arXiv subjects

Hajime Sotani

Publications and source records attributed to Hajime Sotani.

At least 73 records · Page 4Linked to original sources

arXiv2017

Probing crustal structures from neutron star compactness

With various sets of the parameters that characterize the equation of state (EOS) of nuclear matter, we systematically examine the thickness of a neutron star crust and of the pasta phases contained therein. Then, with respect to the thickness of the phase of spherical nuclei, the thickness of the cylindrical phase, and the crust thickness, we successfully derive fitting formulas that express the ratio of each thickness to the star's radius as a function of the star's compactness, the incompressibility of symmetric nuclear matter, and the density dependence of the symmetry energy. In particular, we find that the thickness of the phase of spherical nuclei has such a strong dependence on the stellar compactness as the crust thickness, but both of them show a much weaker dependence on the EOS parameters. Thus, via determination of the compactness, the thickness of the phase of spherical nuclei as well as the crust thickness can be constrained reasonably, even if the EOS parameters remain to be well-determined.

astro-ph.HE↗

arXiv2017

On the maximum mass limit of neutron stars in scalar-tensor gravity

The maximum mass limits of neutron stars in scalar-tensor gravity is discussed and compared with the limits set by general relativity. The limit is parametrized with respect to the combination of the nuclear saturation parameters and the maximum sound velocity in the core. It is shown that, for smaller values of the sound velocity in the core, the maximum mass limit of the scalarized neutron stars is larger than that of in general relativity. However, for stiff equations of state with sound velocity higher than $79\%$ of the velocity of light, the maximum mass limit in general relativity is larger than that of in scalar-tensor gravity. The results suggest that future observations of massive neutron stars, may constrain the maximum sound velocity as well as the coupling parameter in scalar-tensor gravity.

gr-qc↗

arXiv2017

Quark matter with strong magnetic field and possibility of the third family of compact stars

We consider the possibility for the existence of the third family of compact objects, considering the effect of strong magnetic fields inside the hybrid stars. As a result, we demonstrate such new sequences of stable equilibrium configurations for some hadronic equations of state. Through the analysis of the adiabatic index inside stars, we find the conditions for appearing the third family of compact objects, i.e., for hadronic stars without quarks, that the maximum mass should be small, the central density for the maximum mass should be also small, and the radius for the the maximum mass should be large. Even for soft hadronic equations of state, the two solar-mass stars might survive as the third family of compact objects, once quark matter with strong magnetic field, such as $\sim {\cal O}(10^{19} {\rm G})$, is taken into account. It might give a hint to solve the so-called hyperon puzzle in nuclear physics.

astro-ph.HE↗

arXiv2017

Effect of nuclear saturation parameters on possible maximum mass of neutron stars

In order to systematically examine the possible maximum mass of neutron stars, which is one of the important properties characterizing the physics in high-density region, I construct neutron star models by adopting phenomenological equations of state with various values of nuclear saturation parameters for low-density region, which are connected to the equation of state for high-density region characterized by the possible maximum sound velocity in medium. I derive an empirical formula for the possible maximum mass of neutron star. If massive neutron stars are observed, it could be possible to get a constraint on the possible maximum sound velocity for high-density region.

astro-ph.HE↗

arXiv2016

Probing nuclear bubble structure via neutron star asteroseismology

We consider torsional oscillations that are trapped in a layer of spherical-hole (bubble) nuclear structure, which is expected to occur in the deepest region of the inner crust of a neutron star. Because this layer intervenes between the phase of slab nuclei and the outer core of uniform nuclear matter, torsional oscillations in the bubble phase can be excited separately from usual crustal torsional oscillations. We find from eigenmode analyses for various models of the equation of state of uniform nuclear matter that the fundamental frequencies of such oscillations are almost independent of the incompressibility of symmetric nuclear matter, but strongly depend on the slope parameter of the nuclear symmetry energy $L$. Although the frequencies are also sensitive to the entrainment effect, i.e., what portion of nucleons outside bubbles contribute to the oscillations, by having such a portion fixed, we can successfully fit the calculated fundamental frequencies of torsional oscillations in the bubble phase inside a star of specific mass and radius as a function of $L$. By comparing the resultant fitting formula to the frequencies of quasi-periodic oscillations (QPOs) observed from the soft-gamma repeaters, we find that each of the observed low-frequency QPOs can be identified either as a torsional oscillation in the bubble phase or as a usual crustal oscillation, given generally accepted values of $L$ for all the stellar models considered here.

