arXiv subjects

Shijun Yoshida

Publications and source records attributed to Shijun Yoshida.

At least 37 records · Page 2Linked to original sources

arXiv2007

Gravitational radiation from collapsing magnetized dust

In this article we study the influence of magnetic fields on the axial gravitational waves emitted during the collapse of a homogeneous dust sphere. We found that while the energy emitted depends weakly on the initial matter perturbations it has strong dependence on the strength and the distribution of the magnetic field perturbations. The gravitational wave output of such a collapse can be up to an order of magnitude larger or smaller calling for detailed numerical 3D studies of collapsing magnetized configurations.

gr-qc↗

arXiv2007

The g-mode Excitation in the Proto Neutron Star by the Standing Accretion Shock Instability

The so-called "acoustic revival mechanism" of core-collapse supernova proposed recently by the Arizona group is an interesting new possibility. Aiming to understand the elementary processes involved in the mechanism, we have calculated the eigen frequencies and eigen functions for the g-mode oscillations of a non-rotating proto neutron star. The possible excitation of these modes by the standing accretion shock instability, or SASI, is discussed based on these eigen functions. We have formulated the forced oscillations of $g$-modes by the external pressure perturbations exerted on the proto neutron star surface. The driving pressure fluctuations have been adopted from our previous computations of the axisymmetric SASI in the non-linear regime. We have paid particular attention to low l modes, since these are the modes that are dominant in SASI and that the Arizona group claimed played an important role in their acoustic revival scenario. Here l is the index of the spherical harmonic functions, $Y_l^m$. Although the frequency spectrum of the non-linear SASI is broadened substantially by non-linear couplings, the typical frequency is still much smaller than those of g-modes, the fact leading to a severe impedance mismatch. As a result, the excitations of various $g$-modes are rather inefficient and the energy of the saturated g-modes is $\sim 10^{50}$erg or smaller, with the g_2-mode being the largest in our model. Here the g_2-mode has two radial nodes and is confined to the interior of the convection region. The energy transfer rate from the g-modes to out-going sound waves is estimated from the growth of the g-modes and found to be $\sim 10^{51}$erg/s in the model studied in this paper.

astro-ph↗

arXiv2006

Classical instability of Kerr-AdS black holes and the issue of final state

It is now established that small Kerr-Anti-de Sitter (Kerr-AdS) black holes are unstable against scalar perturbations, via superradiant amplification mechanism. We show that small Kerr-AdS black holes are also unstable against gravitational perturbations and we compute the features of this instability. We also describe with great detail the evolution of this instability. In particular, we identify its endpoint state. It corresponds to a Kerr-AdS black hole whose boundary is an Einstein universe rotating with the light velocity. This black hole is expected to be slightly oblate and to co-exist in equilibrium with a certain amount of outside radiation.

hep-th↗

arXiv2005

The Fate of a Five-Dimensional Rotating Black Hole via Hawking Radiation

We study the evolution of a five-dimensional rotating black hole emitting scalar field radiation via the Hawking process for arbitrary initial values of the two rotation parameters $a$ and $b$. It is found that any such black hole whose initial rotation parameters are both nonzero evolves toward an asymptotic state $a/M^{1/2}=b/M^{1/2}={\rm const}(\neq 0)$, where this constant is independent of the initial values of $a$ and $b$.

hep-th↗

arXiv2005

Perturbations and absorption cross-section of infinite-radius black rings

We study scalar field perturbations on the background of non-supersymmetric black rings and of supersymmetric black rings. In the infinite-radius limit of these geometries, we are able to separate the wave equation, and to study wave phenomena in its vicinities. In this limit, we show that (i) both geometries are stable against scalar field perturbations, (ii) the absorption cross-section for scalar fields is equal to the area of the event horizon in the supersymmetric case, and proportional to it in the non-supersymmetric situation.

