arXiv subjects

Keitaro Takahashi

Publications and source records attributed to Keitaro Takahashi.

At least 127 records · Page 7Linked to original sources

arXiv2010

Systematic Study of Gravitational Waves from Galaxy Merger

A systematic study of gravitational waves from galaxy mergers, through N-body simulations, was performed. In particular, we investigated the relative importance of galaxy components (disk, bulge and halo) and effects of initial relative velocity, relative angular momentum and mass ratio of the galaxies. We found that the features of light curve of gravitational waves, such as peak width and luminosity, are reliably simulated with particle numbers larger than ~10^4. Dominant contribution to gravitational wave emission came from the halo component, while peak luminosity amounted to 10^31 erg/sec for the collision of two halos with mass 3.8 x10^12Msun/h. We also found that the initial relative velocity in the direction of the initial separation did not significantly affect gravitational wave emission, while the initial relative angular momentum broadened the peak width and suppressed the luminosity. Mass dependence of the peak luminosity was also investigated, and we obtained evidence that the luminosity is proportional to the cubic mass when the scaling relation is satisfied. This behavior was considered by a simple analysis.

astro-ph.CO↗

arXiv2010

Generation of Seed Magnetic Fields in Primordial Supernova Remnants

Origin of the magnetic field ubiquitous in the Universe is studied based on the Biermann mechanism, which is expected to work in the non-barotropic region. We perform a series of two-dimensional MHD simulations of the first generation supernova remnant (SNR) expanding in the inhomogeneous interstellar matter (ISM) and study the Biermann mechanism working in the interior of the SNR. Especially, we pay attention to the relaxation process of electron and ion temperatures via the Coulomb interaction. In the early SNR in which the electron temperature is much lower than the ion temperature, the Biermann mechanism is ineffective, since the gradient of electron pressure is small. Magnetic fields begin to be generated just behind the shock front when the electron temperature is sufficiently relaxed. Assuming the explosion energy of 10^52 erg, the total magnetic energy generated reaches about 10^26 erg and does not depend strongly on the parameters of either SNR or ISM. Analytic expression to estimate the magnetic total energy is presented and it is shown this agrees well with our numerical results. Finally we evaluate the expected amplitude of magnetic fields in protogalaxies as ~10^-19 G, which is sufficient for seed fields of the observed galactic magnetic fields.

astro-ph.CO↗

arXiv2010

Analytic formulae of the CMB bispectra generated from non-Gaussianity in the tensor and vector perturbations

We present a complete set of formulae for calculating the bispectra of CMB temperature and polarization anisotropies generated from non-Gaussianity in the vector and tensor mode perturbations. In the all-sky analysis it is found that the bispectrum formulae for the tensor and vector-mode non-Gaussianity formally take complicated forms compared to the scalar mode one because the photon transfer functions in the tensor and vector modes depend on the azimuthal angle between the direction of the wave number vector of the photon's perturbation and that of the line of sight. We demonstrate that flat-sky approximations remove this difficulty because this kind of azimuthal angle dependence apparently vanishes in the flat-sky limit. Through the flat-sky analysis, we also find that the vector or tensor bispectrum of $B$-mode polarization vanishes in the squeezed limit, unless the cosmological parity is violated at the nonlinear level.

astro-ph.CO↗

arXiv2010

Can we probe the Lorentz factor of gamma-ray bursts from GeV-TeV spectra integrated over internal shocks?

We revisit the high-energy spectral cutoff originating from the electron-positron pair creation in the prompt phase of gamma-ray bursts (GRBs) with numerical and analytical calculations. We show that the conventional exponential and/or broken power law cutoff should be drastically modified to a shallower broken power-law in practical observations that integrate emissions from different internal shocks. Since the steepening is tiny for observations, this "smearing" effect can generally reduce the previous estimates of the Lorentz factor of the GRB outflows. We apply our formulation to GRB 080916C, recently detected by the Large Area Telescope detector on the Fermi satellite, and find that the minimum Lorentz factor can be ~600 (or even smaller values), which is below but consistent with the previous result of ~900. Observing the steepening energy (so-called "pair-break energy") is crucial to diagnose the Lorentz factor and/or the emission site in the future observations, especially current and future Cherenkov telescopes such as MAGIC, VERITAS, and CTA.

