arXiv subjects

Hajime Sotani

Publications and source records attributed to Hajime Sotani.

At least 91 records · Page 5Linked to original sources

arXiv2014

Observational discrimination of Eddington-inspired Born-Infeld gravity from general relativity

Direct observations of neutron stars could tell us an imprint of modified gravity. However, it is generally difficult to resolve the degeneracy due to the uncertainties in equation of state (EOS) of neutron star matter and in gravitational theories. In this paper, we are successful to find the observational possibility to distinguish Eddington-inspired Born-Infeld gravity (EiBI) from general relativity. We show that the radii of neutron stars with $0.5M_{sun}$ are strongly correlated with the neutron skin thickness of ${}^{208}$Pb independently of EOS, while this correlation depends on the coupling constant in EiBI. As a result, via the direct observations of radius of neutron star with $0.5M_{sun}$ and the measurements of neutron skin thickness of ${}^{208}$Pb by the terrestrial experiments, one could not only discriminate EiBI from general relativity but also estimate the coupling constant in EiBI.

astro-ph.HE↗

arXiv2014

Mass and radius formulas for low-mass neutron stars

Neutron stars, produced at the death of massive stars, are often regarded as giant neutron-rich nuclei. This picture is especially relevant for low-mass (below about solar mass) neutron stars, where non-nucleonic components are not expected to occur. Due to the saturation property of nucleonic matter, leading to the celebrated liquid-drop picture of atomic nuclei, empirical nuclear masses and radii can be approximately expressed as function of atomic mass number. It is, however, not straightforward to express masses and radii of neutron stars even in the low-mass range where the structure is determined by a balance between the pressure of neutron-rich nucleonic matter and the gravity. Such expressions would be of great use given possible simultaneous mass and radius measurements. Here we successfully construct theoretical formulas for the masses and radii of low-mass neutron stars from various models that are consistent with empirical masses and radii of stable nuclei. In this process, we discover a new equation-of-state parameter that characterizes the structure of low-mass neutron stars. This parameter, which plays a key role in connecting the mass-radius relation of the laboratory nuclei to that of the celestial objects, could be constrained from future observations of low-mass neutron stars.

astro-ph.HE↗

arXiv2014

Scalar gravitational waves from relativistic stars in scalar-tensor gravity

Unlike general relativity, the scalar gravitational waves can be excited due to the radial oscillations in scalar-tensor gravity. To examine the scalar gravitational waves in scalar-tensor gravity, we derive the evolution equations of the radial oscillations of neutron stars and determine the specific oscillation frequencies of the matter oscillations and scalar gravitational waves, where we adopt two different numerical approaches, i.e., the mode analysis and direct time evolution. As a result, we observe the spontaneous scalarization even in the radial oscillations. Depending on the background scalar field and coupling constant, the total energy radiated by the scalar gravitational waves dramatically changes, where the specific oscillation frequencies are completely same as the matter oscillations. That is, via the direct observations of scalar gravitational waves, one can not only reveal the gravitational theory, but also extract the radial oscillations of neutron stars.

astro-ph.HE↗

arXiv2014

Electromagnetic waves from neutron stars and black holes driven by polar gravitational perturbations

Neutron stars and black holes are the most compact astrophysical objects we can think of and as a consequence they are the main sources of gravitational waves. There are many astrophysically relevant scenarios in which these objects are immersed in or endowed with strong magnetic fields, in such a way that gravitational perturbations can couple to electromagnetic ones and can potentially trigger synergistic electromagnetic signatures. In a recent paper we derived the main equations for gravito-electromagnetic perturbations and studied in detail the case of polar electromagnetic perturbations driven by axial gravitational perturbations. In this paper we deal with the case of axial electromagnetic perturbations driven by polar black-hole or neutron stars oscillations, in which the energy emitted in case is considerably larger than in the previous case. In the case of neutron stars the phenomenon lasts considerably longer since the fluid acts as an energy reservoir that shakes the magnetic field for a timescale of the order of secs.