hep-th↗

arXiv2005

Superradiant instabilities of rotating black branes and strings

Black branes and strings are generally unstable against a certain sector of gravitational perturbations. This is known as the Gregory-Laflamme instability. It has been recently argued that there exists another general instability affecting many rotating extended black objects. This instability is in a sense universal, in that it is triggered by any massless field, and not just gravitational perturbations. Here we investigate this novel mechanism in detail. For this instability to work, two ingredients are necessary: (i) an ergo-region, which gives rise to superradiant amplification of waves, and (ii) ``bound'' states in the effective potential governing the evolution of the particular mode under study. We show that the black brane Kerr_4 x R^p is unstable against this mechanism, and we present numerical results for instability timescales for this case. On the other hand, and quite surprisingly, black branes of the form Kerr_d x R^p are all stable against this mechanism for d>4. This is quite an unexpected result, and it stems from the fact that there are no stable circular orbits in higher dimensional black hole spacetimes, or in a wave picture, that there are no bound states in the effective potential. We also show that it is quite easy to simulate this instability in the laboratory with acoustic black branes.

hep-th↗

arXiv2004

R-mode oscillations of rapidly rotating barotropic stars in general relativity: Analysis by the relativistic Cowling approximation

We develop a numerical scheme for obtaining the $r$-mode oscillations of rapidly rotating relativistic stars. In the present scheme, we neglect all metric perturbations and only take account of the dynamics of the fluid in the background spacetime of the unperturbed star (the relativistic Cowling approximation). We also assume the star is barotropic, i.e. neutrally stable against convection under the assumption of adiabatic oscillations. Our numerical scheme is based on the Yoshida-Eriguchi formulation for the analysis of the general relativistic f-mode oscillations in the Cowling approximation and a general relativistic generalization of the Karino-Yoshida-Yoshida-Eriguchi's numerical scheme for obtaining oscillations of rapidly rotating Newtonian stars. By this new numerical scheme, frequencies of the $r$-mode oscillations are obtained as functions of the ratio of the rotational energy to the absolute value of the gravitational energy $T/|W|$ along sequences of polytropic equilibrium stars whose ratio of the pressure to the total energy density at the center of the star and the polytropic index are kept constant. It is found that the dimensionless oscillation frequency $σ/Ω$ is a linearly decreasing function of $T/|W|$, where $σ$ and $Ω$ are the oscillation frequency and the angular velocity of the star measured in an inertial frame at spatial infinity. We also find that oscillation frequencies of the $r$-modes are highly dependent on the relativistic factor $M/R$ of the star as already found in previous studies in which the slow rotation approximation has been used. Here $M$ and $R$ denote the mass and radius of the star, respectively.

astro-ph↗

arXiv2004

Scalar perturbations of higher dimensional rotating and ultra-spinning black holes

We investigate the stability of higher dimensional rotating black holes against scalar perturbations. In particular, we make a thorough numerical and analytical analysis of six-dimensional black holes, not only in the low rotation regime but in the high rotation regime as well. Our results suggest that higher dimensional Kerr black holes are stable against scalar perturbations, even in the ultra-spinning regime.

hep-th↗

arXiv2004

Quasinormal modes and stability of the rotating acoustic black hole: numerical analysis

The study of the quasinormal modes (QNMs) of the 2+1 dimensional rotating draining bathtub acoustic black hole, the closest analogue found so far to the Kerr black hole, is performed. Both the real and imaginary parts of the quasinormal (QN) frequencies as a function of the rotation parameter B are found through a full non-linear numerical analysis. Since there is no change in sign in the imaginary part of the frequency as B is increased we conclude that the 2+1 dimensional rotating draining bathtub acoustic black hole is stable against small perturbations.

gr-qc↗

arXiv2004

The black hole bomb and superradiant instabilities

A wave impinging on a Kerr black hole can be amplified as it scatters off the hole if certain conditions are satisfied giving rise to superradiant scattering. By placing a mirror around the black hole one can make the system unstable. This is the black hole bomb of Press and Teukolsky. We investigate in detail this process and compute the growing timescales and oscillation frequencies as a function of the mirror's location. It is found that in order for the system black hole plus mirror to become unstable there is a minimum distance at which the mirror must be located. We also give an explicit example showing that such a bomb can be built. In addition, our arguments enable us to justify why large Kerr-AdS black holes are stable and small Kerr-AdS black holes should be unstable.

hep-th↗

arXiv2004

Numerical analysis of quasinormal modes in nearly extremal Schwarzschild-de Sitter spacetimes

