arXiv subjects

Kei Kotake

Publications and source records attributed to Kei Kotake.

At least 109 records · Page 6Linked to original sources

arXiv2009

The pasta structure in the hadron-quark phase transition and the effects on magnetized compact stars

We report the effects of quark-hadron phase transition on the structures of general relativistic stars with purely toroidal magnetic field. For the mixed phase, we take into account of the finite-size effects, which lead to non-uniform "Pasta" structures. Our study is based on axisymetric and stationary formalism including purely toroidal magnetic field. For hybrid stars, we find the characteristic distribution of magnetic field, which has a discontinuity originated in the quark-hadron mixed phase. These distributions of magnetic field will change astrophysical phenomena, such as cooling processes.

astro-ph.HE↗

arXiv2009

Relativistic hybrid stars with super-strong toroidal magnetic fields: An evolutionary track with QCD phase transition

We investigate structures of hybrid stars, which feature quark core surrounded by a hadronic matter mantle, with super-strong toroidal magnetic fields in full general relativity. Modeling the equation of state (EOS) with a first order transition by bridging the MIT bag model for the description of quark matter and the nuclear EOS by Shen et al., we numerically construct thousands of the equilibrium configurations to study the effects of the phase transition. It is found that the appearance of the quark phase can affect distributions of the magnetic fields inside the hybrid stars, making the maximum field strength about up to 30 % larger than for the normal neutron stars. Using the equilibrium configurations, we explore the possible evolutionary paths to the formation of hybrid stars due to the spin-down of magnetized rotating neutron stars. We find that the energy release by the phase transition to the hybrid stars is quite large ($\la 10^{52} \rm erg$) even for super strongly magnetized compact stars. Our results suggest that the strong gravitational-wave emission and the sudden spin-up signature could be observable signals of the QCD phase transition, possibly for a source out to Megaparsec distances.

astro-ph.HE↗

arXiv2009

Ray-Tracing Analysis of Anisotropic Neutrino Radiation for Estimating Gravitational Waves in Core-Collapse Supernovae

We propose a ray-tracing method to estimate gravitational waves (GWs) generated by anisotropic neutrino emission in supernova cores. To calculate the gravitational waveforms, we derive analytic formulae in a useful form, which are applicable also for three-dimensional computations. Pushed by evidence of slow rotation prior to core-collapse, we focus on asphericities in neutrino emission and matter motions outside the protoneutron star. Based on the two-dimensional (2D) models, which mimic SASI-aided neutrino heating explosions, we compute the neutrino anisotropies via the ray-tracing method in a post-processing manner and calculate the resulting waveforms. With these computations, it is found that the waveforms exhibit more variety in contrast to the ones previously estimated by the ray-by-ray analysis (e.g., Kotake et al. (2007)). In addition to a positively growing feature, which was predicted to determine the total wave amplitudes predominantly, the waveforms are shown to exhibit large negative growth for some epochs during the growth of SASI. These features are found to stem from the excess of neutrino emission in lateral directions, which can be precisely captured by the ray-tracing calculation. Due to the negative contributions and the neutrino absorptions appropriately taken into account by the ray-tracing method, the wave amplitudes become more than one-order-of magnitude smaller than the previous estimation, thus making their detections very hard for a galactic source.On the other hand, it is pointed out that the GW spectrum from matter motions have its peak near $\sim 100$ Hz, which could be characteristic for the SASI-induced supernova explosions.(abridged)

astro-ph.HE↗

arXiv2009

Neutrino oscillations in magnetically driven supernova explosions

We investigate neutrino oscillations from core-collapse supernovae that produce magnetohydrodynamic (MHD) explosions. By calculating numerically the flavor conversion of neutrinos in the highly non-spherical envelope, we study how the explosion anisotropy has impacts on the emergent neutrino spectra through the Mikheyev-Smirnov-Wolfenstein effect. In the case of the inverted mass hierarchy with a relatively large theta_(13), we show that survival probabilities of electron type neutrinos and antineutrinos seen from the rotational axis of the MHD supernovae (i.e., polar direction), can be significantly different from those along the equatorial direction. The event numbers of electron type antineutrinos observed from the polar direction are predicted to show steepest decrease, reflecting the passage of the magneto-driven shock to the so-called high-resonance regions. Furthermore we point out that such a shock effect, depending on the original neutrino spectra, appears also for the low-resonance regions, which leads to a noticeable decrease in the electron type neutrino signals. This reflects a unique nature of the magnetic explosion featuring a very early shock-arrival to the resonance regions, which is in sharp contrast to the neutrino-driven delayed supernova models. Our results suggest that the two features in the electron type antineutrinos and neutrinos signals, if visible to the Super-Kamiokande for a Galactic supernova, could mark an observational signature of the magnetically driven explosions, presumably linked to the formation of magnetars and/or long-duration gamma-ray bursts.

