Dense Neutron Stars Research Articles

Constraining compactness and magnetic field geometry of X-ray pulsars from the statistics of their pulse profiles

<i>Context. <i/>The light curves observed from X-ray pulsars and magnetars reflect the radiation emission pattern, the geometry of the magnetic field, and the neutron star compactness. <i>Aims. <i/>We study the statistics of X-ray pulse profiles in order to constrain the neutron star compactness and the magnetic field geometry.<i>Methods. <i/>We collect the data for 124 X-ray pulsars, which are mainly in high-mass X-ray binary systems, and classify their pulse profiles according to the number of observed peaks seen during one spin period, dividing them into two classes, single- and double-peaked. We find that the pulsars are distributed about equally between both groups. We also compute the probabilities predicted by the theoretical models of two antipodal point-like spots that emit radiation according to the pencil-like emission patterns. These are then compared to the observed fraction of pulsars in the two classes. <i>Results. <i/>Assuming a blackbody emission pattern, it is possible to constrain the neutron star compactness if the magnetic dipole has arbitrary inclinations to the pulsar rotational axis. More realistic pencil-beam patterns predict that 79% of the pulsars are double-peaked independently of their compactness. The theoretical predictions can be made consistent with the data if the magnetic dipole inclination to the rotational axis has an upper limit of 40° <i>±<i/> 4°. We also discuss the effect of limited sensitivity of the X-ray instruments to detect weak pulses, which lowers the number of detected double-peaked profiles and makes the theoretical predictions to be consistent with the data even if the magnetic dipole does have random inclinations. This shows that the statistics of pulse profiles does not allow us to constrain the neutron star compactness. In contrast to the previous claims by Bulik et al. (2003, A&A, 404, 1023), the data also do not require the magnetic inclination to be confined in a narrow interval.

COMPACT STARS IN HADRON AND QUARK-HADRON MODELS

We investigate strongly interacting dense matter and neutron stars using a flavor-SU(3) approach based on a nonlinear realization of chiral symmetry. We study chiral symmetry restoration and the equation of state of stellar matter and determine neutron star properties using different sets of degrees of freedom. Finally, we include quarks in the model approach. We show the resulting phase diagram as well as hybrid star solutions for this model.

R-modes in low temperature color-flavor-locked superconducting quark stars

We present the first multi-fluid analysis of a dense neutron star core with a deconfined colour-flavour-locked superconducting quark component. Accounting only for the condensate and (finite temperature) phonons, we make progress by taking over results for superfluid $^4$He. The resultant two-fluid model accounts for a number of additional viscosity coefficients (compared to the Navier-Stokes equations) and we show how they enter the dissipation analysis for an oscillating star. We provide simple estimates for the gravitational-wave driven r-mode instability, demonstrating that the various phonon processes that we consider are not effective damping agents. Even though the results are likely of little direct astrophysical importance (since we consider an overly simplistic stellar model) our analysis represents significant technical progress, laying the foundation for more detailed numerical studies and preparing the ground for the inclusion of additional aspects (in particular associated with kaons) of the problem.

Imprints of the nuclear symmetry energy on gravitational waves from the axialw-modes of neutron stars

The eigenfrequencies of the axial $w$-modes of oscillating neutron stars are studied using the continued fraction method with an equation of state (EOS) partially constrained by the recent terrestrial nuclear laboratory data. It is shown that the density dependence of the nuclear symmetry energy ${E}_{\mathrm{sym}}(\ensuremath{\rho})$ affects significantly both the frequencies and the damping times of these modes. Besides confirming the previously found universal behavior of the mass-scaled eigenfrequencies as functions of the compactness of neutron stars, we explored several alternative universal scaling functions. Moreover, the ${w}_{\mathit{II}}$-mode is found to exist only for neutron stars having a compactness of $M/R\ensuremath{\geqslant}0.1078$ independent of the EOS used.

Measuring the neutron star equation of state with gravitational wave observations

We report the results of a first study that uses numerical simulations to estimate the accuracy with which one can use gravitational wave observations of double neutron star inspiral to measure parameters of the neutron-star equation of state. The simulations use the evolution and initial-data codes of Shibata and Uryu to compute the last several orbits and the merger of neutron stars, with matter described by a parametrized equation of state. Previous work suggested the use of an effective cutoff frequency to place constraints on the equation of state. We find, however, that greater accuracy is obtained by measuring departures from the point-particle limit of the gravitational waveform produced during the late inspiral. As the stars approach their final plunge and merger, the gravitational wave phase accumulates more rapidly for smaller values of the neutron star compactness (the ratio of the mass of the neutron star to its radius). We estimate that realistic equations of state will lead to gravitational waveforms that are distinguishable from point particle inspirals at an effective distance (the distance to an optimally oriented and located system that would produce an equivalent waveform amplitude) of 100 Mpc or less. As Lattimer and Prakash observed, neutron-star radius is closely tied to the pressure at density not far above nuclear. Our results suggest that broadband gravitational wave observations at frequencies between 500 and 1000 Hz will constrain this pressure, and we estimate the accuracy with which it can be measured. Related first estimates of radius measurability show that the radius can be determined to an accuracy of ~1 km at 100 Mpc.