astro-ph.HE↗

arXiv2010

Probing Early Cosmic Magnetic Fields through Pair Echos from High-Redshift GRBs

We discuss the expected properties of pair echo emission from gamma-ray bursts (GRBs) at high redshifts ($z \gtrsim 5$), their detectability, and the consequent implications for probing intergalactic magnetic fields (IGMFs) at early epochs. Pair echos comprise inverse Compton emission by secondary electron-positron pairs produced via interactions between primary gamma-rays from the GRB and low-energy photons of the diffuse intergalactic radiation, arriving with a time delay that depends on the nature of the intervening IGMFs. At sufficiently high $z$, the IGMFs are unlikely to have been significantly contaminated by astrophysical outflows, and the relevant intergalactic radiation may be dominated by the well-understood cosmic microwave background (CMB). Pair echoes from luminous GRBs at $z \sim 5-10$ may be detectable by future facilities such as the Cherenkov Telescope Array or the Advanced Gamma-ray Imaging System, as long as the GRB primary emission extends to multi-TeV energies, the comoving IGMFs at these redshifts are $B \sim 10^{-16}-10^{-15}~{\rm Gauss}$, and the non-CMB component of the diffuse intergalactic radiation is relatively low. Observations of pair echos from high-$z$ GRBs can provide a unique, in-situ probe of weak IGMFs during the epochs of early structure formation and cosmic reionization.

astro-ph.HE↗

arXiv2010

Analytical model for CMB temperature angular power spectrum from cosmic (super-)strings

We present a new analytical method to calculate the small angle CMB temperature angular power spectrum due to cosmic (super-)string segments. In particular, using our method, we clarify the dependence on the intercommuting probability $P$. We find that the power spectrum is dominated by Poisson-distributed string segments. The power spectrum for a general value of $P$ has a plateau on large angular scales and shows a power-law decrease on small angular scales. The resulting spectrum in the case of conventional cosmic strings is in very good agreement with the numerical result obtained by Fraisse et al.. Then we estimate the upper bound on the dimensionless tension of the string for various values of $P$ by assuming that the fraction of the CMB power spectrum due to cosmic (super-)strings is less than ten percents at various angular scales up to $\ell=2000$. We find that the amplitude of the spectrum increases as the intercommuting probability. As a consequence, strings with smaller intercommuting probabilities are found to be more tightly constrained.

astro-ph.CO↗

arXiv2010

Skewness in CMB temperature fluctuations from curved cosmic (super-)strings

We compute the one-point probability distribution function of small-angle cosmic microwave background temperature fluctuations due to curved cosmic (super-)strings with a simple model of string network by performing Monte Carlo simulations. Taking into account of the correlation between the curvature and the velocity of string segments, there appear non-Gaussian features, specifically non-Gaussian tails and a skewness, in the one-point pdf. The obtained sample skewness for the conventional field-theoretic cosmic strings is $g_1\approx -0.14$, which is consistent with the result reported by Fraisse et al.. We also discuss the dependence of the pdf on the intercommuting probability. We find that the standard deviation of the Gaussian part increases and non-Gaussian features are suppressed as the intercommuting probability decreases. For sufficiently small intercommuting probability, the skewness is given by $\lesssim$ (a\ few) $\times 10^{-2}$.

astro-ph.CO↗

arXiv2009

Non-Gaussianity in Cosmic Microwave Background Temperature Fluctuations from Cosmic (Super-)Strings

We compute analytically the small-scale temperature fluctuations of the cosmic microwave background from cosmic (super-)strings and study the dependence on the string intercommuting probability $P$. We develop an analytical model which describes the evolution of a string network and calculate the numbers of string segments and kinks in a horizon volume. Then we derive the probability distribution function (pdf) which takes account of finite angular resolution of observation. The resultant pdf consists of a Gaussian part due to frequent scatterings by long string segments and a non-Gaussian tail due to close encounters with kinks. The dispersion of the Gaussian part is reasonably consistent with that obtained by numerical simulations by Fraisse et al.. On the other hand, the non-Gaussian tail contains two phenomenological parameters which are determined by comparison with the numerical results for P=1. Extrapolating the pdf to the cases with $P<1$, we predict that the non-Gaussian feature is suppressed for small $P$.

astro-ph↗

arXiv2009

Cosmological Constraints from calibrated Yonetoku and Amati relation implies Fundamental plane of Gamma-ray bursts

We consider two empirical relations using data only from the prompt emission of Gamma-Ray Bursts (GRBs), peak energy ($E_p$) - peak luminosity ($L_p$) relation (so called Yonetoku relation) and $E_p$-isotropic energy ($E_{\rm iso}$) relation (so called Amati relation). We first suggest the independence of the two relations although they have been considered similar and dependent. From this viewpoint, we compare constraints on cosmological parameters, $Ω_m$ and $Ω_Λ$, from the Yonetoku and Amati relations calibrated by low-redshift GRBs with $z < 1.8$. We found that they are different in 1-$σ$ level, although they are still consistent in 2-$σ$ level. This and the fact that both Amati and Yonetoku relations have systematic errors larger than statistical errors suggest the existence of a hidden parameter of GRBs. We introduce the luminosity time $T_L$ defined by $T_L\equiv E_{\rm iso}/L_p$ as a hidden parameter to obtain a generalized Yonetoku relation as $(L_p/{10^{52} \rm{erg s^{-1}}}) = 10^{-3.88\pm0.09}(E_p/{\rm{keV}})^{1.84\pm0.04} (T_L/{\rm{s}})^{-0.34\pm0.04}$. The new relation has much smaller systematic error, 30%, and can be regarded as "Fundamental plane" of GRBs. We show a possible radiation model for this new relation. Finally we apply the new relation for high-redshift GRBs with $1.8 < z < 5.6$ to obtain $(Ω_m,Ω_Λ) = (0.16^{+0.04}_{-0.06},1.20^{+0.03}_{-0.09})$, which is consistent with the concordance cosmological model within 2-$σ$ level.