We calculate high-order quasinormal modes with large imaginary frequencies for electromagnetic and gravitational perturbations in nearly extremal Schwarzschild-de Sitter spacetimes. Our results show that for low-order quasinormal modes, the analytical approximation formula in the extremal limit derived by Cardoso and Lemos is a quite good approximation for the quasinormal frequencies as long as the model parameter $r_1κ_1$ is small enough, where $r_1$ and $κ_1$ are the black hole horizon radius and the surface gravity, respectively. For high-order quasinormal modes, to which corresponds quasinormal frequencies with large imaginary parts, on the other hand, this formula becomes inaccurate even for small values of $r_1κ_1$. We also find that the real parts of the quasinormal frequencies have oscillating behaviors in the limit of highly damped modes, which are similar to those observed in the case of a Reissner-Nordstr{\" o}m black hole. The amplitude of oscillating ${\rm Re(ω)}$ as a function of ${\rm Im}(ω)$ approaches a non-zero constant value for gravitational perturbations and zero for electromagnetic perturbations in the limit of highly damped modes, where $ω$ denotes the quasinormal frequency. This means that for gravitational perturbations, the real part of quasinormal modes of the nearly extremal Schwarzschild-de Sitter spacetime appears not to approach any constant value in the limit of highly damped modes. On the other hand, for electromagnetic perturbations, the real part of frequency seems to go to zero in the limit.

gr-qc↗

arXiv2004

Highly Damped Quasinormal Modes of Kerr Black Holes: A Complete Numerical Investigation

We compute for the first time very highly damped quasinormal modes of the (rotating) Kerr black hole. Our numerical technique is based on a decoupling of the radial and angular equations, performed using a large-frequency expansion for the angular separation constant_{s}A_{l m}. This allows us to go much further in overtone number than ever before. We find that the real part of the quasinormal frequencies approaches a non-zero constant value which does not depend on the spin s of the perturbing field and on the angular index l: ω_R=m\varpi(a). We numerically compute \varpi(a). Leading-order corrections to the asymptotic frequency are likely to be of order 1/ω_I. The imaginary part grows without bound, the spacing between consecutive modes being a monotonic function of a.

gr-qc↗

arXiv2003

Scalar-gravitational perturbations and quasinormal modes in the five dimensional Schwarzschild black hole

We calculate the quasinormal modes (QNMs) for gravitational perturbations of the Schwarzschild black hole in the five dimensional (5D) spacetime with a continued fraction method. For all the types of perturbations (scalar-gravitational, vector-gravitational, and tensor-gravitational perturbations), the QNMs associated with l=2, l=3, and l=4 are calculated. Our numerical results are summarized as follows: (i) The three types of gravitational perturbations associated with the same angular quantum number l have a different set of the quasinormal (QN) frequencies; (ii) There is no purely imaginary frequency mode; (iii) The three types of gravitational perturbations have the same asymptotic behavior of the QNMs in the limit of the large imaginary frequencies. In Hawking temperature units these frequencies are given by log3 + i2pi(n+1/2) as n goes to infinity, where n is the mode number.

hep-th↗

arXiv2003

Quasinormal modes of Schwarzschild black holes in four and higher dimensions

We make a thorough investigation of the asymptotic quasinormal modes of the four and five-dimensional Schwarzschild black hole for scalar, electromagnetic and gravitational perturbations. Our numerical results give full support to all the analytical predictions by Motl and Neitzke, for the leading term. We also compute the first order corrections analytically, by extending to higher dimensions, previous work of Musiri and Siopsis, and find excellent agreement with the numerical results. For generic spacetime dimension number D the first-order corrections go as $\frac{1}{n^{(D-3)/(D-2)}}$. This means that there is a more rapid convergence to the asymptotic value for the five dimensional case than for the four dimensional case, as we also show numerically.