Deconfinement phase transition in neutron star matter

The transition from hadron phase to strange quark phase in dense matter is investigated. Instead of using the conventional bag model in quark sect, we achieve the confinement by a density-dependent quark mass derived from in-medium chiral condensates, with a thermodynamic problem improved. In nuclear slot, we adopt the equation of state from Brueckner–Bethe–Goldstone approach with three-body force. It is found that the mixed phase can occur, for reasonable confinement parameter, near the normal saturation density, and transit to pure quark matter at 4–5 times the saturation, which is quite different from the previous results from other quark models that pure quark phase can not appear at neutron-star densities.

Influence of theσ−ωmeson interaction on neutron star matter

Relativistic mean field theory with nonlinear scalar self-interaction and isoscalar scalar-vector cross-interaction is used to study the properties of neutron star matter in $\ensuremath{\beta}$ equilibrium with and without hyperons. The influence of $\ensuremath{\sigma}\text{\ensuremath{-}}\ensuremath{\omega}$ meson cross-interaction on the properties of neutron star matter and the mass-radius relation of neutron stars is examined with attractive and repulsive \ensuremath{\Sigma} potential, respectively. The calculated result indicates that the cross-interaction softens the equation of state (EOS) of nuclear (hadronic) matter and reduces the maximum mass of neutron stars. It also decreases the densities for hyperonization to occur and lowers the center density of neutron stars. The increase of the cross-interaction strength enhances the softening effect of hyperons on the EOS. Meanwhile the repulsive \ensuremath{\Sigma} potential stiffens slightly the EOS and influences obviously the composition of neutron star matter.

The Density‐Dependent Correlations among Observables in Nuclear Matter and Hyperon‐Rich Neutron Stars

The conserving σ‐ω‐ρ mean‐field approximation with nonlinear interactions of hadrons has been applied to examine properties of nuclear matter and hyperonic neutron stars. The nonlinear interactions that will produce density‐dependent effective masses and coupling constants of hadrons are included in order to examine density correlations among properties of nuclear matter and neutron stars such as binding energy, incompressibility, K, symmetry energy, a4, hyperon‐onset density, and maximum masses of neutron stars. The conditions of conserving approximations in order to maintain thermodynamic consistency to an approximation are essential for the analysis of density‐dependent correlations.

Electrostatic drift waves in nonuniform quantum magnetized plasmas

Electrostatic drift waves (EDWs) in nonuniform quantum magnetized plasmas are described by the quantum hydrodynamic model. Electrons are viewed as a low-temperature Fermi gas. Analytical expression of the dispersion relationship of the quantum EDW is presented. Quantum effects are shown to affect the dispersion of the EDW significantly. The effects on the dispersion relation due to the magnetic field and spatial inhomogeneity give rise to results similar to the classical case. Our results should be relevant to dense astrophysical objects, e.g., neutron stars, magnet-stars, and white dwarfs.

Hypernuclear physics for neutron stars

The role of hypernuclear physics for the physics of neutron stars is delineated. Hypernuclear potentials in dense matter control the hyperon composition of dense neutron star matter. The three-body interactions of nucleons and hyperons determine the stiffness of the neutron star equation of state and thereby the maximum neutron star mass. Two-body hyperon–nucleon and hyperon–hyperon interactions give rise to hyperon pairing which exponentially suppresses cooling of neutron stars via the direct hyperon URCA processes. Nonmesonic weak reactions with hyperons in dense neutron star matter govern the gravitational wave emissions due to the r-mode instability of rotating neutron stars.

Universal thermodynamics of strongly interacting Fermi gases

Strongly interacting Fermi gases are of great current interest. Not only are fermions the most common particles in the universe, but they are also thought to have a universal thermodynamic behavior for strong interactions \cite{heiselberg,carlson,ho}. Recent experiments on ultra-cold Fermi gases provide an unprecedented opportunity to test universality in the laboratory \cite{hara,thomas,partridge,stewart,luo}. In principle this allows - for example - the interior properties of hot, dense neutron stars to be investigated on earth. Here we carry out a detailed test of this prediction. We analyze results from three ultra-cold fermion experiments involving two completely distinct atomic species in different kinds of atomic trap environments \cite{partridge,stewart,luo}. The data is compared with the predictions of a recent strong interaction theory \cite{hldepl,hldpra}. Excellent agreement is obtained, with no adjustable parameters. By extrapolating to zero temperature, we show that the experimental measurements yield a many-body parameter} $\mathbf{\beta\simeq-0.59\pm.07}$, \textbf{describing the universal energy of strongly interacting Fermi gases.

