arXiv subjects

Hajime Sotani

Publications and source records attributed to Hajime Sotani.

103 records · Page 6Linked to original sources

arXiv2009

Probing Tensor-Vector-Scalar Theory with Gravitational Wave Asteroseismology

In order to examine the gravitational waves emitted from the neutron stars in the tensor-vector-scalar (TeVeS) theory, we derive the perturbation equations for relativistic stars, where for simplicity we omit the perturbations of vector field. That is, we consider the perturbations of scalar and tensor fields. With this assumption, we find that the axial gravitational waves, which are corresponding to the oscillations of spacetime ($w$ modes), are independent from the perturbations of scalar field and the effects of scalar field can be mounted only via the background properties. Using two different equations of state, we calculate the complex eigenfrequencies of axial $w$ modes and find that the dependences of frequencies on the stellar compactness are almost independent from the adopted equation of state and the parameter in TeVeS. Additionally, these dependences of frequencies of axial $w$ modes in TeVeS is obviously different from those expected in the general relativity. Thus the direct observations of gravitational waves could reveal the gravitational theory in the strong-field regime.

gr-qc↗

arXiv2009

Gravitational Radiation from Collapsing Magnetized Dust. II -- Polar Parity Perturbation --

With the gauge-invariant perturbation theory, we study the effects of the stellar magnetic fields on the polar gravitational waves emitted during the homogeneous dust collapse. We found that the emitted energy in gravitational waves depends strongly not only on the initial stellar radius but also on the rasio between the poloidal and toroidal magnetic components. The polar gravitational wave output of such a collapse can be easily up to a few order of magnitude larger than what we get from the nonmagnetized collapse. The changes due to the existence of magnetic field could be helpful to extract some information of inner magnetic profiles of progenitor from the detection of the gravitational waves radiated during the black hole formation, which results from the stellar collapse.

gr-qc↗

arXiv2009

Stellar Oscillations in Tensor-Vector-Scalar Theory

An alternative theory of gravity has recently been proposed by Bekenstein, named Tensor-Vector-Scalar (TeVeS) theory, which can explain many galactic and cosmological observations without the need for dark matter. Whilst this theory passes basic solar system tests, and has been scrutinized with considerable detail in other weak-field regimes, comparatively little has been done in the strong-field limit of the theory. In this article, with Cowling approximation, we examine the oscillation spectra of neutron stars in TeVeS. As a result, we find that the frequencies of fundamental modes in TeVeS could become lager than those expected in general relativity, while the dependence of frequency of higher overtone on gravitational theory is stronger than that of lower modes. These imprints of TeVeS make it possible to distinguish the gravitational theory in strong-field regime via the observations of gravitational waves, which can provide unique confirmation of the existence of scalar field.

gr-qc↗

arXiv2008

Structure of Neutron Stars in Tensor-Vector-Scalar Theory

Bekenstein's Tensor-Vector-Scalar (TeVeS) theory has had considerable success in explaining various phenomena without the need for dark matter. However, it is difficult to observationally discern the differences between TeVeS and predictions made within the Lambda-cold dark matter concordance model. This implies that alternative tests are required that independently verify which theory is correct. For this we turn to the strong-field regime of TeVeS. In particular, we solve the spherically symmetric equations of hydrostatic equilibrium for a perfect fluid with a realistic equation of state to build models of neutron stars in TeVeS. We show that causality within the neutron star is only maintained for certain cosmological values of the scalar field, which allows us to put constraints on this value independently of cosmological observations. We also discuss in detail the internal structure of neutron stars and how each of the free parameters in the theory effects the overall size and mass of the neutron stars. In particular, the radii of neutron stars in TeVeS can significantly differ from those in General Relativity for certain values of the vector field coupling, which allows us to also place extra constraints on this parameter. Finally, we discuss future observations of neutron stars using both the electromagnetic and gravitational wave spectrums that will allow for tests of the appropriate theory of gravity.

astro-ph↗

arXiv2007

Alfvén QPOs in Magnetars

We investigate torsional Alfvén oscillations of relativistic stars with a global dipole magnetic field, via two-dimensional numerical simulations. We find that a) there exist two families of quasi-periodic oscillations (QPOs) with harmonics at integer multiples of the fundamental frequency, b) the lower-frequency QPO is related to the region of closed field lines, near the equator, while the higher-frequency QPO is generated near the magnetic axis, c) the QPOs are long-lived, d) for the chosen form of dipolar magnetic field, the frequency ratio of the lower to upper fundamental QPOs is ~0.6, independent of the equilibrium model or of the strength of the magnetic field, and e) within a representative sample of equations of state and of various magnetar masses, the Alfvén QPO frequencies are given by accurate empirical relations that depend only on the compactness of the star and on the magnetic field strength. The lower and upper QPOs can be interpreted as corresponding to the edges or turning points of an Alfvén continuum, according to the model proposed by Levin (2007). Several of the low-frequency QPOs observed in the X-ray tail of SGR 1806-20 can readily be identified with the Alfvén QPOs we compute. In particular, one could identify the 18Hz and 30Hz observed frequencies with the fundamental lower and upper QPOs, correspondingly, while the observed frequencies of 92Hz and 150Hz are then integer multiples of the fundamental upper QPO frequency (three times and five times, correspondingly). With this identification, we obtain an upper limit on the strength of magnetic field of SGR 1806-20 (if is dominated by a dipolar component) between ~3 and $7\times 10^{15}$G. Furthermore, we discuss the implications for the high-density EOS of compact stars. (Abridged)

