Properties Of Neutron Star Matter Research Articles

Elastic properties of nuclear pasta in a fully three-dimensional geometry

Realistic estimations on the elastic properties of neutron star matter are carried out with a large strain (ε≲0.5) in the framework of relativistic-mean-field model with Thomas-Fermi approximation, where various crystalline configurations are considered in a fully three-dimensional geometry with reflection symmetry. Our calculation confirms the validity of assuming Coulomb crystals for the droplet phase above neutron drip density, which nonetheless does not work at large densities since the elastic constants are found to be decreasing after reaching their peaks. Similarly, the analytic formulae derived in the incompressible liquid-drop model give excellent description for the rod phase at small densities, which overestimates the elastic constants at larger densities. For slabs, due to the negligence on the variations of their thicknesses, the analytic formulae from liquid-drop model agree qualitatively but not quantitatively with our numerical estimations. By fitting to the numerical results, these analytic formulae are improved by introducing dampening factors. The impacts of nuclear symmetry energy are examined adopting two parameter sets, corresponding to the slope of symmetry energy L=41.34 and 89.39 MeV. Even with the uncertainties caused by the anisotropy in polycrystallines, the elastic properties of neutron star matter obtained with L=41.34 and 89.39 MeV are distinctively different, results in detectable differences in various neutron star activities.

Long range plans to study the nuclear equation-of-state from sub- to supra-saturation densities with heavy-ion collisions

We cover here the present state-of-the-art in constraining the nuclear equation-of-state (EoS) and the symmetry energy using heavy-ion collisions (HIC), from sub- to supra-saturation densities, from Fermi to (ultra-) relativistic beam energies. We also discuss how HIC constraints on the EoS contribute to the knowledge of thermodynamical properties of neutron star matter. Necessary improvements and challenges are outlined, in particular in the perspective, for HICs, of staying competitive with future astrophysical multimessenger observations.

Quark condensate and chiral symmetry restoration in neutron stars

Based on an equivparticle model, we investigate the in-medium quark condensate in neutron stars. Carrying out a Taylor expansion of the nuclear binding energy to the order of ρ3, we obtain a series of EOSs for neutron star matter, which are confronted with the latest nuclear and astrophysical constraints. The in-medium quark condensate is then extracted from the constrained properties of neutron star matter, which decreases non-linearly with density. However, the chiral symmetry is only partially restored with non-vanishing quark condensates, which may vanish at a density that is out of reach for neutron stars.

Uncertainty limits on neutron star radius measurements with gravitational waves

Upcoming observing campaigns with improved detectors will yield numerous detections of gravitational waves from neutron star binary inspirals. Rare loud signals together with numerous signals of moderate strength promise stringent constraints on the properties of neutron star matter, with a projected radius statistical uncertainty of $50-200$m with ${\cal{O}}(2000)$ sources. Given this precision we revisit all analysis assumptions and identify sources of systematic errors, quantify their impact on radius extraction, and discuss their relative importance and ways to mitigate them.

Hyperons in Neutron Stars

I review the issues related to the appearance of hyperons in neutron star matter, focusing in particular on the problem of the maximum mass supported by hyperonic equations of state. I discuss the general mechanism that leads to the formation of hyperons in the core of neutron stars and I review the main techniques and many-body methods used to construct an appropriate equation of state to describe the strongly interacting system of hadrons hosted in the core of neutron stars. I outline the consequences on the structure and internal composition of neutron stars and also discuss the possible signatures of the presence of hyperons in astrophysical dynamical systems like supernova explosions and binary neutron star mergers. Finally, I briefly report about the possible important role played by hyperons in the transport properties of neutron star matter and on the consequences of neutron star cooling and gravitational wave instabilities induced by the presence of hyperons.

Nonparametric constraints on neutron star matter with existing and upcoming gravitational wave and pulsar observations

