Central Speed of Sound, Trace Anomaly and Observables of Neutron Stars from Perturbative Analyses of Scaled TOV Equations

Abstract

The central speed of sound (SS) measures the stiffness of the Equation of State (EOS) of superdense neutron star (NS) matter. Its variations with density and radial coordinate in NSs in conventional analyses often suffer from uncertainties of the specific nuclear EOSs used. Using the central SS and NS mass/radius scaling obtained from solving perturbatively the scaled Tolman-Oppenheimer-Volkoff (TOV) equations, we study the variations of SS, trace anomaly and several closely related properties of NSs in an EOS-model independent manner. We find that the SS increases with the reduced central pressure ${\hat{P}}_{\mathrm{c}}\equiv P_{\mathrm{c}}/{\epsilon }_{\mathrm{c}}$ (scaled by the central energy density ${\epsilon }_{\mathrm{c}}$), and the conformal bound for SS tends to break down for NSs with masses higher than about 1.9$M_{\odot }$. The ratio $P/\epsilon$ is upper bounded as $P/\epsilon \lesssim 0.374$ around the centers of stable NSs. We demonstrate that it is an intrinsic property of strong-field gravity and is more relevant than the perturbative QCD bound on it. While a sharp phase transition at high densities characterized by a sudden vanishing of SS in cores of massive NSs are basically excluded, the probability for a continuous crossover signaled by a peaked radial profile of SS is found to be enhanced as ${\hat{P}}_{\mathrm{c}}$ decreases, implying it likely happens near the centers of massive NSs. Moreover, a new and more stringent causality boundary as $R_{\max }/\mathrm{km}\gtrsim 4.73{\mathrm{M}}_{\mathrm{NS}}^{\max }/{\mathrm{M}}_{\odot }+1.14$ for NS M-R curve is found to be excellently consistent with observational data on NS masses and radii. Furthermore, new constraints on the ultimate energy density and pressure allowed in NSs before collapsing into black holes are obtained and compared with earlier predictions in the literature.