Relativistic astrophysics and cosmology in Uzbekistan

AbstractThe theoretical results obtained in Uzbekistan in the field of relativistic astrophysics and cosmology are presented. In particular electrostatic plasma modes along the open field lines of a rotating neutron star and Goldreich-Julian charge density in general relativity are analyzed for the rotating and oscillating magnetized neutron stars. The impact that stellar oscillations of different type (radial, toroidal and spheroidal ones) have on electric and magnetic fields external to a relativistic magnetized star has been investigated. A study of the dynamical evolution and the number of stellar encounters in globular clusters with a central black hole is presented. Perturbation features and instabilities of the large-scale oscillations on the background of the non-linearly pulsating isotropic and isotropic Ω-models are studied. The non-stationary dispersion equation of the sectorial perturbations for the general case and the results of certain oscillation mode analysis are given. The model composed as the linear superposition of two other models was constructed and the stability of this model is studied. In a cosmological setting the theory of macroscopic gravity as a large-distance scale generalization of general relativity has been developed. Exact cosmological solutions to the equations of macroscopic gravity for a flat spatially homogeneous, isotropic space-time are found. The gravitational correlation terms in the averaged Einstein equations have the form of spatial curvature, dark matter and dark energy (cosmological constant) with particular equations of state for each correlation regime. Interpretation of these cosmological models to explain the observed large-scale structure of the accelerating Universe with a significant amount of the nonluminous (dark) matter is discussed.

Neutron star equation of state and the possibility of complex self-energy in Landau theory of a Fermi liquid in the presence of a strong, quantizing magnetic field

Using a relativistic version of Landau theory of Fermi liquid with $\sigma-\omega$ and $\rho$ mesons exchange, we have obtained an equation of state for dense neutron star matter in presence of strong quantizing magnetic field. It is found that in this scenario the self energies of both neutron and proton components of dense neutron star matter become complex under certain physical conditions. To be more specific, it is observed that in the exchange diagrams of $\sigma$, $\omega$ and neutral $\rho$ transfer processes and in the direct interaction diagram with $\rho_\pm$ transfer reactions, the nucleon self-energies become complex in nature.

Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state

Spectra of the spreading layers on the neutron star surface are calculated on the basis of the Inogamov-Sunyaev model taking into account general relativity correction to the surface grav- ity and considering various chemical composition of the accreting matter. Local (at a given latitude) spectra are similar to the X-ray burst spectra and are described by a diluted blackbody. Total spreading layer spectra are integrated accounting for the light bending, gravitational red- shift and the relativistic Doppler effect and aberration. They depend slightly on the inclination angle and on the luminosity. These spectra also can be fitted by a diluted blackbody with the colour temperature depending mainly on a neutron star compactness. Owing to the fact that the flux from the spreading layer is close to the critical Eddington, we can put constraints on a neutron star radius without the need to know precisely the emitting region area or the distance to the source. The boundary layer spectra observed in the luminous low-mass X-ray binaries, and described by a blackbody of colour temperature T c = 2.4 ± 0.1 keV, restrict the neutron star radii to R = 14.8 ± 1.5 km (for a 1.4-M ⊙ star and solar composition of the accreting matter), which corresponds to the hard equation of state.

Strange Star Equations of State Revisited

AbstractMotivated by recent suggestions that strange stars can be responsible for glitches and other observational features of pulsars, we review some possible equations of state and their implications for models of neutron, hybrid, and strange stars. We consider the MIT bag model and also strange matter in the colour–flavour locked phase. The central energy densities for strange stars are higher than the central densities of ordinary neutron stars. Strange stars are bound by the strong force and so can also rotate much faster than neutron stars. These results are only weakly dependent on the model used for the quark matter. If just one of the existing mass-to-radius ratio constraint is valid, most neutron stars equations of state are ruled out, but all the strange stars equations of state presented in this work remain consistent with the constraint.