astro-ph.HE↗

arXiv2016

Gravitational wave asteroseismology with protoneutron stars

We examine the time evolution of the frequencies of the gravitational wave after the bounce within the framework of relativistic linear perturbation theory using the results of one dimensional numerical simulations of core-collapse supernovae. Protoneutron star models are constructed in such a way that the mass and radius of protoneutron star become equivalent to the results obtained from the numerical simulations. Then, we find that the frequencies of gravitational waves radiating from protoneutron stars strongly depend on the mass and radius of protoneutron stars, but almost independently of the profiles of electron fraction and entropy per baryon inside the star. Additionally, we find that the frequencies of gravitational waves can be characterized by the square root of the average density of protoneutron star irrespectively the progenitor models, which are completely different from the empirical formula for cold neutron stars. The dependence of the spectra on the mass and radius is different from that of the $g$-mode: the oscillations around the surface of protoneutron stars due to the convection and the standing accretion-shock instability. Careful observations of the these modes of gravitational waves can determine the evolution of the mass and radius of protoneutron stars after core-bounce. Furthermore, the expected frequencies of gravitational waves are around a few hundred hertz in the early stage after bounce, which must be a good candidate for the ground-based gravitational wave detectors.

astro-ph.HE↗

arXiv2016

Low-mass neutron stars: universal relations, the nuclear symmetry energy and gravitational radiation

The lowest neutron star masses currently measured are in the range $1.0-1.1~M_\odot$, but these measurement have either large uncertainties or refer to isolated neutron stars. The recent claim of a precisely measured mass $M/M_{\odot} = 1.174 \pm 0.004$ by Martinez et al [Astrophys. J. 812, 143 (2015)] in a double neutron star system suggests that low-mass neutron stars may be an interesting target for gravitational-wave detectors. Furthermore, Sotani et al [PTEP 2014, 051E01 (2014)] recently found empirical formulas relating the mass and surface redshift of nonrotating neutron stars to the star's central density and to the parameter $η\equiv (K_0 L^2)^{1/3}$, where $K_0$ is the incompressibility of symmetric nuclear matter and $L$ is the slope of the symmetry energy at saturation density. Motivated by these considerations, we extend the work by Sotani et al to slowly rotating and tidally deformed neutron stars. We compute the moment of inertia, quadrupole moment, quadrupole ellipticity, tidal and rotational Love number and apsidal constant of slowly rotating neutron stars by integrating the Hartle-Thorne equations at second order in rotation, and we fit all of these quantities as functions of $η$ and of the central density. These fits may be used to constrain $η$, either via observations of binary pulsars in the electromagnetic spectrum, or via near-future observations of inspiralling compact binaries in the gravitational-wave spectrum.

astro-ph.HE↗

arXiv2016

Empirical formula of crustal torsional oscillations

Crustal torsional oscillations depend on not only crust properties but also the stellar mass and radius. Thus, one could extract stellar information by identifying the observed frequencies of stellar oscillations with the crustal torsional oscillations. Owing to the confinement of torsional oscillations inside the crust region of neutron stars, we successfully derive an empirical formula for the fundamental crustal torsional oscillations as a function of the stellar mass, radius, the so-called slope parameter of the nuclear symmetry energy, and the angular index of oscillations, with which one can estimate the frequencies with high accuracy. This empirical formula could be valuable in both the astrophysics and nuclear physics communities.

astro-ph.HE↗

arXiv2015

Testing General Relativity with Present and Future Astrophysical Observations

One century after its formulation, Einstein's general relativity has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that general relativity should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of general relativity. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einstein's theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

gr-qc↗

arXiv2015

Torsional oscillations of neutron stars with highly tangled magnetic fields

To determine the frequencies of magnetic oscillations in neutron stars with highly tangled magnetic fields, we derive the perturbation equations. We assume that the field strength of the global magnetic structure is so small that such fields are negligible compared with tangled fields, which may still be far from a realistic configuration. Then, we systematically examine the spectra of the magnetic oscillations, as varying the magnetic field strength and stellar mass. The frequencies without crust elasticity are completely proportional to the strength of the magnetic field, whose proportionality constant depends strongly on the stellar mass. On the other hand, the oscillation spectra with crust elasticity become more complicated, where the frequencies even for weak magnetic fields are different from the crustal torsional oscillations without magnetic fields. For discussing spectra, the critical field strength can play an important role, and it is determined in such a way that the shear velocity is equivalent to the Alfvén velocity at the crust basis. Additionally, we find that the effect of the crust elasticity can be seen strongly in the fundamental oscillations with a lower harmonic index, $\ell$. Unlike the stellar models with a pure dipole magnetic field, we also find that the spectra with highly tangled magnetic fields become discrete, where one can expect many of the eigenfrequencies. Maybe these frequencies could be detected after the violent phenomena breaking the global magnetic field structure.