Phys. Rev. D (2023) in press

References in corpus (78)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Shapiro delay measurement of a two solar mass neutron star
• A Massive Pulsar in a Compact Relativistic Binary
• GW190814: Gravitational Waves from the Coalescence of a 23 M${}_{\odot }$ Black Hole with a 2.6 M${}_{\odot }$ Compact Object
• PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
• Color superconductivity in dense quark matter
• Neutron Star Observations: Prognosis for Equation of State Constraints
• A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy
• Refined Mass and Geometric Measurements of the High-Mass PSR J0740+6620
• Constraining the Maximum Mass of Neutron Stars From Multi-Messenger Observations of GW170817
• Neutron-Rich Nuclei in Heaven and Earth
• GW170817: Joint Constraint on the Neutron Star Equation of State from Multimessenger Observations
• Neutron-star radius constraints from GW170817 and future detections
• Cold Quark Matter
• The Skyrme Interaction in finite nuclei and nuclear matter
• Nonparametric constraints on neutron star matter with existing and upcoming gravitational wave and pulsar observations
• Constraining Neutron-Star Matter with Microscopic and Macroscopic Collisions
• Sound velocity bound and neutron stars
• How well do we know the neutron-matter equation of state at the densities inside neutron stars? A Bayesian approach with correlated uncertainties
• Neutron Stars and the Nuclear Equation of State
• New Hyperon Equations of State for Supernovae and Neutron Stars in Density-dependent Hadron Field Theory
• On the Sound Speed in Neutron Stars
• Impact of the PSR J0740+6620 radius constraint on the properties of high-density matter
• Towards Understanding Astrophysical Effects of Nuclear Symmetry Energy
• Chiral Effective Field Theory and the High-Density Nuclear Equation of State
• Multimessenger constraints for ultra-dense matter
• Limiting masses and radii of neutron stars and their implications
• Neutron-star Radius from a Population of Binary Neutron Star Mergers
• Trace anomaly as signature of conformality in neutron stars
• Neutron Star Equation of State in light of GW190814
• Equation of state constraints from the threshold binary mass for prompt collapse of neutron star mergers
• Direct Astrophysical Tests of Chiral Effective Field Theory at Supranuclear Densities
• Quarkyonic Matter Equation of State in Beta-Equilibrium
• Was GW190814 a black hole -- strange quark star system?
• Ab-initio QCD calculations impact the inference of the neutron-star-matter equation of state
• The Radius of PSR J0740+6620 from NICER with NICER Background Estimates
• A bound on the speed of sound from holography
• Reaching percolation and conformal limits in neutron stars
• Combining Electromagnetic and Gravitational-Wave Constraints on Neutron-Star Masses and Radii
• Bayesian Inference of the Symmetry Energy of Super-Dense Neutron-Rich Matter from Future Radius Measurements of Massive Neutron Stars
• Extreme Matter meets Extreme Gravity: Ultra-heavy neutron stars with crossovers and first-order phase transitions
• Bayesian analysis of neutron-star properties with parameterized equations of state: the role of the likelihood functions
• Hard-core deconfinement and soft-surface delocalization from nuclear to quark matter
• Merger and post-merger of binary neutron stars with a quark-hadron crossover equation of state
• Implications of NICER for neutron star matter: the QHC21 equation of state
• Implications of the mass $M=2.17_{-0.10}^{+0.11}$M${}_{\odot }$ of PSR~J0740+6620 on the Equation of State of Super-Dense Neutron-Rich Nuclear Matter
• Inference of the sound speed and related properties of neutron stars
• A general, scale-independent description of the sound speed in neutron stars
• Phase transitions in neutron stars and their links to gravitational waves
• Direct Imaging Discovery and Dynamical Mass of a Substellar Companion Orbiting an Accelerating Hyades Sun-like Star with SCExAO/CHARIS
• Gravitational Wave Signal for Quark Matter with Realistic Phase Transition
• Bayesian inference of dense matter EOS encapsulating a first-order hadron-quark phase transition from observables of canonical neutron stars
• Plausible presence of new state in neutron stars with masses above $0.98M_{\mathrm{TOV}}$
• Holography and the speed of sound at high temperatures
• Investigating Signatures of Phase Transitions in Neutron-Star Cores
• Hot quark matter and (proto-) neutron stars
• Future Physics Perspectives on the Equation of State from Heavy Ion Collisions to Neutron Stars
• Impact of large-mass constraints on the properties of neutron stars
• The slope, the hill, the drop, and the swoosh: Learning about the nuclear matter equation of state from the binary Love relations
• Equation of state of dense matter in the multimessenger era
• What neutron stars tell about the hadron-quark phase transition: a Bayesian study
• Probing the incompressibility of nuclear matter at ultra-high density through the prompt collapse of asymmetric neutron star binaries
• Bayesian inference of strange star equation of state using the GW170817 and GW190425 data
• Bayesian model-selection of neutron star equation of state using multi-messenger observations
• Constraints on the maximum densities of neutron stars from postmerger gravitational waves with third-generation observations
• Bayesian nonparametric inference of neutron star equation of state via neural network
• Large and massive neutron stars: Implications for the sound speed in dense QCD
• Impact of NICER's Radius Measurement of PSR J0740+6620 on Nuclear Symmetry Energy at Suprasaturation Densities
• Equation of state of cold and dense QCD matter in resummed perturbation theory
• GW190814: Circumstantial Evidence for Up-Down Quark Star
• Degeneracy in the inference of phase transitions in the neutron star equation of state from gravitational wave data
• Impact of Symmetry Energy on Sound Speed and Spinodal Decomposition in Dense Neutron-Rich Matter
• On the Speed of Sound in Hyperonic Stars
• Core States of Neutron Stars from Anatomizing their Scaled Structure Equations
• Properties of First-Order Hadron-Quark Phase Transition from Inverting Neutron Star Observables
• Impact of Multiple Phase Transitions in Dense QCD on Compact Stars
• Optimized Statistical Approach for Comparing Multi-Messenger Neutron Star Data

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• Bayesian Inference of Fine-Features of Nuclear Equation of State from Future Neutron Star Radius Measurements to 0.1km Accuracy
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• Analyzing the speed of sound in neutron star with machine learning
• Novel Scalings of Neutron Star Properties from Analyzing Dimensionless Tolman--Oppenheimer--Volkoff Equations
• Strong Gravity Extruding Peaks in Speed of Sound Profiles of Massive Neutron Stars
• Bayesian Quantification of Observability and Equation of State of Twin Stars
• Correlations between the Neutron Star Mass-Radius Relation and the Equation of State of Dense Matter
• Unraveling Trace Anomaly of Supradense Matter via Neutron Star Compactness Scaling
• Bayesian Inference of Hybrid Star Properties from Future High-Precision Measurements of Their Radii
• New Insights into Supradense Matter from Dissecting Scaled Stellar Structure Equations
• Anisotropic pressure effect on central EOS of PSR J0740+6620 in the light of dimensionless TOV equation
• Is The Trace Anomaly at its Minimum Value at Neutron Star Centers?
• Bayesian Constraints on the Neutron Star Equation of State with a Smooth Hadron-Quark Crossover