astro-ph.HE↗

arXiv2009

Long-Term Evolution of Slowly Rotating Collapsar in Special Relativistic Magnetohydrodynamics

We present our numerical results of two-dimensional magnetohydrodynamic (MHD) simulations of the collapse of rotating massive stars in light of the collapsar model of gamma-ray bursts (GRBs). Pushed by recent evolution calculations of GRB progenitors, we focus on lower angular momentum of the central core than the ones taken mostly in previous studies. By performing special relativistic simulations including both realistic equation of state and neutrino coolings, we follow a unprecedentedly long-term evolution of the slowly rotating collapsars up to $\sim$ 10 s, accompanied by the formation of jets and accretion disks. Our results show that for the GRB progenitors to function as collapsars, there is a critical initial angular momentum, below which matter is quickly swallowed to the central objects, no accretion disks and no MHD outflows are formed. When larger than the criteria, we find the launch of the MHD jets in the following two ways. For models with stronger initial magnetic fields, the magnetic pressure amplified inside the accretion disk can drive the MHD outflows, which makes the strong magnetic explosions like a 'magnetic tower' (type II). For the models with weaker initial magnetic fields, the magnetic tower stalls first and the subsequent MHD outflows are produced by some eventual inflows of the accreting material from the equator to the polar regions (type I). Regardless of type I or II, the jets can attain only mildly relativistic speeds with the explosion energy less than $10^{49} \erg$, which could possibly be related to the X-ray flashes. Due to high opacity for neutrinos inside the disk, we find that the luminosities of $ν_e$ and $\barν_e$ become almost comparable, which is advantageous for making the energy deposition rate larger.

astro-ph.HE↗

arXiv2009

Stochastic Nature of Gravitational Waves from Supernova Explosions with Standing Accretion Shock Instability

We study properties of gravitational waves based on the three-dimensional simulations, which demonstrate the neutrino-driven explosions aided by the standing accretion shock instability (SASI). Pushed by evidence supporting slow rotation prior to core-collapse, we focus on the asphericities in neutrino emissions and matter motions outside the protoneutron star. By performing a ray-tracing calculation in 3D, we estimate accurately the gravitational waveforms from anisotropic neutrino emissions. In contrast to the previous work assuming axisymmetry, we find that the gravitational waveforms vary much more stochastically because the explosion anisotropies depend sensitively on the growth of the SASI which develops chaotically in all directions. Our results show that the gravitational-wave spectrum has its peak near $\sim 100$ Hz, reflecting the SASI-induced matter overturns of $\sim O(10)$ ms. We point out that the detection of such signals, possibly visible to the LIGO-class detectors for a Galactic supernova, could be an important probe into the long-veiled explosion mechanism.

astro-ph.HE↗

arXiv2009

General relativistic compact stars with exotic matter

We study structures of general relativistic compact stars with exotic matter. Our study is based on axisymetric and stationary formalism including purely toroidal magnetic field. We also study the finite size effects of quark-hadron mixed phase on structures of magnetars. For hybrid stars, we find a characteristic distribution of magnetic field, which has a discontinuity originated in the quark-hadron mixed phase. These distributions of magnetic field will change astrophysical phenomena, such as cooling processes.

astro-ph.HE↗

arXiv2009

Relativistic stars with purely toroidal magnetic fields with realistic equations of state

