Dense Matter In Neutron Stars Research Articles

Triplet Pairing in Neutron Matter

The presence of superfluidity in neutron star interiors can affect the cooling of neutron stars in intricate ways, enhancing certain mechanisms and suppressing others. Model calculations employing realistic nuclear potentials in Bardeen–Cooper–Schrieffer theory generally suggest the development of a 3P2–3F2 pairing gap, and therefore the presence of superfluidity in dense neutron star matter. Improved models that go beyond conventional mean-field calculations by including polarization effects suggest a suppression of the triplet gap. We have evaluated the pairing interaction by summing the “parquet” Feynman diagrams, which include both ladder and ring diagrams systematically, plus a set of important nonparquet diagrams, making this the most comprehensive diagram-based approach presently available. Our results suggest a radical suppression of the 3P2–3F2 triplet pairing gap and an enhancement of 3P0 pairing.

Compact Stars in the vBag Model and Its f-Mode Oscillations

Electromagnetic (EM) observations and gravitational wave (GW) measurements enable us to determine the mass and radius of neutron stars (NSs) and their tidal deformability, respectively. These parameters offer valuable insights into the properties of dense matter in NSs. In this study, the vector-interaction-enhanced bag model (vBag model) is employed to investigate strange and hybrid stars’ properties. The parameters of the vBag model are constrained using multi-messenger observations, revealing that strange stars are incompatible with current observations. In contrast, hybrid stars can exhibit a substantial mixed phase region and a thin hadronic shell. Furthermore, we present the frequencies and damping time of fundamental mode (f-mode) oscillations of hybrid stars and test their universal relations with compactness and tidal deformability. The findings indicate that the presence of mixed phase components leads to larger frequencies and shorter damping time of the f-mode oscillation of hybrid stars, and the softer equation of state (EoS) affects this behavior more significantly. The universal relations of hybrid stars in the vBag model can be described by fourth-order/seventh-order polynomials, which do not break the previous results.

Deducing the EOS of dense neutron star matter with machine learning

AbstractThe interior of a neutron star is a unique astrophysical laboratory for studying matter at extreme densities and pressures beyond what is replicable in terrestrial experiments. While there is no direct way to simulate the interior of these stars, one promising avenue to learning more about the equation of state (EOS) of such matter is through X‐rays emitted from the star's surface. The current state‐of‐the‐art method for inference of EOS from a star's X‐ray spectra uses piece‐wise, simulation‐based likelihoods that rely on theoretical assumptions complicated by systematic uncertainties. To reduce the dimensionality of the problem, this method infers macroscopic properties of the star (mass and radius) from emitted X‐ray spectra, and from those quantities infers the EOS. This work approaches the same problem using machine learning techniques, demonstrating a series of enhancements to the current state‐of‐the‐art by realistic uncertainty quantification and reducing the need for theoretical assumptions. We also demonstrate novel inference of the EOS directly from high‐dimensional simulated X‐ray spectra from neutron stars that negate the need for a piece‐wise approach. This inference allows for a natural propagation of uncertainties from the X‐ray spectra by conditioning the discussed networks on realistic sources of uncertainty for each star.

Confirming the Existence of Twin Stars in a NICER Way

We demonstrate that future radius measurements of the NICER mission have the potential to reveal the existence of a strong phase transition in dense neutron star matter by confirming the existence of so-called twin stars, compact star configurations with the same mass but different radii. The latest radius constraints from NICER for the pulsars J0740+6620 as well as J0030+0451 are discussed using relativistic mean field equations of state with varying stiffness, connected with a first-order phase transition to quark matter. We show that twin star solutions are compatible with the new radius constraint but are located at radii below the present constraints from NICER, serving as a smoking gun for a strong phase transition in neutron star matter. This scenario is realized if a strong phase transition takes place in neutron stars of the first branch with masses above 2 M ⊙.