Observations of neutron stars, whether in binaries or in isolation, provide information about the internal structure of the most extreme material objects in the Universe. In this work, we combine information from recent observations to place joint constraints on the properties of neutron star matter. We use (i) lower limits on the maximum mass of neutron stars obtained through radio observations of heavy pulsars, (ii) constraints on tidal properties inferred through the gravitational waves neutron star binaries emit as they coalesce, and (iii) information about neutron stars' masses and radii obtained through X-ray emission from surface hot spots. In order to combine information from such distinct messengers while avoiding the kind of modeling systematics intrinsic to parametric inference schemes, we employ a nonparametric representation of the neutron-star equation of state based on Gaussian processes conditioned on nuclear theory models. We find that existing astronomical observations imply $R_{1.4}=12.32^{+1.09}_{-1.47}\,\mathrm{km}$ for the radius of a $1.4\,M_{\odot}$ neutron star and $p(2\rho_\mathrm{nuc})=3.8^{+2.7}_{-2.9}\times10^{34}\,\mathrm{dyn}/\mathrm{cm}^2$ for the pressure at twice nuclear saturation density at the 90% credible level. The upper bounds are driven by the gravitational wave observations, while X-ray and heavy pulsar observations drive the lower bounds. Additionally, we compute expected constraints from potential future astronomical observations and find that they can jointly determine $R_{1.4}$ to ${\cal{O}}(1)\,\mathrm{km}$ and $p(2\rho_\mathrm{nuc})$ to 80% relative uncertainty in the next five years.

PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter

Abstract Neutron stars are not only of astrophysical interest, but are also of great interest to nuclear physicists because their attributes can be used to determine the properties of the dense matter in their cores. One of the most informative approaches for determining the equation of state (EoS) of this dense matter is to measure both a star’s equatorial circumferential radius R e and its gravitational mass M. Here we report estimates of the mass and radius of the isolated 205.53 Hz millisecond pulsar PSR J0030+0451 obtained using a Bayesian inference approach to analyze its energy-dependent thermal X-ray waveform, which was observed using the Neutron Star Interior Composition Explorer (NICER). This approach is thought to be less subject to systematic errors than other approaches for estimating neutron star radii. We explored a variety of emission patterns on the stellar surface. Our best-fit model has three oval, uniform-temperature emitting spots and provides an excellent description of the pulse waveform observed using NICER. The radius and mass estimates given by this model are km and (68%). The independent analysis reported in the companion paper by Riley et al. explores different emitting spot models, but finds spot shapes and locations and estimates of R e and M that are consistent with those found in this work. We show that our measurements of R e and M for PSR J0030+0451 improve the astrophysical constraints on the EoS of cold, catalyzed matter above nuclear saturation density.

Argonne family potentials and neutron star matter equation of state

Properties of neutron star matter are studied using different microscopic equation of states, derived from the lowest order constrained variational (LOCV) many-body theory using the two-body nuclear force AV8' , and AV6' and also implementing the three-nucleon interaction in AV18, AV8' , and AV6' . The maximum mass configuration of the neutron star is obtained, \( M_{\max}= 2.11, 2.03, 2.12 M_{\odot}\), with AV18+TNI, AV8' +TNI and AV6' +TNI, respectively, which is an excellent agreement with the accepted maximum mass of non-rotating neutron star range \( 2.01 M_{\odot} < M_{\max} < 2.16 M_{\odot}\).

Frequency domain model of f -mode dynamic tides in gravitational waveforms from compact binary inspirals

The recent detection of gravitational waves (GWs) from the neutron star binary inspiral GW170817 has opened a unique avenue to probe matter and fundamental interactions in previously unexplored regimes. Extracting information on neutron star matter from the observed GWs requires robust and computationally efficient theoretical waveform models. We develop an approximate frequency-domain GW phase model of a main GW signature of matter: dynamic tides associated with the neutron stars' fundamental oscillation modes ($f$-modes). We focus on nonspinning objects on circular orbits and demonstrate that, despite its mathematical simplicity, the new "$f$-mode tidal" (fmtidal) model is in good agreement with the effective-one-body dynamical tides model up to GW frequencies of $\gtrsim 1$ kHz and gives physical meaning to part of the phenomenology captured in tidal models tuned to numerical-relativity. The advantages of the fmtidal model are that it makes explicit the dependence of the GW phasing on the characteristic equation-of-state parameters, i.e., tidal deformabilities and $f$-mode frequencies; it is computationally efficient; and it can readily be added to any frequency-domain baseline waveform. The fmtidal model is easily amenable to future improvements and provides the means for a first step towards independently measuring additional fundamental properties of neutron star matter beyond the tidal deformability as well as performing novel tests of general relativity from GW observations.

Neutron Star Matter and Baryonic Interactions

The properties of neutron-star matter up to the appearance of hyperons are calculated with the aid of Skyrme potentials. The conditions for the appearance of Λ hyperons in matter and values of the density at the point of their appearance are analyzed for various parametrizations of nucleon-nucleon and hyperon-nucleon interactions. The dependence of the results on the magnitude of density-dependent forces, the degree of nonlocality, the behavior of the symmetry energy, and incompressibility of nuclear matter is examined.