astro-ph.HE↗

arXiv2014

Electron screening effects on crustal torsional oscillations

We systematically examine the crustal torsional oscillations as varying the stellar mass and radius, where we take into account the effect of electron screening due to the inhomogeneity of electron distribution. In the examinations, we adopt two different equations of state (EOSs) for the inner crust region of neutron stars. As a result, we find that the frequencies depend obviously on the EOS, even if the neutron star models are almost independent of the EOS. That is, one could solve the degeneracy of the stellar models with different EOS via the observations of shear oscillations frequencies. Additionally, we find that the fundamental frequencies of the $l$-th order torsional oscillations can reduce 6% due to the effect of electron screening, which is independent of the adopted EOS and stellar models. This reduction of frequencies can be crucial to make a constraints on the density dependence of the nuclear symmetry energy, $L$, i.e., the constraint on $L$ can reduce ~15% by the electron screening effect.

astro-ph.HE↗

arXiv2013

Possible Constraints on the Density Dependence of the Nuclear Symmetry Energy from Quasiperiodic Oscillations in Soft Gamma Repeaters

We systematically examine the fundamental frequencies of shear torsional oscillations in neutron star crusts in a manner that is dependent on the parameter $L$ characterizing the poorly known density dependence of the symmetry energy. The identification of the lowest quasiperiodic oscillation (QPO) among the observed QPOs from giant flares in soft-gamma repeaters as the $/ell=2$ fundamental torsional oscillations enables us to constrain the parameter $L$ as $L/ge 47.4$ MeV, which is the most conservative restriction on $L$ obtained in the present work that assumes that the mass and radius of the flaring neutron stars range 1.4-1.8 $M_{/odot}$ and 10-14 km. Next, we identify one by one a set of the low-lying frequencies observed in giant flares as the fundamental torsional oscillations. The values of $L$ that can reproduce all the observed frequencies in terms of the torsional oscillations coupled with a part of dripped neutrons via entrainment effects are then constrained as 101.1 MeV $/le L /le$ 131.0 MeV. Alternatively, if only the second lowest frequency observed in SGR 1806$-$20 has a different origin, one obtains relatively low $L$ values ranging 58.0 MeV $/le L /le$ 85.3 MeV, which seem more consistent with other empirical constraints despite large uncertainties.

astro-ph.HE↗

arXiv2013

Gravitationally Driven Electromagnetic Perturbations of Neutron Stars and Black Holes

Gravitational perturbations of neutron stars and black holes are well known sources of gravitational radiation. If the compact object is immersed in or endowed with a magnetic field, the gravitational perturbations would couple to electromagnetic perturbations and potentially trigger synergistic electromagnetic signatures. We present a detailed analytic calculation of the dynamics of coupled gravitational and electromagnetic perturbations for both neutron stars and black holes. We discuss the prospects for detecting the electromagnetic waves in these scenarios and the potential that these waves have for providing information about their source.

astro-ph.HE↗

arXiv2013

Shear oscillations in the hadron-quark mixed phase

Inside neutron stars, the hadron-quark mixed phase is expected during the first order phase transition from the hadron phase to the quark phase. The geometrical structure of the mixed phase strongly depends on the surface tension at the hadron-quark interface. We evaluate the shear modulus which is one of the specific properties of the hadron-quark mixed phase. As an application, we study shear oscillations due to the hadron-quark mixed phase in neutron stars. We find that the frequencies of shear oscillations depend strongly on the surface tension; with a fixed stellar mass, the fundamental frequencies are almost proportional to the surface tension. Thus, one can estimate the value of surface tension via the observation of stellar oscillations with the help of the information on the stellar mass.