Symmetry energy dilemma within a relativistic quark model

We develop the equation of state of dense neutron star matter with nucleons, nucleons and hyperons, nucleons and delta, as well as including strange interactions through strange meson-hyperon couplings to assess the effect of both the soft and stiff symmetry energy, determined in the recent PREX-II experiment, on the properties of neutron stars. The macroscopic properties of neutron stars such as the mass, radius and tidal deformability are a direct consequence of the underlying equation of state of dense neutron star matter. Given the recent advances in constraining the above macroscopic observations from gravitational wave analysis, NICER and radio observations, we study here the effect of a variation in the symmetry energy on such observational properties of neutron stars with different possible compositions in a relativistic quark model. We observe that the stiff symmetry energy satisfies the current constraints on maximum mass and radius, but creates tension with tidal deformability values.

Neutron Stars and Gravitational Waves: The Key Role of Nuclear Equation of State

Neutron stars are the densest known objects in the universe and an ideal laboratory for the strange physics of super-condensed matter. Theoretical studies in connection with recent observational data of isolated neutron stars, as well as binary neutron stars systems, offer an excellent opportunity to provide robust solutions on the dense nuclear problem. In the present work, we review recent studies concerning the applications of various theoretical nuclear models on a few recent observations of binary neutron stars or neutron-star–black-hole systems. In particular, using a simple and well-established model, we parametrize the stiffness of the equation of state with the help of the speed of sound. Moreover, in comparison to the recent observations of two events by LIGO/VIRGO collaboration, GW170817 and GW190425, we suggest possible robust constraints. We also concentrate our theoretical study on the resent observation of a compact object with mass ∼2.59−0.09+0.08M⊙ (GW190814 event), as a component of a system where the main companion was a black hole with mass ∼23M⊙. There is scientific debate concerning the identification of the low mass component, as it falls into the neutron-star–black-hole mass gap. This is an important issue since understanding the nature of GW190814 event will offer rich information concerning the upper limit of the speed of sound in dense matter and the possible phase transition into other degrees of freedom. We systematically study the tidal deformability of a possible high-mass candidate existing as an individual star or as a component in a binary neutron star system. Finally, we provide some applications of equations of state of hot, dense nuclear matter in hot neutron stars (nonrotating and rapidly rotating with the Kepler frequency neutron stars), protoneutron stars, and binary neutron star merger remnants.

Multiple configurations of neutron stars containing quark matter * * Supported by National Natural Science Foundation of China (11903013)

The main purpose of this study is to interpret the possibilities of hybrid star configurations under different phase transition paths and provide a general description of the conditions and features of the different configurations. We assume that there are two possible phase transition paths, i.e., from a nuclear phase to a 2flavor(2f)/3flavor(3f) quark phase directly, or first from a nuclear phase to a 2f quark phase, and then from that phase to a 3f quark phase sequentially. In addition, we consider Maxwell and Gibbs constructions based on the assumption of a first-order transition, which yields multiple configurations of hybrid stars: N-2f, N-3f, and N-2f-3f for a Maxwell construction, and N-2fmix-2f, N-3fmix-3f, N-2f3fmix, and N-2fmix-3f for a Gibbs construction. From the radii analysis of different hybrid star configurations with the same mass of , the appearance of the quark matter (from nuclear to 2f or 3f quark matter) causes a radius difference of 0.5km~2km and provides the possibility of detection by NICER in the future. However, the sequential transition from 2f to 3f quark matter is difficult to detect because the transition does not lead to too high of a change in radius (far smaller than ). The dependence solely on the measurements of the stellar radii to probe the equation of state of dense matter in neutron stars causes difficulties. Multi-messenger observations can help us to infer the interior of a neutron star in the future.