Transport properties of the Fermi hard-sphere system

The transport properties of neutron star matter play an important role in many astrophysical processes. We report the results of a calculation of the shear viscosity and thermal conductivity coefficients of the hard-sphere fermion system of degeneracy $\nu$=2, that can be regarded as a model of pure neutron matter. Our approach is based on the effective interaction obtained from the formalism of correlated basis functions and the cluster expansion technique. The resulting transport coefficients show a strong sensitivity to the quasiparticle effective mass, reflecting the effect of second order contributions to the self-energy that are not taken into account in nuclear matter studies available in the literature.

NEUTRON STARS WITH KAON CONDENSATION IN RELATIVISTIC EFFECTIVE MODEL

Relativistic mean-field theory with parameter sets FSUGold and IU-FSU is extended to study the properties of neutron star matter in β equilibrium by including Kaon condensation. The mixed phase of normal baryons and Kaon condensation cannot exist in neutron star matter for the FSUGold model and the IU-FSU model. In addition, it is found that when the optical potential of the K- in normal nuclear matter UK ≳ -100 MeV , the Kaon condensation phase is absent in the inner cores of the neutron stars.

Constraints the properties of neutron star matter from the mass of neutron star PSR J1614-2230

The constraints on the properties of neutron star matter from the mass of neutron star PSR J1614-2230 are examined in the framework of the relativistic mean field theory. We find that there are little differences between the σ potentials of large mass neutron star and those of canonnical mass neutron star. For potentials of ω, ρ, neutrons and electrons, the values corresponding to the large mass neutron star are larger than those to the canonnical mass neutron star as the baryon number density is more than a certain value. We also find that for the relative particle number density of electrons, muons, neutrons and protons and the pressure of the neutron star, the values corresponding to the large mass neutron star are far larger than those to the canonnical mass neutron star. For the relative particle number density of hyperons Λ, Σ−, Σ0, Σ+ and Ξ−, the values corresponding to the large mass neutron star are far smaller than those to the canonnical mass neutron star. These mean that the larger mass of neutron star is more advantageous to the production of protons but is not advantageous to the production of hyperons.

The equation of state of neutron star matter and the symmetry energy

We present an overview of microscopic calculations of the Equation of State (EOS) of neutron matter performed using Quantum Monte Carlo techniques. We focus on the role of the model of the three-neutron force in the high-density part of the EOS up to a few times the saturation density. We also discuss the interplay between the symmetry energy and the neutron star mass-radius relation.The combination of theoretical models of the EOS with recent neutron star observations permits us to constrain the value of the symmetry energy and its slope. We show that astrophysical observations are starting to provide important insights into the properties of neutron star matter.

Transport properties of a quark-hadron Coulomb lattice in the cores of neutron stars

Already more that 40 years ago, it has been suggested that because of the enormous mass densities in the cores of neutron stars, the hadrons in the centers of neutron stars may undergo a phase transition to deconfined quark matter. In this picture, neutron stars could contain cores made of pure (up, down, strange) quark matter which are surrounded by a mixed phase of quarks and hadrons. More than that, because of the competition between the Coulomb and the surface energies associated with the positively charged regions of nuclear matter and negatively charged regions of quark matter, the mixed phase may develop geometrical structures, similarly to what is expected of the sub-nuclear liquid-gas phase transition. In this paper we restrict ourselves to considering the formation of rare phase blobs in the mixed quark-hadron phase. The influence of rare phase blobs on the thermal and transport properties of neutron star matter is investigated. The total specific heat, $c_V$, thermal conductivity, $\kappa$, and electron-blob Bremsstrahlung neutrino emissivities, $\epsilon_{\nu,\text{BR}}$, of quark-hybrid matter are computed and the results are compared with the associated thermal and transport properties of standard neutron star matter. Our results show that the contribution of rare phase blobs to the specific heat is negligibly small. This is different for the neutrino emissivity from electron-blob Bremsstrahlung scattering, which turns out to be of the same order of magnitude as the total contributions from other Bremsstrahlung processes for temperatures below about $10^8$ K.

The Effects of δ Meson on the Neutron Star Cooling

In the framework of the relativistic mean field theory, the isovector scalar interaction is considered by exchanging δ meson to study the influence of δ meson on the cooling properties of neutron star matter. The calculation results show that with the inclusion of δ meson, the neutrino emissivity of the direct Urca processes increases, and thus enhances the cooling of neutron star matter. When strong proton superfluidity is considered, the theoretical cooling curves agree with the observed thermal radiation for isolated neutron stars.

