworld have been found so far. The problem of finding a static black hole solution is reduced to a boundary value problem. We solve it by means of a numerical method, and show numerical examples of a localized black hole whose horizon radius is small compared to the bulk curvature scale. The sequence of small localized black holes exhibits a smooth transition from a five-dimensional Schwarzschild black hole, which is a solution in the limit of small horizon radius. The localized black hole tends to flatten as its horizon radius increases. However, it becomes difficult to find black hole solutions as its horizon radius increases.

gr-qc↗
arXiv2003

Cosmological X-ray Flashes in the Off-Axis Jet Model

The of the cosmological X-ray flashes detected by Wide Field Cameras on BeppoSAX is calculted theoretically in a simple jet model. The total emission energy from the jet is assumed to be constant. We find that if the jet opening half-angle is smaller than 0.03 radian, the theoretical is less than ~0.4, which is consistent with the recently reported observational value of 0.27+/-0.16 at the 1-sigma level. This suggests that the off-axis GRB jet with the small opening half-angle at the cosmological distance can be identified as the cosmological X-ray flash.

astro-ph↗
arXiv2003

General Relativistic Numerical Simulation on Coalescing Binary Neutron Stars and Gauge-Invariant Gravitational Wave Extraction

We are developing 3 dimensional simulation codes for coalescing binary neutron stars. A code using the maximal slicing condition is obtained. To evaluate the gravitational radiation, we implemented a gauge-invariant wave extraction and compared the wave forms with the metric tensors at the wave zone. The energy spectrum of the waves was also evaluated to investigate the possibility that the excitation of the quasi-normal modes of the black hole, which may be formed after the merger of two stars, can be observed.

astro-ph↗
arXiv2003

Wave Effects in Gravitational Lensing of Gravitational Waves from Chirping Binaries

In the gravitational lensing of gravitational waves, the wave optics should be used instead of the geometrical optics when the wavelength $λ$ of the gravitational waves is longer than the Schwarzschild radius of the lens mass $M_L$. For the gravitational lensing of the chirp signals from the coalescence of the super massive black holes at the redshift $z_S\sim 1$ relevant to LISA, the wave effects become important for the lens mass smaller than $\sim 10^8 M_{\odot}$. For such cases, we compute how accurately we can extract the mass of the lens and the source position from the lensed signal. We consider two simple lens models: the point mass lens and the SIS (Singular Isothermal Sphere). We find that the lens mass and the source position can be determined within $\sim 0.1% [(S/N)/10^3]^{-1}$ for the lens mass larger than $10^8 M_{\odot}$ and $\gsim 10% [(S/N)/10^3]^{-1}$ for the lens mass smaller than $10^7 M_{\odot}$ due to the diffraction effect, where $(S/N)$ is the signal to noise ratio of the unlensed chirp signals. For the SIS model, if the source position is outside the Einstein radius, only a single image exists in the geometrical optics approximation so that the lens parameters can not be determined. While in the wave optics cases we find that the lens mass can be determined even for $M_L < 10^8 M_{\odot}$. For the point mass lens, one can extract the lens parameters even if the source position is far outside the Einstein radius. As a result, the lensing cross section is an order of magnitude larger than that for the usual strong lensing of light.

astro-ph↗
arXiv2003

Delayed Flashes from Counter Jets of Gamma Ray Bursts

If X-ray flashes are due to the forward jet emissions from gamma ray bursts (GRBs) observed with large viewing angles, we show that a prompt emission from a counter jet should be observed as a delayed flash in the UV or optical band several hours to a day after the X-ray flash. Ultraviolet and Optical Telescope on Swift can observe the delayed flashes within ~13 Mpc, so that (double-sided) jets of GRBs may be directly confirmed. Since the event rate of delayed flashes detected by Swift may be as small as 6*10^{-5}events/yr, we require more sensitive detectors in future experiments.

astro-ph↗
arXiv2003

Peak Luminosity-Spectral Lag Relation Caused by the Viewing Angle of the Collimated Gamma-Ray Bursts

We compute the kinematical dependence of the peak luminosity, the pulse width and the spectral lag of the peak luminosity on the viewing angle $θ_v$ of a jet. For appropriate model parameters we obtain the peak luminosity-spectral lag relation similar to the observed one including GRB980425. A bright (dim) peak with short (long) spectral lag corresponds to a jet with small (large) viewing angle. This suggests that the viewing angle of the jet might cause various relations in GRBs such as the peak luminosity-variability relation and the luminosity-width relation. Our model also suggests that X-ray rich GRBs (or X-ray flushes or Fast X-ray transients) are typical GRBs observed from large $θ_v$ with large spectral lag and low variability.