The Equation of State and Some Key Parameters of Neutron Stars: Constraints from GW170817, the Nuclear Data, and the Low-mass X-Ray Binary Data

Abstract In this work we parameterize the equation of state of dense neutron star (NS) matter with four pressure parameters of and then set the combined constraints with the data of GW170817 and the data of six low-mass X-ray binaries (LMXBs) with thermonuclear burst or alternatively the symmetry energy of the nuclear interaction. We find that the nuclear data effectively narrow down the possible range of , the gravitational-wave data plays the leading role in bounding , and the LMXB data as well as the lower bound on the maximal gravitational mass of non-rotating NSs govern the constraints on and . Using posterior samples of pressure parameters and some universal relations, we further investigate how the current data sets can advance our understanding of tidal deformability (Λ), moment of inertia (I), and binding energy (BE) of NSs. For a canonical mass of 1.4 M ⊙, we have , , , and if the constraints from the nuclear data and the gravitational-wave data have been jointly applied. For the joint analysis of gravitational-wave data and the LMXB data, we have , , , and . These results suggest that the current constraints on Λ and R still suffer from significant systematic uncertainties, while I 1.4 and BE1.4 are better constrained.

Equation of state and sound velocity in hybrid stars with a Dyson–Schwinger quark model

We investigate the equation of state (EOS) and the corresponding sound velocity of the dense matter in hybrid neutron stars. For the hadron matter, the Bruckner–Hartree–Fock (BHF) many-body theory with Bonn-B potential is adopted. For the quark matter, the Dyson–Schwinger quark model is adopted, with the rainbow approximation and the Gaussian type effective interaction for quark–gluon vertex and gluon propagator. The phase transition is considered with both Maxwell condition and Gibbs condition. Then, the sound velocity in the pure hadron phase, pure quark phase and the mixed phase are obtained. The causality requirement [Formula: see text] puts a strong constraint on the EOS from BHF theory. In quark matter, it is found that [Formula: see text] and varies slowly. In the mixed phase with Gibbs condition, the sound velocity varies strongly and nonmonotonically.

Going from asymmetric nuclei to neutron stars to tidal polarizability in gravitational waves

Among the five-year government-funded World Class University (WCU) Projects in Korea, the category-3 program approved at Hanyang University in Seoul led to an exploratory effort to go from neutron-rich nuclei to dense matter in neutron stars. The principal results in what transpired in the effort — and what followed afterwards — are described with the focus on the possibly important, hitherto unexplored, role played in nuclear dynamics of topology and hidden symmetries of QCD. The potential link to the proton mass problem is pointed out.

Inhomogeneous chiral symmetry breaking in dense neutron-star matter

An increasing number of model results suggests that chiral symmetry is broken inhomogeneously in a certain window at intermediate densities in the QCD phase diagram. This could have significant effects on the properties of compact stars, possibly leading to new astrophysical signatures. In this contribution we discuss this idea by reviewing recent results on inhomogeneous chiral symmetry breaking under an astrophysics-oriented perspective. After introducing two commonly studied spatial modulations of the chiral condensate, the chiral density wave and the real kink crystal, we focus on their properties and their effect on the equation of state of quark matter. We also describe how these crystalline phases are affected by different elements which are required for a realistic description of a compact star, such as charge neutrality, the presence of magnetic fields, vector interactions and the interplay with color-superconductivity. Finally, we discuss possible signatures of inhomogeneous chiral symmetry breaking in the core of compact stars, considering the cases of mass-radius relations and neutrino emissivity explicitly.

Massive hybrid stars with a first-order phase transition

We develop our previous study of the transition to deconfined quark phase in neutron stars, including the interaction in the quark equation of state to the leading order in the perturbative expansion within the confinement density-dependent mass model. Using the Gibbs conditions the hadron-quark mixed phase is constructed matching the latter with the hadron equation of state derived from the microscopic Brueckner-Hartree-Fock approximation. The influence of quark interaction parameters on threshold properties and phase diagram of dense neutron star matter are discussed in detail. We find that the leading-order quark interaction expands the density range of the mixed phase, pushing forward the disappearance of the hadron phase. Moreover, since the equation of state could turn out to be stiffer, a high-mass hybrid star is possible with mixed-phase core with typical parameter sets.

