Role of the symmetry energy on hybrid stars

TL;DR

This paper investigates how the symmetry energy influences the structure and observable signals of hybrid stars, considering nuclear physics constraints and different models, and finds that phase transitions can reconcile observations from GW170817 and NICER.

Contribution

It introduces a parameterized phase transition model for hybrid stars and demonstrates its role in aligning theoretical predictions with astrophysical observations.

Findings

Phase transition models reduce tension between GW170817 and NICER data.

GW170817 waveform best fits a hybrid star with a low-density stiff quark matter onset.

Symmetry energy significantly impacts the nature and observational signatures of compact stars.

Abstract

The impact of the symmetry energy on the properties of compact stars is analyzed considering constraints from nuclear physics and astrophysics. A compact star can be a neutron star composed only of nuclear matter or a hybrid star with a quark core. Two typical models (soft and stiff) are considered for the nuclear equation of state, and for the hybrid one, a parameterized first-order phase transition approach, completed with a linear quark matter equation of state, is implemented. We show that the phase transition reduces the tension between GW170817 and NICER observations, and we illustrate the impact of the symmetry energy for the understanding of the nature of the binary system in GW170817. We also confirm our previous findings that the GW170817 waveform is best described as a binary HS with a low-density onset of stiff quark matter. This could also be interpreted as a quarkyonic cross-over.