astro-ph.HE↗

arXiv2012

Slowly Rotating Relativistic Stars in Scalar-Tensor Gravity

We consider the slowly rotating relativistic stars with a uniform angular velocity in the scalar-tensor gravity, and examine the rotational effect around such compact objects. For this purpose, we derive a 2nd order differential equation describing the frame dragging in the scalar-tensor gravity and solve it numerically. As a result, we find that the total angular momentum is proportional to the angular velocity even in the scalar-tensor gravity. We also show that one can observe the spontaneous scalarization in rotational effects as well as the other stellar properties, if the cosmological value of scalar field is zero. On the other hand, if the cosmological value of scalar field is nonzero, the deviation from the general relativity can be seen in a wide range of the coupling constant. Additionally, we find that the deviation from the general relativity becomes larger with more massive stellar models, which is independent of the cosmological value of scalar field. Thus, via precise observations of astronomical phenomena associated with rotating relativistic stars, one may be possible to probe not only the gravitational theory in the strong-field regime, but also the existence of scalar field.

astro-ph.HE↗

arXiv2012

Effect of superfluidity on neutron star crustal oscillations

We consider how superfluidity of dripped neutrons in the crust of a neutron star affects the frequencies of the crust's fundamental torsional oscillations. A nonnegligible superfluid part of dripped neutrons, which do not comove with nuclei, act to reduce the enthalpy density and thus enhance the oscillation frequencies. By assuming that the quasi-periodic oscillations observed in giant flares of soft gamma repeaters arise from the fundamental torsional oscillations and that the mass and radius of the neutron star is in the range of 1.4 < M/M_SUN < 1.8 and 10 km < R < 14 km, we constrain the density derivative of the symmetry energy as 100 MeV < L < 130 MeV, which is far severer than the previous one, L > 50 MeV, derived by ignoring the superfluidity.

astro-ph.HE↗

arXiv2012

Thermodynamical description of hadron-quark phase transition and its implications on compact-star phenomena

One of the most promising possibilities may be the appearance of quark matter in astrophysical phenomena in the light of recent progress in observations. The mechanism of deconfinement is not well understood, but the thermodynamical aspects of the hadron-quark (HQ) phase transition have been extensively studied in recent years. Then the mixed phase of hadron and quark matter becomes important; the proper treatment is needed to describe the HQ phase transition and derive the equation of state (EOS) for the HQ matter, based on the Gibbs conditions for phase equilibrium. We here adopt a EOS based on the baryon-baryon interactions including hyperons for the hadron phase, while we use rather simple EOS within the MIT bag model in the quark phase. For quark matter we further try to improve the previous EOS by considering other effective models of QCD. One of the interesting consequences may be the appearance of the inhomogeneous structures called "pasta", which are brought about by the surface and the Coulomb interaction effects. We present here a comprehensive review of our recent works about the HQ phase transition in various astrophysical situations: cold catalyzed matter, hot matter and neutrino-trapped matter. We show how the pasta structure becomes unstable by the charge screening of the Coulomb interaction, thermal effect or the neutrino trapping effect. Such inhomogeneous structure may affect astrophysical phenomena through its elasticity or thermal properties. Here we also discuss some implications on supernova explosion, gravitational wave and cooling of compact stars.

astro-ph.HE↗

arXiv2012

Probing the Equation of State of Nuclear Matter via Neutron Star Asteroseismology

We general relativistically calculate the frequency of fundamental torsional oscillations of neutron star crusts, where we focus on the crystalline properties obtained from macroscopic nuclear models in a way depending on the equation of state of nuclear matter. We find that the calculated frequency is sensitive to the density dependence of the symmetry energy, but almost independent of the incompressibility of symmetric nuclear matter. By identifying the lowest-frequency quasi-periodic oscillation in giant flares observed from soft gamma-ray repeaters as the fundamental torsional mode and allowing for the dependence of the calculated frequency on stellar models, we provide a lower limit of the density derivative of the symmetry energy as $L\simeq 50$ MeV.

astro-ph.HE↗

arXiv2012

NR/HEP: roadmap for the future

Physics in curved spacetime describes a multitude of phenomena, ranging from astrophysics to high energy physics. The last few years have witnessed further progress on several fronts, including the accurate numerical evolution of the gravitational field equations, which now allows highly nonlinear phenomena to be tamed. Numerical relativity simulations, originally developed to understand strong field astrophysical processes, could prove extremely useful to understand high-energy physics processes like trans-Planckian scattering and gauge-gravity dualities. We present a concise and comprehensive overview of the state-of-the-art and important open problems in the field(s), along with guidelines for the next years. This writeup is a summary of the "NR/HEP Workshop" held in Madeira, Portugal from August 31st to September 3rd 2011.