We investigate equilibrium sequences of relativistic stars containing purely toroidal magnetic fields with four kinds of realistic equations of state (EOSs) of SLy (Douchin et al.), FPS (Pandharipande et al.), Shen (Shen et al.), and LS (Lattimer & Swesty). We numerically construct thousands of equilibrium configurations. Particularly we pay attention to the equilibrium sequences of constant baryon mass and/or constant magnetic flux, which model evolutions of an isolated neutron star. Important properties obtained in this study are summarized as follows ; (1) The dependence of the mass-shedding angular velocity on the EOSs is determined from that of the non-magnetized case. The stars with Shen(FPS) EOS reach the mass-shedding limit at the smallest(largest) angular velocity, while the stars with SLy or Lattimer-Swesty EOSs take the moderate values. (2) For the supramassive sequences, the equilibrium configurations are found to be generally oblate for the realistic EOSs in sharp contrast to the polytropic stars. For FPS(LS) EOS, the parameter region which permits the prolately deformed stars is widest(narrowest). For SLy and Shen EOS, it is in medium. Furthermore, the angular velocities $Ω_{\rm up}$, above which the stars start to spin up as they lose angular momentum, are found to depend sharply on the realistic EOSs. Our analysis indicates that the hierarchy of this spin up angular velocity is $Ω_{\rm up,SLy} > Ω_{\rm up,FPS} > Ω_{\rm up,LS}>Ω_{\rm up,Shen}$ and this relation holds even if the sequences have strong magnetic fields. Our results suggest the EOSs within the relativistic stars containing purely toroidal magnetic fields can be constrained by observing the angular velocity, the gravitational wave, and the signature of the spin up.

astro-ph.HE↗

arXiv2008

Special Relativistic Simulations of Magnetically-dominated Jets in Collapsing Massive Stars

We perform a series of two-dimensional magnetohydrodynamic core-collapse simulations of rapidly rotating and strongly magnetized massive stars. To study the properties of magnetic explosions for a longer time stretch of postbounce evolution, we develop a new code under the framework of special relativity including a realistic equation of state with a multiflavor neutrino leakage scheme. Our results show the generation of the magnetically-dominated jets in the two ways. One is launched just after the core-bounce in a prompt way and another is launched at $ \sim 100 $ ms after the stall of the prompt shock. We find that the shock-revival occurs when the magnetic pressure becomes strong, due to the field wrapping, enough to overwhelm the ram pressure of the accreting matter. The critical toroidal magnetic fields for the magnetic shock-revival are found to be universal of $\sim 10^{15}\mathrm{G}$ behind the jets. We point out that the time difference before the shock-revival has a strong correlation with the explosions energies. Our results suggest that the magnetically dominated jets are accompanied by the formation of the magnetars. Since the jets are mildly relativistic, we speculate that they might be the origin of some observed X-ray flashes.

astro-ph↗

arXiv2008

Effects of Rotation on Standing Accretion Shock Instability in Nonlinear Phase for Core-Collapse Supernovae

We studied the effects of rotation on standing accretion shock instability (SASI) by performing three-dimensional hydrodynamics simulations. Taking into account a realistic equation of state and neutrino heating/cooling, we prepared a spherically symmmetric and steady accretion flow through a standing shock wave onto a proto-neutron star (PNS). When the SASI entered the nonlinear phase, we imposed uniform rotation on the flow advecting from the outer boundary of the iron core, whose specific angular momentum was assumed to agree with recent stellar evolution models. Using spherical harmonics in space and Fourier decompositions in time, we performed mode analysis of the nonspherical deformed shock wave to observe rotational effects on the SASI in the nonlinear phase. We found that rotation imposed on the axisymmetric SASI did not make any spiral modes and hardly affected sloshing modes, except for steady l=2, m=0 modes. In contrast, rotation imposed on the non-axisymmetric flow increased the amplitude of spiral modes so that some spiral flows accreting on the PNS were more clearly formed inside the shock wave than without rotation. The amplitudes of spiral modes increased significantly with rotation in the progressive direction.

astro-ph↗

arXiv2008

Determination of the angular momentum distribution of supernovae from gravitational wave observations

Significant progress has been made in the development of an international network of gravitational wave detectors, such as TAMA300, LIGO, VIRGO, and GEO600. For these detectors, one of the most promising sources of gravitational waves are core collapse supernovae especially in our Galaxy. Recent simulations of core collapse supernovae, rigorously carried out by various groups, show that the features of the waveforms are determined by the rotational profiles of the core, such as the rotation rate and the degree of the differential rotation prior to core-collapse. Specifically, it has been predicted that the sign of the second largest peak in the gravitational wave strain signal is negative if the core rotates cylindrically with strong differential rotation. The sign of the second peak could be a nice indicator that provides us with information about the angular momentum distribution of the core, unseen without gravitational wave signals. Here we present a data analysis procedure aiming at the detection of the second peak using a coherent network analysis and estimate the detection efficiency when a supernova is at the sky location of the Galactic center. The simulations showed we were able to determine the sign of the second peak under an idealized condition of a network of gravitational wave detectors if a supernova occurs at the Galactic center.

