Relativistic hybrid stars in light of the NICER PSR J0740+6620 radius measurement

TL;DR

This paper investigates how recent neutron star radius measurements and nuclear experiments inform the internal structure of hybrid stars with phase transitions, suggesting the possible existence of twin stars and their compatibility with gravitational wave observations.

Contribution

It combines observational data with theoretical models to constrain the equation of state and explore the existence of twin stars and hybrid configurations in neutron stars.

Findings

Reconciles GW170817 constraints with PREX-II data via phase transition models.

Identifies narrow mass and radius ranges for twin star configurations.

Shows twin stars can be hybrid stars compatible with GW190814 observations.

Abstract

We explore the implications of the recent radius determination of PSR J0740+6620 by the NICER experiment combined with the neutron skin measurement by the PREX-II experiment and the associated inference of the slope of symmetry energy, for the structure of hybrid stars with a strong first-order phase transition from nucleonic to quark matter. We combine a covariant density-functional nucleonic equation of state (EOS) with a constant-speed-of-sound EOS for quark matter. We show that the radius and tidal deformability ranges obtained from GW170817 can be reconciled with the implication of the PREX-II experiment if there is a phase transition to quark matter in the low-mass compact star. In the high-mass segment, the EoS needs to be stiff to comply with the large-radius inference for PSR J0740+6620 and J0030+0451 with masses $M\simeq 2M_{\odot}$ and $M\simeq 1.4M_{\odot}$. We show that twin stars are not excluded, but the mass and radius ranges (with $M \geq M_\odot$) are restricted to narrow domains $\Delta M_{\rm twin} \lesssim 0.05 M_\odot$ and $\Delta R_{\rm twin} \sim 1.0$~km. We also show that the existence of twin configurations is compatible with the light companion in the GW190814 event being a hybrid star in the case of values of the sound-speed square $s=0.6$ and $s=1/3$.