astro-ph.HE↗

arXiv2015

Strong gravitational lensing by an electrically charged black hole in Eddington-inspired Born-Infeld gravity

We systematically examine the properties of null geodesics around an electrically charged, asymptotically flat black hole in Eddington-inspired Born-Infeld gravity, varying the electric charge of black hole and the coupling constant in the theory. We find that the radius of the unstable circular orbit for massless particle decreases with the coupling constant, if the value of the electrical charge is fixed. Additionally, we consider the strong gravitational lensing around such a black hole. We show that the deflection angle, the position angle of the relativistic images, and the magnification due to the light bending in strong gravitational field are quite sensitive to the parameters determining the black hole solution. Thus, through the accurate observations associated with the strong gravitational lensing, it might be possible to reveal the gravitational theory in a strong field regime.

gr-qc↗

arXiv2015

Possible identifications of newly observed magnetar quasi-periodic oscillations as crustal shear modes

Quasi-periodic oscillations (QPOs) discovered in soft-gamma repeaters (SGRs) are expected to help us to study the properties of matter in neutron stars. In earlier investigations, we identified the QPOs of frequencies below $\sim100$ Hz observed in giant flares of SGR 1806$-$20 and SGR 1900+14 as the crustal torsional oscillations. For this purpose, we calculated the frequencies of the fundamental torsional oscillations with various angular indices $\ell$, by changing the stellar mass and radius. In this work, we try to explain the additional QPO frequencies recently reported by Huppenkothen et al. (2014a,b) within the same framework as before except that we newly take into account the effect of electron screening, which acts to decrease the frequencies by a small amount. Those QPOs were discovered in two different SGRs, i.e., SGR 1806$-$20 and SGR J1550$-$5418. Then, we find that the newly observed QPO frequency in SGR 1806$-$20 can be still identified as one of the frequencies of the fundamental torsional oscillations, while those in SGR J1550$-$5418 can also be explained in terms of the torsional oscillations although the relevant angular indices are difficult to identify.

astro-ph.HE↗

arXiv2015

Magnetized relativistic stellar models in Eddington-inspired Born-Infeld gravity

We consider the structure of the magnetic fields inside the neutron stars in Eddington-inspired Born-Infeld (EiBI) gravity. In order to construct the magnetic fields, we derive the relativistic Grad-Shafranov equation in EiBI, and numerically determine the magnetic distribution in such a way that the interior magnetic fields should be connected to the exterior distribution. Then, we find that the magnetic distribution inside the neutron stars in EiBI is qualitatively similar to that in general relativity, where the deviation of magnetic distribution in EiBI from that in general relativity is almost comparable to uncertainty due to the equation of state (EOS) for the neutron star matter. However, we also find that the magnetic fields in the crust region are almost independent of the coupling constant in EiBI, which suggests a possibility to obtain the information about the crust EOS independently of the gravitational theory via the observations of the phenomena associated with the crust region. In any case, since the imprint of EiBI gravity on the magnetic fields is weak, the magnetic fields could be a poor probe of gravitational theories, considering many magnetic uncertainties.

astro-ph.HE↗

arXiv2015

Constraining the density dependence of the nuclear symmetry energy from an X-ray bursting neutron star

Neutrons stars lighter than the Sun are basically composed of nuclear matter of density up to around twice normal nuclear density. In our recent analyses, we showed that possible simultaneous observations of masses and radii of such neutron stars could constrain $η\equiv(K_0L^2)^{1/3}$, a combination of the incompressibility of symmetric nuclear matter $K_0$ and the density derivative of the nuclear symmetry energy $L$ that characterizes the theoretical mass-radius relation. In this paper, we focus on the mass-radius constraint of the X-ray burster 4U 1724-307 given by Suleimanov et al. (2011). We therefrom obtain the constraint that $η$ should be larger than around 130 MeV, which in turn leads to $L$ larger than around 110, 98, 89, and 78 MeV for $K_0=180$, 230, 280, and 360 MeV. Such a constraint on $L$ is more or less consistent with that obtained from the frequencies of quasi-periodic oscillations in giant flares observed in soft-gamma repeaters.