astro-ph↗

arXiv2009

Possible Effects of Pair Echoes on Gamma-Ray Burst Afterglow Emission

High-energy emission from gamma-ray bursts (GRBs) is widely expected but had been sparsely observed until recently when the Fermi satellite was launched. If >TeV gamma rays are produced in GRBs and can escape from the emission region, they are attenuated by the cosmic infrared background photons, leading to regeneration of GeV-TeV secondary photons via inverse-Compton scattering. This secondary emission can last for a longer time than the duration of GRBs, and it is called a pair echo. We investigate how this pair echo emission affects spectra and light curves of high energy afterglows, considering not only prompt emission but also afterglow as the primary emission. Detection of pair echoes is possible as long as the intergalactic magnetic field (IGMF) in voids is weak. We find (1) that the pair echo from the primary afterglow emission can affect the observed high-energy emission in the afterglow phase after the jet break, and (2) that the pair echo from the primary prompt emission can also be relevant, but only when significant energy is emitted in the TeV range, typically E_{gamma, >0.1 TeV} > (Y/(1+Y)) epsilon_e E_k. Even non-detections of the pair echoes could place interesting constraints on the strength of IGMF. The more favorable targets to detect pair echoes may be the "naked" GRBs without conventional afterglow emission, although energetic naked GRBs would be rare. If the IGMF is weak enough, it is predicted that the GeV emission extends to >30-300 s.

astro-ph↗

arXiv2008

Probing Intergalactic Magnetic Fields in the GLAST Era through Pair Echo Emission from TeV Blazars

More than a dozen blazars are known to be emitters of multi-TeV gamma rays, often with strong and rapid flaring activity. By interacting with photons of the cosmic microwave and infrared backgrounds, these gamma rays inevitably produce electron-positron pairs, which in turn radiate secondary inverse Compton gamma rays in the GeV-TeV range with a characteristic time delay that depends on the properties of the intergalactic magnetic field (IGMF). For sufficiently weak IGMF, such "pair echo" emission may be detectable by the Gamma-ray Large Area Space Telescope (GLAST), providing valuable information on the IGMF. We perform detailed calculations of the time-dependent spectra of pair echos from flaring TeV blazars such as Mrk 501 and PKS 2155-304, taking proper account of the echo geometry and other crucial effects. In some cases, the presence of a weak but non-zero IGMF may enhance the detectability of echos. We discuss the quantitative constraints that can be imposed on the IGMF from GLAST observations, including the case of non-detections.

astro-ph↗

arXiv2008

Constraints on $w_0$ and $w_a$ of Dark Energy from High Redshift Gamma Ray Bursts

We extend the Hubble diagram up to $z = 5.6$ using 63 gamma-ray bursts (GRBs) via peak energy-peak luminosity relation (so called Yonetoku relation), and obtain constraints on cosmological parameters including dynamical dark energy parametrized by $P/ρ\equiv w(z) = w_0 + w_a \cdot z/(1+z)$. It is found that the current GRB data are consistent with the concordance model, ($Ω_m = 0.28, Ω_Λ = 0.72, w_0 = -1, w_a = 0$), within two sigma level. Although constraints from GRBs themselves are not so strong, they can improve the conventional constraints from SNeIa because GRBs have much higher redshifts. Further we estimate the constraints on the dark-energy parameters expected by future observations with GLAST (Gamma-ray Large Area Space Telescope) and \swift by Monte-Carlo simulation. Constraints would improve substantially with another 150 GRBs.

astro-ph↗

arXiv2008

Detectability of Pair Echos from Gamma-Ray Bursts and Intergalactic Magnetic Fields

