Crossover Equation of State Constrained by Astronomical Observations and pQCD

TL;DR

This paper develops a neutron star equation of state by combining hadronic and quark models constrained by astrophysical data and pQCD, revealing significant effects on star properties and potential observational signatures of quark matter.

Contribution

It introduces a method to constrain the quark matter coupling constants in the EOS using pQCD and astrophysical observations, and explores the impact on neutron star properties.

Findings

Diquark coupling is tightly constrained to H ≈ 1.5G_s.

Vector coupling G_v is restricted to G_v ≲ 1.1G_s.

Crossover EOSs increase maximum neutron star mass and affect radial oscillation frequencies.

Abstract

The hadron--quark crossover equation of state (EOS) of neutron star (NS) matter is investigated by combining relativistic mean-field (RMF) hadronic models with the Nambu--Jona-Lasinio (NJL) model for quark matter. The vector and diquark coupling constants of the NJL model are constrained using perturbative QCD (pQCD) calculations at high density through a scale-averaging likelihood approach, together with constraints from NS observations and the causality condition on the speed of sound. It is found that the diquark coupling is tightly constrained to $H \simeq 1.5G_s$, while the vector coupling is restricted to $G_v \lesssim 1.1G_s$ by the combined pQCD and astrophysical constraints. Crossover EOSs are constructed based on three hadronic RMF parameter sets, and their thermodynamic properties, sound speed behaviour, and trace anomaly are analysed. The resulting EOSs are applied to calculate NS global and dynamical properties, including mass--radius relations, tidal deformabilities, and fundamental radial oscillation frequencies. Compared with pure hadronic EOSs, the hadron--quark crossover is shown to significantly enhance the maximum NS mass, particularly for softer hadronic EOSs, while remaining consistent with observational bounds. It is further shown that the fundamental radial oscillation frequencies predicted by different EOSs exhibit pronounced differences, especially for intermediate-mass NSs, indicating that radial modes may provide a sensitive probe of the internal composition of NSs. These results indicate that quantitative NS observables may provide potential signatures of quark matter in NS interiors.