hep-th↗

arXiv2011

Torsional oscillations in tensor-vector-scalar theory

With the Cowling approximation, the torsional oscillations on relativistic stars in tensor-vector-scalar (TeVeS) theory are examined. The spectrum features in TeVeS are very similar to those in general relativity (GR), but the torsional frequencies in TeVeS become larger than those expected in GR. We find that, compared with the fluid oscillations with polar parity, the torsional frequencies depend strongly on the gravitational theory. Since the dependences of fundamental frequencies on the gravitational theory and on the equation of state are different from those of overtone, it could be possible to distinguish TeVeS from GR in the strong-field regime via observations of this type of oscillations with the help of the observation of stellar mass.

astro-ph.HE↗

arXiv2011

Constraints on Pasta Structure of Neutron Stars from Oscillations in Giant Flares

We show that the shear modes in the neutron star crust are quite sensitive to the existence of nonuniform nuclear structures, so-called "pasta". Due to the existence of pasta phase, the frequencies of shear modes are reduced, where the dependence of fundamental frequency is different from that of overtones. Since the torsional shear frequencies depend strongly on the structure of pasta phase, through the observations of stellar oscillations, one can probe the pasta structure in the crust, although that is quite difficult via the other observations. Additionally, considering the effect of pasta phase, we show the possibility to explain the all observed frequencies in the SGR 1806-20 with using only crust torsional shear modes.

astro-ph.HE↗

arXiv2010

Signatures of hadron-quark mixed phase in gravitational waves

We calculate stellar oscillations including the hadron-quark mixed phase considering the finite size effects. We find that it is possible to distinguish whether the density discontinuity exists or not in the stars, even if one will observe the gravitational waves of the fundamental mode. Additionally, the normalized eigenfrequencies of pressure modes depend strongly on the stellar mass and on the adopted equation of state. Especially, in spite of the fact that the radius of the neutron star with $1.4M_\odot$, which is standard mass, is almost independent from the equation of state with quark matter, the frequencies of pressure modes depend on the adopted equation of state. Thus, via observing the many kinds of gravitational waves, it will be possible to make a restriction on the equation of state.

astro-ph.HE↗

arXiv2010

Toroidal oscillations of slowly rotating relativistic star in tensor-vector-scalar theory

We examine the toroidal oscillations on the slowly rotating relativistic stars in tensor-vector-scalar (TeVeS) theory with the Cowling approximation. As a result, we find that perturbation equations describing the toroidal oscillations are same equation form as in general relativity (GR). Although the frequencies of toroidal oscillations in TeVeS are not so different from those in GR, the momentum inertia depends strongly on the gravitational theory. Thus, observing the frequencies of toroidal oscillations and momentum inertia with high accuracy might reveal the gravitational theory in the strong-field regime.

astro-ph.HE↗

arXiv2010

Slowly Rotating Relativistic Stars in Tensor-Vector-Scalar Theory

In order to examine the rotational effect around neutron star in tensor-vector-scalar (TeVeS) theory, we consider the slowly rotating relativistic stars with a uniform angular velocity. As a result, we find that similar to the case in general relativity (GR), the angular momentum is proportional to the angular velocity. Additionally, as the value of coupling constant $K$ becomes higher, the frame dragging in TeVeS becomes quite different distribution from that in GR, where we can also see the deviation even in the interior of star. While with smaller value of $K$, although the frame dragging approaches to that expected in GR, the induced vector field due to the rotation does not vanish and still exists. Thus, through the observations associated with relativistic object, one could be possible to distinguish the gravitational theory in strong field regime even in the case that the value of coupling constant $K$ is quite small.

astro-ph.HE↗