High-energy emission from gamma-ray bursts (GRBs) can give rise to pair echos, i.e. delayed inverse Compton emission from secondary $e^{\pm}$ pairs produced in $γ-γ$ interactions with intergalactic background radiation. We investigate the detectability of such emission with modern-day gamma-ray telescopes. The spectra and light curves are calculated for a wide range of parameters, applying the formalism recently developed by Ichiki et al. The flux depends strongly on the unknown magnitude and coherence length of intergalactic magnetic fields, and we delineate the range of field strength and redshift that allow detectable echos. Relevant uncertainties such as the high-energy cutoff of the primary gamma-ray spectrum and the intensity of the cosmic infrared background are addressed. GLAST and MAGIC may be able to detect pair echo emission from GRBs with redshift $\lesssim 1$ if the primary spectra extend to $\sim 10 ~ {\rm TeV}$.

astro-ph↗

arXiv2008

Electromagnetic Properties of the Early Universe

Detailed physical processes of magnetic field generation from density fluctuations in the pre-recombination era are studied. Solving Maxwell equations and the generalized Ohm's law, the evolutions of the net charge density, the electric current and the electromagnetic field are solved. Unlike most of previous works, we treat electrons and photons as separate components under the assumption of tight coupling. We find that generation of the magnetic field due to density fluctuations takes place only from the second order of both perturbation theory and the tight coupling approximation.

astro-ph↗

arXiv2008

Behavior of magnetic fields around the cosmic recombination

Several mechanisms have been proposed to generate primordial magnetic fields and it is often assumed that magnetic fields are not affected by a sharp drop in ionization rate due to the cosmic recombination. We investigate the validity of the assumption by studying the behavior of magnetic fields and fluid motion around recombination. Fluid equations including the effect of recombination are considered for protons, electrons and neutral hydrogens separately, combining Maxwell equations. We find that the residual ionization rate required for the conservation of magnetic field at cosmological scales is about $10^{-10}$, which is much smaller than the standard value $\sim 10^{-4}$. Further we will show the acceleration of protons and electrons in the process of recombination which conpensates the decrease in the carrier of electric current in order to preserve electric current and then magnetic fields.

astro-ph↗

arXiv2008

Probing the Nature of the Weakest Intergalactic Magnetic Fields with the High Energy Emission of Gamma-Ray Bursts

We investigate the delayed, secondary GeV-TeV emission of gamma-ray bursts and its potential to probe the nature of intergalactic magnetic fields. Geometrical effects are properly taken into account for the time delay between primary high energy photons and secondary inverse Compton photons from electron-positron pairs, which are produced in $γ$-$γ$ interactions with background radiation fields and deflected by intervening magnetic fields. The time-dependent spectra of the delayed emission are evaluated for a wide range of magnetic field strengths and redshifts. The typical flux and delay time of secondary photons from bursts at $z \sim 1$ are respectively $\sim 10^{-8}$ GeV cm$^{-2}$ s$^{-1}$ and $\sim 10^4$ s if the field strengths are $\sim 10^{-18}$ G, as might be the case in intergalactic void regions. We find crucial differences between the cases of coherent and tangled magnetic fields, as well as dependences on the field coherence length.

astro-ph↗

arXiv2007

Gravitational Wave Spectrum Induced by Primordial Scalar Perturbations

We derive the complete spectrum of gravitational waves induced by primordial scalar perturbations ranging over all observable wavelengths. This scalar-induced contribution can be computed directly from the observed scalar perturbations and general relativity and is, in this sense, independent of the cosmological model for generating the perturbations. The spectrum is scale-invariant on small scales, but has an interesting scale-dependence on large and intermediate scales, where scalar-induced gravitational waves do not redshift and are hence enhanced relative to the background density of the Universe. This contribution to the tensor spectrum is significantly different in form from the direct model-dependent primordial tensor spectrum and, although small in magnitude, it dominates the primordial signal for some cosmological models. We confirm our analytical results by direct numerical integration of the equations of motion.

hep-th↗

arXiv2007

Cosmological magnetic fields from nonlinear effects

In the standard cosmological model, magnetic fields and vorticity are generated during the radiation era via second-order density perturbations. In order to clarify the complicated physics of this second-order magnetogenesis, we use a covariant approach and present the electromagneto-dynamical equations in the nonlinear regime. We use the tight-coupling approximation to analyze Thomson and Coulomb scattering. At the zero-order limit of exact tight-coupling, we show that the vorticity is zero and no magnetogenesis takes place at any nonlinear order. We show that magnetogenesis also fails at all orders if either protons or electrons have the same velocity as the radiation, and momentum transfer is neglected. Then we prove a key no-go result: at first-order in the tight-coupling approximation, magnetic fields and vorticity still cannot be generated even via nonlinear effects. The tight-coupling approximation must be broken at first order, for the generation of vorticity and magnetic fields, and we derive a closed set of nonlinear evolution equations that governs this generation. We estimate that the amplitude of the magnetic field at recombination on the horizon scale is $\sim 10^{-27} $G.

astro-ph↗