astro-ph.HE↗

arXiv2014

Massive Hybrid Quark Stars with Strong Magnetic Field

We estimate the critical magnetic field strength at which the lowest or second Landau levels play an important role in the quark phase inside the hybrid stars, and show that the magnetic field should be in the order of $10^{19}$ Gauss at the nuclear density $\sim0.16$ fm$^{-3}$. We also find that the pressure of quark matter settling only in the lowest Landau level can be expressed as a function of the energy density independently of the magnetic field strength, which corresponds to the causality limit of a stiff equation of state. Adiabatic index of quark matter well exceeds 4/3 in the core, and we find a possibility to construct massive hybrid star models occupied in large part by quark matter, whose maximum mass becomes larger than two solar mass.

astro-ph.HE↗

arXiv2014

Properties of an electrically charged black hole in Eddington-inspired Born-Infeld gravity

We systematically examine the properties of an electrically charged black hole in Eddington-inspired Born-Infeld gravity with not only the positive but also the negative coupling constant in the theory. As a result, we numerically find that the black hole solution exists even with the negative coupling constant, where the electric charge of black hole can be larger than the black hole mass. We also clarify the parameter space where the black hole solution exists. On the other hand, to examine the particle motion around such black hole, we derive the geodesic equation. The behavior of the effective potential for the radial particle motion is almost the same as that in general relativity, but the radius of the innermost stable circular orbit and the angular momentum giving the innermost stable circular orbit can be changed, depending on the coupling constant. In particular, we find that the radius of innermost stable circular orbit with the specific value of the coupling constant can be smaller than that for the extreme case in general relativity. Such a particle can release the gravitational binding energy more than the prediction in general relativity, which could be important from the observational point of view.

gr-qc↗

arXiv2014

Torsional oscillations of neutron stars in scalar-tensor theory of gravity

We study torsional oscillations of neutron stars in the scalar-tensor theory of gravity using the relativistic Cowling approximation. We compute unperturbed neutron star models adopting realistic equations of state for the neutron star's core and crust. For scalar-tensor theories that allow for spontaneous scalarization, the crust thickness can be significantly smaller than in general relativity. We derive the perturbation equation describing torsional oscillations in scalar-tensor theory, and we solve the corresponding eigenvalue problem to find the oscillation frequencies. The fundamental mode (overtone) frequencies become smaller (larger) than in general relativity for scalarized stellar models. Torsional oscillation frequencies may yield information on the crust microphysics if microphysics effects are not degenerate with strong-gravity effects, such as those due to scalarization. To address this issue, we consider two different models for the equation of state of the crust and we look at the effects of electron screening. The effect of scalarization on torsional oscillation frequencies turns out to be smaller than uncertainties in the microphysics for all spontaneous scalarization models allowed by binary pulsar observations. Our study shows that the observation of quasi-periodic oscillations (QPOs) following giant flares can be used to constrain the microphysics of neutron star crusts, whether spontaneous scalarization occurs or not.

gr-qc↗

arXiv2014

Stellar oscillations in Eddington-inspired Born-Infeld gravity

We consider the stellar oscillations of relativistic stars in the Eddington-inspired Born-Infeld gravity (EiBI). In order to examine the specific frequencies, we derive the perturbation equations governing the stellar oscillations in EiBI by linearizing the field equations, and numerically determine the oscillation frequencies as changing the coupling parameter in EiBI, $κ$, and stellar models. As a result, we find that the frequencies depend strongly on the value of $κ$, where the frequencies in EiBI with negative $κ$ become higher and those with positive $κ$ become lower than the expectations in general relativity. We also find that, via the observation of the fundamental frequency, one could distinguish EiBI with $8πε_0|κ|\gtrsim 0.03$ from general relativity, independently of the equation of state (EOS) for neutron star matter, where $ε_0$ denotes the nuclear saturation density and $ε_0κ$ become dimensionless parameter. With the further constraints on EOS, one might distinguish EiBI even with $8πε_0|κ|\lesssim 0.03$ from general relativity.

astro-ph.HE↗