F‐Mode Secular Instabilities in Deleptonizing Fizzlers

Fizzlers are intermediate states that may form between white dwarf and neutron star densities during the collapse of massive rotating stars. This paper studies the gravitational radiation reaction (GRR) driven f-mode secular instabilities of fizzlers with angular momentum distributions h(mc) appropriate to the core collapse of massive rotating stars, where h is the specific angular momentum and mc is the cylindrical mass fraction. For core collapses that maintain axial symmetry, the h(mc) of the remnant reflects the conditions in the precollapse stellar core, and, thus, the h(mc) will resemble that of a uniformly rotating star supported by the pressure of relativistically degenerate electrons. Such an h(mc) concentrates most angular momentum toward the equatorial region of the object. The onset of f-mode secular instabilities in such fizzlers is affected strongly by the h(mc), whereas instability depends only weakly on compressibility. For a broad range of fizzler equations of state and the core h(mc), the f-mode secular instability thresholds drop to T/ ≈ 0.034-0.042, 0.019-0.021, and 0.012-0.0135, for m = 2, 3, and 4, respectively. These same thresholds with the Maclaurin spheroid h(mc) are T/ = 0.13-0.15, 0.10-0.11, and 0.08-0.09, respectively. The growth times τgw for GRR-driven m = 2 modes are long. For fizzlers with specific angular momentum J/M ≈ 1.5 × 1016 cm2 s-1 and T/ 0.24 (ρc 1014 g cm-3), τgw > 400 s. For these fizzlers, τgw τde, the deleptonization timescale, and GRR-driven secular instabilities will not grow along a deleptonizing fizzler sequence except, possibly, at T/ near the dynamic bar mode instability threshold, T/ ≈ 0.27.

Merger of binary neutron stars of unequal mass in full general relativity

We present results of three dimensional numerical simulations of the merger of unequal-mass binary neutron stars in full general relativity. A $\Gamma$-law equation of state $P=(\Gamma-1)\rho\epsilon$ is adopted, where $P$, $\rho$, $\varep$, and $\Gamma$ are the pressure, rest mass density, specific internal energy, and the adiabatic constant, respectively. We take $\Gamma=2$ and the baryon rest-mass ratio $Q_M$ to be in the range 0.85--1. The typical grid size is $(633,633,317)$ for $(x,y,z)$ . We improve several implementations since the latest work. In the present code, the radiation reaction of gravitational waves is taken into account with a good accuracy. This fact enables us to follow the coalescence all the way from the late inspiral phase through the merger phase for which the transition is triggered by the radiation reaction. It is found that if the total rest-mass of the system is more than $\sim 1.7$ times of the maximum allowed rest-mass of spherical neutron stars, a black hole is formed after the merger irrespective of the mass ratios. The gravitational waveforms and outcomes in the merger of unequal-mass binaries are compared with those in equal-mass binaries. It is found that the disk mass around the so formed black holes increases with decreasing rest-mass ratios and decreases with increasing compactness of neutron stars. The merger process and the gravitational waveforms also depend strongly on the rest-mass ratios even for the range $Q_M= 0.85$--1.

SU(2) Chiral Sigma Model Study of Phase Transition in Hybrid Stars

We use a modified SU(2) chiral sigma model to study nuclear matter component and simple bag model for quark matter constituting a neutron star. We also study the phase transition of nuclear matter to quark matter with the mixed phase characterized by two conserved charges in the interior of highly dense neutron stars. Stable solutions of Tolman–Oppenheimer–Volkoff equations representing hybrid stars are obtained with a maximum mass of 1.67M⊙ and radius around 8.9 km.

Hadron-quark mixed phase in neutron stars

Possibility of structured mixed phases at first order phase transitions is examined by taking into account of charge screening and surface effects. Hadron-quark phase transition in dense neutron star interiors is considered as a concrete example.

Antikaon condensation and in-medium (anti)kaon production inβequilibrium nuclear matter

The density dependent relativistic mean field (DDRMF) model in the new parametrization used recently is extended to investigate antikaon condensation and in-medium (anti)kaon production in $\ensuremath{\beta}$ equilibrium nuclear matter, and the obtained results are compared for different models. Our results show that the mean critical density for antikaon condensation is $\ensuremath{\approx}2.8{\ensuremath{\rho}}_{0}$ in the DDRMF model with the two model parameter sets and in the nonlinear $\ensuremath{\sigma}\ensuremath{-}\ensuremath{\omega}$ model with the parameter set Tm1, while a little larger critical density $3.3{\ensuremath{\rho}}_{0}$ is obtained in chiral perturbation theory. The subthreshold energies for (anti)kaon production are calculated, and indicate that the in-medium effects predicted by the DDRMF model are essential for explaining the antikaon enhanced production, while kaon production rates are possibly enhanced only in the high density region. As an application for $\ensuremath{\beta}$ equilibrium nuclear matter, the masses against the central densities of neutron stars are studied in the DDRMF model too.