Constraints on strong phase transitions in neutron stars

TL;DR

This paper investigates constraints on strong first-order phase transitions in neutron star matter, combining theoretical modeling with astrophysical data to explore their effects on neutron star properties and possible solutions.

Contribution

It introduces a large ensemble of polytropic EOS models constrained by nuclear physics and astrophysical data to analyze phase transition effects in neutron stars.

Findings

Phase transitions can lead to larger neutron star radii if they occur below twice nuclear saturation density.

A significant parameter space for phase transitions remains unexplored by current numerical-relativity studies.

Some neutron stars may contain sizeable high-density cores beyond the phase transition, especially more massive ones.

Abstract

We study current bounds on strong first-order phase transitions (PTs) along the equation of state (EOS) of dense strongly interacting matter in neutron stars, under the simplifying assumption that on either side of the PT the EOS can be approximated by a simple polytropic form. We construct a large ensemble of possible EOSs of this form, anchor them to chiral effective field theory calculations at nuclear density and perturbative QCD at high densities, and subject them to astrophysical constraints from high-mass pulsars and gravitational-wave observations. Within this setup, we find that a PT permits neutron-star solutions with larger radii, but only if the transition begins below twice nuclear saturation density. We also identify a large parameter space of allowed PTs currently unexplored by numerical-relativity studies. Additionally, we locate a small region of parameter space allowing twin-star solutions, though we find them to only marginally pass the current astrophysical constraints. Finally, we find that sizeable cores of high-density matter beyond the PT may be located in the centers of some stable neutron stars, primarily those with larger masses.