astro-ph↗
arXiv2002

X-ray Flashes from Off-axis Gamma-Ray Bursts

Ioka and Nakamura (2001) proposed a simple jet model that is compatible with the peak luminosity-spectral lag relation, the peak luminosity-variability relation and various other relations in the Gamma-Ray Bursts. If the viewing angle is much larger than the collimation angle of the jet in the model by Ioka and Nakamura, for appropriate model parameters we obtain the observational characteristics of the X-ray flashes, such as the peak flux ratio and the fluence ratio between the $γ$-ray (50-300 keV) and the X-ray band (2-10 keV), the X-ray photon index, the typical duration and the event rate $\sim 100 {\rm yr}^{-1}$. In our model, if the distance to the X-ray flashes is much larger than $\sim 1$ Gpc (or $z \gtrsim 0.2$) they are too dim to be observed, so the spatial distribution of the X-ray flashes should be homogeneous and isotropic.

astro-ph↗
arXiv2002

Possible Formation Scenario of the Quark Star of Maximum Mass around 0.7 solar mass

If there exists the quark star of maximum mass $\sim 0.7\sol$ as suggested by recent Chandra observations, we show that the general relativistic collapse of a neutron star of mass greater than the maximum mass of the neutron star with angular momentum parameter $q\equiv cJ/GM^2 > 1$ may lead to such a strange star. Here $J$ and $M$ are the angular momentum and the gravitational mass of the neutron star, respectively. Under the cosmic censorship hypothesis, such a star can not be a black hole directly. The jet formed in the soft core might explode the outer envelope and leave the quark star of mass $\sim 0.7\sol$. The remnant quark star has $\lesssim 10^{53}$ erg rotational energy so that the formation of the quark star may be related to the central engine of GRBs. The detailed numerical simulations are urgent to confirm or refute this scenario.

astro-ph↗
arXiv2002

Possible Origin of Lognormal Distributions in Gamma-Ray Bursts

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 becomes almost lognormal including the redshift effect because of the central limit theorem. The excess from the lognormal distribution at the low break energy is possibly due to the high redshift GRBs. The same argument may also apply to the pulse duration, the interval between pulses and so on.

astro-ph↗
arXiv2001

Possibility of Direct Measurement of the Acceleration of the Universe Using 0.1 Hz Band Laser Interferometer Gravitational Wave Antenna in Space

It may be possible to construct a laser interferometer gravitational wave antenna in space with $h_{rms}\sim 10^{-27}$ at $ f\sim 0.1{\rm Hz}$ in this century. We show possible specification of this antenna which we call DECIGO. Using this antenna we show that 1) typically $ 10^5$ ($10^4\sim 10^6$) chirp signals of coalescing binary neutron stars per year may be detected with S/N $\sim 10^4$. 2) We can directly measure the acceleration of the universe by ten years observation of binary neutron stars. 3) The stochastic gravitational waves of $Ω_{GW}\gsim 10^{-20}$ predicted by the inflation may be detected by correlation analysis for which effects of the recent cosmic acceleration would become highly important. Our formula for phase shift due to accelerating motion might be also applied for binary sources of LISA.

astro-ph↗
arXiv2001

Oscillating scalar-field dark matter in supergravity

We show that an oscillating scalar field in supergravity of mass of the order of $\sim$ TeV with a nonzero vacuum expectation value ($\sim 10^{10}$ GeV) can be a candidate of cold dark matter (CDM). To avoid the gravitino problem, we need a low reheating temperature after the primordial inflation. Then, the energy density of the oscillating scalar field satisfies all the requirements for CDM at present in the universe.

hep-ph↗
arXiv2001

Microlensing of collimated Gamma-Ray Burst afterglows

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'' image), which is maximally amplified as it crosses the lens. We find that the image of the collimated afterglow becomes quite uniform (``disk-like'' image) after the jet break time (after the Lorentz factor of the jet drops below the inverse of the jet opening angle). Consequently, the amplification peak in the light curve after the break time is lower and broader. Therefore detailed monitoring of the amplification history will be able to test whether the afterglows are jets or not, i.e., ``disk-like'' or not, if the lensing occurs after the break time. We also show that some proper motion and polarization is expected, peaking around the maximum amplification. The simultaneous detection of the proper motion and the polarization will strengthen that the brightening of the light curve is due to microlensing.

astro-ph↗
arXiv2000

Possible direct method to determine the radius of a star from the spectrum of gravitational wave signals II : Spectra for various cases

We compute the spectrum and the waveform of gravitational waves generated by the inspiral of a disk or a spherical like dust body into a Kerr black hole. We investigate the effect of the radius R of the body on gravitational waves and conclude that the radius is inferred from the gravitational wave signal irrespective of (1) the form of the body (a disk or a spherical star) (2) the location where the shape of the body is determined, (3) the orbital angular momentum of the body, and (4) a black hole rotation. We find that when $R$ is much larger than the characteristic length of the quasinormal mode frequency, the spectrum has several peaks and the separation of the troughs $Δω$ is proportional to $R^{-1}$. Thus, we may directly determine the radius of a star in a coalescing binary black hole - star system from the observed spectrum of gravitational waves. For example, both trough frequency of neutron stars and white dwarfs are within the detectable frequency range of some laser interferometers and resonant type detectors so that this effect can be observed in the future. We therefore conclude that the spectrum of gravitational waves may provide us important signals in gravitational wave astronomy as in optical astronomy.

astro-ph↗