astro-ph↗

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↗

arXiv2006

Gravitational waves from a dust disk around a Schwarzschild black hole

We investigate gravitational waves from a dust disk around a Schwarzschild black hole to focus on whether we can extract any of its physical properties from a direct detection of gravitational waves. We adopt a black hole perturbation approach in a time domain, which is a satisfactory approximation to illustrate a dust disk in a supermassive black hole. We find that we can determine the radius of the disk by using the power spectrum of gravitational waves and that our method to extract the radius works for a disk of arbitrary density distribution. Therefore we believe a possibility exists for determining the radius of the disk from a direct observation of gravitational waves detected by the Laser Interferometer Space Antenna.

gr-qc↗

arXiv2006

The comparison of velocity distribution between Adhesion approximation and the Euler-Jeans-Newton model

For the evolution of density fluctuation in nonlinear cosmological dynamics, adhesion approximation (AA) is proposed as a phenomenological model, which is especially useful for describing nonlinear evolution. However, the origin of the artificial viscosity in AA is not clarified. Recently, Buchert and Dom\'ınguez report if the velocity dispersion of the dust fluid is regarded as isotropic, it works on a principle similar to viscosity or effective pressure, and they consider isotropic velocity dispersion as the origin of the artificial viscosity in AA. They name their model the Euler-Jeans-Newton (EJN) model. In this paper, we focus on the velocity distribution in AA and the EJN model and examine the time evolution in both models. We find the behavior of AA differs from that of the EJN model, i.e., although the peculiar velocity in the EJN model oscillates, that in AA is monotonically decelerated due to viscosity without oscillation. Therefore it is hard to regard viscosity in AA as effective pressure in the EJN model.

astro-ph↗

arXiv2005

Stellar Oscillations in Scalar-Tensor Theory of Gravity

We derive the perturbation equations for relativistic stars in scalar-tensor theories of gravity and study the corresponding oscillation spectrum. We show that the frequency of the emitted gravitational waves is shifted proportionally to the scalar field strength. Scalar waves which might be produced from such oscillations can be a unique probe for the theory, but their detectability is questionable if the radiated energy is small. However we show that there is no need for a direct observation of scalar waves: the shift in the gravitational wave spectrum could unambiguously signal the presence of a scalar field.

gr-qc↗

arXiv2004

Probing Strong-Field Scalar-Tensor Gravity with Gravitational Wave Asteroseismology

We present an alternative way of tracing the existence of a scalar field based on the analysis of the gravitational wave spectrum of a vibrating neutron star. Scalar-tensor theories in strong-field gravity can potentially introduce much greater differences in the parameters of a neutron star than the uncertainties introduced by the various equations of state. The detection of gravitational waves from neutron stars can set constraints on the existence and the strength of scalar fields. We show that the oscillation spectrum is dramatically affected by the presence of a scalar field, and can provide unique confirmation of its existence.

gr-qc↗

arXiv2004

Restricting quark matter models by gravitational wave observation

We consider the possibilities for obtaining information about the equation of state for quark matter by using future direct observational data on gravitational waves. We study the nonradial oscillations of both fluid and spacetime modes of pure quark stars. If we observe the $f$ and the lowest $w_{\rm II}$ modes from quark stars, by using the simultaneously obtained radiation radius we can constrain the bag constant $B$ with reasonable accuracy, independently of the $s$ quark mass.

gr-qc↗

arXiv2003

Nonradial oscillations of quark stars

Recently, it has been reported that a candidate for a quark star may have been observed. In this article, we pay attention to quark stars with radiation radii in the reported range. We calculate nonradial oscillations of $f$-, $w$- and $w_{\rm II}$-modes. Then, we find that the dependence of the $f$-mode quasi-normal frequency on the bag constant and stellar radiation radius is very strong and different from that of the lowest $w_{\rm II}$-mode quasi-normal frequency. Furthermore we deduce a new empirical formula between the $f$-mode frequency of gravitational waves and the parameter of the equation of state for quark stars. The observation of gravitational waves both of the $f$-mode and of the lowest $w_{\rm II}$-mode would provide a powerful probe for the equation of state of quark matter and the properties of quark stars.

gr-qc↗

arXiv2001

Density Discontinuity of a Neutron Star and Gravitational Waves

We calculate quasi-normal f- and g-modes of a neutron star with density discontinuity, which may appear in a phase transition at extreme high density. We find that discontinuity will reflect largely on the f-mode, and that the g-mode could also be important for a less massive star.

gr-qc↗