gr-qc↗

arXiv2008

Impact of Rotation on Neutrino Emission and Relic Neutrino Background from Population III Stars

We study the effects of rotation on the neutrino emission from Population III (Pop III) stars by performing a series of two-dimensional rotational collapse simulations of Pop III stellar cores. Our results show that rotation enhances the neutrino luminosities and the average energies of emitted neutrinos. This is because the thermalized inner core, which is the dominant neutrino source from Pop III stars, can be enlarged, due to rotational flattening, enough to extend the inner core outside the neutrinospheres. This is in sharp contrast to the case of spherical collapse, in which the case of inner core shrinks deeper inside the neutrinospheres before black hole formation, which hinders the efficient neutrino emission. In the case of rotational core-collapse, the emitted neutrino energies are found to become larger in the vicinity near the pole than the ones near the equatorial plane. These factors make the emergent neutrino spectrum broader and harder than the spherical collapse case. By computing the overall neutrino signals produced by the ensemble of individual rotating Pop III stars, we find that the amplitudes of the relic neutrinos, depending on their star formation rates, can dominate over the contributions from ordinary core-collapse supernovae below a few MeV. A detection of this signal could be an important tool to probe star formation history in the early universe.

astro-ph↗

arXiv2008

Search method for coincident events from LIGO and IceCube detectors

We present a coincidence search method for astronomical events using gravitational wave detectors in conjunction with other astronomical observations. We illustrate our method for the specific case of the LIGO gravitational wave detector and the IceCube neutrino detector. LIGO trigger-events and IceCube events which occur within a given time window are selected as time-coincident events. Then the spatial overlap of the reconstructed event directions is evaluated using an unbinned maximum likelihood method. Our method was tested with Monte Carlo simulations based on realistic LIGO and IceCube event distributions. We estimated a typical false alarm rate for the analysis to be 1 event per 435 years. This is significantly smaller than the false alarm rates of the ndividual detectors.

astro-ph↗

arXiv2007

Three-Dimensional Simulations of Standing Accretion Shock Instability in Core-Collapse Supernovae

We have studied non-axisymmetric standing accretion shock instability, or SASI, by 3D hydrodynamical simulations. This is an extention of our previous study on axisymmetric SASI. We have prepared a spherically symmetric and steady accretion flow through a standing shock wave onto a proto-neutron star, taking into account a realistic equation of state and neutrino heating and cooling. This unperturbed model is supposed to represent approximately the typical post-bounce phase of core-collapse supernovae. We then have added a small perturbation (~1%) to the radial velocity and computed the ensuing evolutions. Not only axisymmetric but non-axisymmetric perturbations have been also imposed. We have applied mode analysis to the non-spherical deformation of the shock surface, using the spherical harmonics. We have found that (1) the growth rates of SASI are degenerate with respect to the azimuthal index m of the spherical harmonics Y_l^m, just as expected for a spherically symmetric background, (2) nonlinear mode couplings produce only m=0 modes for the axisymmetric perturbations, whereas m=!0 modes are also generated in the non-axisymmetric cases according to the selection rule for the quadratic couplings, (3) the nonlinear saturation level of each mode is lower in general for 3D than for 2D because a larger number of modes are contributing to turbulence in 3D, (4) low l modes are dominant in the nonlinear phase, (5) the equi-partition is nearly established among different m modes in the nonlinear phase, (6) the spectra with respect to l obey power laws with a slope slightly steeper for 3D, and (7) although these features are common to the models with and without a shock revival at the end of simulation, the dominance of low l modes is more remarkable in the models with a shock revival.

astro-ph↗

arXiv2007

Numerical Simulations of Equatorially-Asymmetric Magnetized Supernovae: Formation of Magnetars and Their Kicks

