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Role of the symmetry energy on hybrid stars

H. Güven, K. Bozkurt, E. Khan, J. Margueron

TL;DR

This paper examines how the density dependence of the nuclear symmetry energy affects the hadron-quark phase transition in neutron stars. It uses two representative nucleonic EoS (soft SLy5 and stiff PKDD) and a parameterized first-order transition to a linear quark-matter EoS, analyzed within a Bayesian framework constrained by GW170817 and NICER observations. The results show that the phase transition helps reconcile radii and tidal deformabilities and that the symmetry energy strongly shapes the QM onset and the observable properties of hybrid stars, with GW170817 most consistent with a binary containing at least one hybrid star, especially in the stiff nuclear-EoS case. The study highlights the critical role of the symmetry energy in interpreting dense-matter observations and guiding future multi-messenger analyses.

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.

Role of the symmetry energy on hybrid stars

TL;DR

This paper examines how the density dependence of the nuclear symmetry energy affects the hadron-quark phase transition in neutron stars. It uses two representative nucleonic EoS (soft SLy5 and stiff PKDD) and a parameterized first-order transition to a linear quark-matter EoS, analyzed within a Bayesian framework constrained by GW170817 and NICER observations. The results show that the phase transition helps reconcile radii and tidal deformabilities and that the symmetry energy strongly shapes the QM onset and the observable properties of hybrid stars, with GW170817 most consistent with a binary containing at least one hybrid star, especially in the stiff nuclear-EoS case. The study highlights the critical role of the symmetry energy in interpreting dense-matter observations and guiding future multi-messenger analyses.

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.
Paper Structure (12 sections, 33 equations, 11 figures, 3 tables)

This paper contains 12 sections, 33 equations, 11 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Theoretical Framework
  3. Crust and outer core
  4. First order phase transition and quark matter equation of state
  5. Neutron star global properties
  6. Uncertainty quantification and constraints on the quark matter phase
  7. Bayesian approach
  8. Neutron versus hybrid stars
  9. Prediction for GW170817
  10. Model parameter PDFs
  11. Two-parameter correlations
  12. Conclusions

Figures (11)

  • Figure 1: Symmetry energy in MeV for SLy5 Chabanat:1998 and PKDD Long:2004 as a function of the nucleon density $n_0$ in fm$^{-3}$.
  • Figure 2: Energy-density (a), baryon density (b), and chemical potential (c) envelopes at 95% confidence interval for hybrid EoS based on PKDD and SLy5 nuclear model (grey area). The two nuclear EoS are shown in back lines while typical examples of hybrid EoS are shown in color lines (with a large and small density jump at the FOPT).
  • Figure 3: Mass-radii (a) and mass-$\Lambda$ (b) solutions of PKDD and SLy5 EoSs with FOPT, where the contours considered here are presented in a 95% confidence interval. Observational data are also represented at 68% and 95% confidence intervals.
  • Figure 4: Posterior PDF for $\Tilde{\Lambda}$ with PKDD (a) and SLy5 (b) EoSs with FOPT. The orange band represents tidal deformability of GW170817 from LIGO-Virgo parameter estimation Abbott:2019.
  • Figure 5: Posterior PDF for $p_{\mathrm{PT}}$ associated with various binary systems (BNS, NSHS, and BHS) and based on PKDD (a) and SLy5 (b) models.
  • ...and 6 more figures