Strange hadronic stars in relativistic mean-field theory with the FSUGold parameter set

Relativistic mean-field theory with parameter set FSUGold that includes the isoscalar-isovector cross interaction term is extended to study the properties of neutron star matter in $\ensuremath{\beta}$ equilibrium by including hyperons. The influence of the attractive and repulsive $\ensuremath{\Sigma}$ potential on the properties of neutron star matter and the maximum mass of neutron stars is examined. We also investigate the equations of state for pure neutron matter and for nonstrange hadronic matter for comparison. For a pure neutron star, the maximum mass is about 1.8${M}_{\text{sun}}$, while for a strange (nonstrange) hadronic star in $\ensuremath{\beta}$ equilibrium, the maximum mass is around 1.35${M}_{\text{sun}}$ (1.7${M}_{\text{sun}}$).

EFFECTS OF IN-MEDIUM MODIFICATION OF WEAKLY INTERACTING LIGHT BOSON MASS IN NEUTRON STARS

The effects of the presence of weakly interacting light boson (WILB) in neutron star matter have been revisited. Direct checking based on the experimental range of symmetric nuclear matter binding energy1 and the fact that the presence of this boson should give no observed effect on the crust properties of neutron star matter, shows that the characteristic scale of WILB [Formula: see text] should be ≤2 GeV-2. The recent observational data with significant low neutron stars radii2 and the recent largest pulsar which has been precisely measured, i.e. J1903+0327 (Ref. 3) indicate that in-medium modification of WILB mass in neutron stars cannot be neglected.

Influence of the isovector-scalar channel interaction on neutron star matter with hyperons and antikaon condensation

The relativistic mean field approach including isovector-scalar channel (i.e., exchanging $\ensuremath{\delta}$ mesons) interaction is taken to study the properties of neutron star matter including hyperons and antikaon condensation. For hyperonic neutron stars, it shows that the $\ensuremath{\delta}$-meson channel interaction stiffens the equation of state at lower densities but it softens the equation of state after hyperons appear. This leads to the neutron star having a lower central density and a larger radius than the one with the same mass but without the $\ensuremath{\delta}$-meson channel interaction. For neutron star matter including both hyperons and antikaon condensation, the $\ensuremath{\delta}$-meson channel interaction increases the onset density of the antikaon condensation. At the same time, the stability of the kaonic neutron star and its dependence on the kaon optical potential are discussed. For stable kaonic neutron stars with larger radii, those with the inclusion of the $\ensuremath{\delta}$-meson channel interaction have larger masses than those without the $\ensuremath{\delta}$-meson interaction, but the result is reversed for those with smaller radii. Calculated results are also compared with neutron star observations. Constraints on the model parameters are then provided.

FINITE TEMPERATURE EFFECTS ON SPIN POLARIZATION OF NEUTRON MATTER IN A STRONG MAGNETIC FIELD

Magnetars are neutron stars possessing a magnetic field of about <TEX>$10^{14}-10^{15}$</TEX> G at the surface. Thermodynamic properties of neutron star matter, approximated by pure neutron matter, are considered at finite temperature in strong magnetic fields up to <TEX>$10^{18}$</TEX> G which could be relevant for the inner regions of magnetars. In the model with the Skyrme effective interaction, it is shown that a thermodynamically stable branch of solutions for the spin polarization parameter corresponds to the case when the majority of neutron spins are oriented opposite to the direction of the magnetic field (i.e. negative spin polarization). Moreover, starting from some threshold density, the self-consistent equations have also two other branches of solutions, corresponding to positive spin polarization. The influence of finite temperatures on spin polarization remains moderate in the Skyrme model up to temperatures relevant for protoneutron stars. In particular, the scenario with the metastable state characterized by positive spin polarization, considered at zero temperature in Phys. Rev. C 80, 065801 (2009), is preserved at finite temperatures as well. It is shown that, above certain density, the entropy for various branches of spin polarization in neutron matter with the Skyrme interaction in a strong magnetic field shows the unusual behavior, being larger than that of the nonpolarized state. By providing the corresponding low-temperature analysis, we prove that this unexpected behavior should be related to the dependence of the entropy of a spin polarized state on the effective masses of neutrons with spin up and spin down, and to a certain constraint on them which is violated in the respective density range.