A series of numerical simulations on magnetorotational core-collapse supernovae are carried out. Dipole-like configurations which are offset northward are assumed for the initially strong magnetic fields together with rapid differential rotations. Aims of our study are to investigate effects of the offset magnetic field on magnetar kicks and on supernova dynamics. Note that we study a regime where the proto-neutron star formed after collapse has a large magnetic field strength approaching that of a ``magnetar'', a highly magnetized slowly rotating neutron star. As a result, equatorially-asymmetric explosions occur with a formation of the bipolar jets. Resultant magnetar's kick velocities are $\sim 300-1000$ km s$^{-1}$. We find that the acceleration is mainly due to the magnetic pressure while the somewhat weaker magnetic tension works toward the opposite direction, which is due to stronger magnetic field in the northern hemisphere. Noted that observations of magnetar's proper motions are very scarce, our results supply a prediction for future observations. Namely, magnetars possibly have large kick velocities, several hundred km s$^{-1}$, as ordinary neutron stars do, and in an extreme case they could have those up to 1000 km s$^{-1}$.

astro-ph↗

arXiv2007

Equilibrium Configurations of Strongly Magnetized Neutron Stars with Realistic Equations of State

We investigate equilibrium sequences of magnetized rotating stars with four kinds of realistic equations of state (EOSs) of SLy (Douchin et al.), FPS (Pandharipande et al.), Shen (Shen et al.), and LS (Lattimer & Swesty). Employing the Tomimura-Eriguchi scheme to construct the equilibrium configurations. we study the basic physical properties of the sequences in the framework of Newton gravity. In addition we newly take into account a general relativistic effect to the magnetized rotating configurations. With these computations, we find that the properties of the Newtonian magnetized stars, e.g., structure of magnetic field, highly depends on the EOSs. The toroidal magnetic fields concentrate rather near the surface for Shen and LS EOSs than those for SLy and FPS EOSs. The poloidal fields are also affected by the toroidal configurations. Paying attention to the stiffness of the EOSs, we analyze this tendency in detail. In the general relativistic stars, we find that the difference due to the EOSs becomes small because all the employed EOSs become sufficiently stiff for the large maximum density, typically greater than $10^{15}\rm{g} \rm{cm}^{-3}$. The maximum baryon mass of the magnetized stars with axis ratio $q\sim 0.7$ increases about up to twenty percents for that of spherical stars. We furthermore compute equilibrium sequences at finite temperature, which should serve as an initial condition for the hydrodynamic study of newly-born magnetars. Our results suggest that we may obtain information about the EOSs from the observation of the masses of magnetars.

astro-ph↗

arXiv2007

Gravitational Wave Background from Population III Stars

We estimate the gravitational wave (GW) background from Population III (Pop III) stars using the results from our hydrodynamic simulations (Suwa et al. 2007). We calculate gravitational waveforms from matter motions and anisotropic neutrino emissions for single Pop III stars. We find that the GW amplitudes from matter motions are dominant until just after bounce, but those from neutrinos dominate later on at frequencies below $\sim 10$ Hz in the GW spectrum. Computing the overall signal produced by the ensemble of such Pop III stars, we find that the resultant density parameter of the GW background peaks at the amplitude of $Ω_\mathrm{GW}\sim 10^{-10}$ in the frequency interval $\sim 1-10$ Hz. We show that such signals, depending on the formation rate of Pop III stars, can be within the detection limits of future planned interferometers such as DECIGO and BBO in the frequency interval of $\sim 0.1-1$ Hz. Our results suggest that the detection of the GW background from Pop III stars can be an important tool to supply the information about the star formation history in the early universe.

astro-ph↗

arXiv2007

Magnetorotational Collapse of Population III Stars

We perform a series of two-dimensional magnetorotational core-collapse simulations of Pop III stars. Changing the initial distributions of rotation and magnetic fields prior to collapse in a parametric manner, we compute 19 models. By so doing, we systematically investigate how rotation and magnetic fields affect the collapse dynamics and explore how the properties of the black-hole formations and neutrino emissions could be affected. As for the microphysics, we employ a realistic equation of state and approximate the neutrino transfer by a multiflavour leakage scheme. With these computations, we find that the jet-like explosions are obtained by the magnetodriven shock waves if the initial magnetic field is as large as $10^{12}$ G. We point out that the black-hole masses at the formation decrease with the initial field strength, on the other hand, increase with the initial rotation rates. As for the neutrino properties, we point out that the degree of the differential rotation plays an important role to determine which species of the neutrino luminosity is more dominant than the others. Furthermore, we find that the stronger magnetic fields make the peak neutrino luminosities smaller, because the magnetic pressure acts to halt the collapse in the central regions, leading to the suppression of the releasable gravitational binding energies.

astro-ph↗