Unified description of dense matter in neutron stars and magnetars

AbstractWe have recently developed a set of equations of state based on the nuclear energy density functional theory providing a unified description of the different regions constituting the interior of neutron stars and magnetars. The nuclear functionals, which were constructed from generalized Skyrme effective nucleon-nucleon interactions, yield not only an excellent fit to essentially all experimental atomic mass data but were also constrained to reproduce the neutron-matter equation of state as obtained from realistic many-body calculations.

Phases of Dense Matter in Supernovae and Neutron Stars

We study the properties of dense matter at finite temperature with various proton fractions for use in supernova simulations. The relativistic mean-field theory is used to describe homogeneous nuclear matter, while the Thomas-Fermi approximation is adopted to describe inhomogeneous matter. We also discuss the equation of state of neutron star matter at zero temperature in a wide density range. The equation of state at high densities can be significantly softened by the inclusion of hyperons.

Asymmetric Neutrino Reaction in Magnetized Proto-Neutron Stars in Fully Relativistic Approach

We calculate asymmetric neutrino absorption and scattering cross sections on hot and dense magnetized neutron-star matter including hyperons in fully relativistic mean-field theory. The absorption/scattering cross sections are suppressed/enhanced incoherently in the direction of the magnetic field B = Bẑ . The asymmetry is 2–4% at the matter density ρ0 ≤ ρB ≤ 3ρ0 and temperature T ≤ 40MeV for B = 2 × 1017 G. Then we solve the Boltzmann equation for the neutrino transport in 1D attenuation approximation, and get the result that the kick velocity becomes about 300 km/s for the proto-neutron star with 168 solar mass at T = 20MeV.

Asymmetric neutrino emission from magnetized proto-neutron star matter including hyperons in relativistic mean field theory

We calculate asymmetric neutrino absorption and scattering cross sections on hot and dense magnetized neutron star matter, including hyperons in fully relativistic mean-field theory. The absorption/scattering cross sections are suppressed/enhanced incoherently in the direction of the magnetic field $\mathbf{B}=B\stackrel{^}{z}$. The asymmetry is 2--4% at the matter density ${\ensuremath{\rho}}_{0}\ensuremath{\le}{\ensuremath{\rho}}_{B}\ensuremath{\le}3{\ensuremath{\rho}}_{0}$ and temperature $T\ensuremath{\le}40\text{ }\text{ }\mathrm{MeV}$ for $B=2\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{G}$. This asymmetry is comparable to the effects owing to parity violation or asymmetric magnetic field topology proposed for the origin of pulsar kicks.

Probing dense matter in neutron stars with axialwmodes

We study the problem of extracting information about composition and equation of state of dense matter in neutron star interior using axial $w$ modes. We determine complex frequencies of axial $w$ modes for a set of equations of state involving hyperons as well as Bose-Einstein condensates of antikaons adopting the continued fraction method. Hyperons and antikaon condensates result in softer equations of state leading to higher frequencies and lower damping times of first axial $w$ modes than those of the nuclear matter case. The presence of condensates may lead to the appearance of a new stable branch of superdense stars beyond the neutron star branch called the third family. The existence of the same mass compact stars in both branches is known as neutron star twins. Further investigation of twins reveals that first axial $w$-mode frequencies of superdense stars in the third family are higher than those of the corresponding twins in the neutron star branch.

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.

Phase Transitions and Equation of State of Highly Dense Matter in Neutron Stars

Phase Transitions and Equation of State of Highly Dense Matter in Neutron Stars

Exotica in rotating compact stars

The determination of mass and radius of a single neutron star EXO 0748-676 has been reported recently. Also, the estimate of radius from the measurement of moment of inertia of pulsar A in double pulsar system PSR J0737-3039 would be possible in near future. Here we construct models of static and uniformly rotating neutron stars involving exotic matter and compare our theoretical calculations with the recent findings from observations to probe dense matter in neutron stars.