gr-qc↗

arXiv2003

Rapidly rotating superfluid neutron stars in Newtonian dynamics

We develop a formulation for constructing and examining rapidly rotating Newtonian neutron star models that contain two superfluids, taking account of the effect of the rotation velocity difference between two superfluids. We assume neutron stars to be composed of the superfluid neutrons and the mixture of the superfluid protons and the normal fluid electrons. To describe Newtonian dynamics of the two superfluids, the Newtonian version of the so-called two-fluid formalism is employed. The effect of the rotation velocity difference on the structure of equilibrium state is treated as a small perturbation to rapidly rotating superfluid stars whose angular velocities of two superfluids are assumed to be exactly the same. We derive basic equations for the perturbed structures of rapidly rotating superfluid stars due to the rotation velocity difference between two superfluids. Assuming the superfluids to obey a simple analytical equation of state proposed by Prix, Comer, and Andersson, we obtain numerical solutions for the perturbations and find that the density distributions of the superfluids are strongly dependent on the parameter $σ$ which appears in the analytical equation of state and characterizes the so-called symmetry energy. It is also found that if Prix et al.'s analytical equation of state is assumed, the perturbations can be represented in terms of the universal functions that are independent of the parameters of the equation of state.

astro-ph↗

arXiv2003

Late-Time Tails of Wave Propagation in Higher Dimensional Spacetimes

We study the late-time tails appearing in the propagation of massless fields (scalar, electromagnetic and gravitational) in the vicinities of a D-dimensional Schwarzschild black hole. We find that at late times the fields always exhibit a power-law falloff, but the power-law is highly sensitive to the dimensionality of the spacetime. Accordingly, for odd D>3 we find that the field behaves as t^[-(2l+D-2)] at late times, where l is the angular index determining the angular dependence of the field. This behavior is entirely due to D being odd, it does not depend on the presence of a black hole in the spacetime. Indeed this tails is already present in the flat space Green's function. On the other hand, for even D>4 the field decays as t^[-(2l+3D-8)], and this time there is no contribution from the flat background. This power-law is entirely due to the presence of the black hole. The D=4 case is special and exhibits, as is well known, the t^[-(2l+3)] behavior. In the extra dimensional scenario for our Universe, our results are strictly correct if the extra dimensions are infinite, but also give a good description of the late time behaviour of any field if the large extra dimensions are large enough.

hep-th↗

arXiv2003

Electromagnetic radiation from collisions at almost the speed of light: an extremely relativistic charged particle falling into a Schwarzschild black hole

We investigate the electromagnetic radiation released during the high energy collision of a charged point particle with a four-dimensional Schwarzschild black hole. We show that the spectra is flat, and well described by a classical calculation. We also compare the total electromagnetic and gravitational energies emitted, and find that the former is supressed in relation to the latter for very high energies. These results could apply to the astrophysical world in the case charged stars and small charged black holes are out there colliding into large black holes, and to a very high energy collision experiment in a four-dimensional world. In this latter scenario the calculation is to be used for the moments just after the black hole formation, when the collision of charged debris with the newly formed black hole is certainly expected. Since the calculation is four-dimensional, it does not directly apply to Tev-scale gravity black holes, as these inhabit a world of six to eleven dimensions, although our results should qualitatively hold when extrapolated with some care to higher dimensions.

gr-qc↗

arXiv2003

Inertial modes of neutron stars with the superfluid core

We investigate the modal properties of inertial modes of rotating neutron stars with the core filled with neutron and proton superfluids, taking account of entrainment effects between the superfluids. In this paper, the entrainment effects are modeled by introducing a parameter $η$ so that no entrainment state is realized at $η=0$. We find that inertial modes of rotating neutron stars with the superfluid core are split into two families, which we call ordinary fluid inertial modes ($i^o$-mode) and superfluid inertial modes ($i^s$-mode). The two superfluids in the core counter-move for the $i^s$-modes. For the $i^o$-modes, $κ_0=\lim_{Ω\to 0}ω/Ω$ is only weakly dependent on the entrainment parameter $η$, where $Ω$ and $ω$ are the angular frequency of rotation and the oscillation frequency observed in the corotating frame of the star, respectively. For the $i^s$-modes, on the other hand, $|κ_0|$ almost linearly increases as $η$ increases. Avoided crossings as functions of $η$ are therefore quite common between $i^o$- and $i^s$-modes. We find that some of the $i^s$-modes that are unstable against the gravitational radiation reaction at $η=0$ become stable when $η$ is larger than $η_{crit}$, the value of which depends on the mode. Since the radiation driven instability associated with the current multipole radiation is quite weak for the inertial modes and the mutual friction damping in the superfluid core is strong, the instability caused by the inertial modes will be easily suppressed unless the entrainment parameter $η$ is extremely small and the mutual friction damping is sufficiently weak.

astro-ph↗