Nuclear Pasta and Crustal Quasi-Periodic Oscillations in Neutron Star

TL;DR

This study explores how nuclear pasta affects neutron star crust structure and torsional oscillations, using Bayesian models constrained by nuclear physics and observations, and finds correlations between QPO frequencies and symmetry energy properties.

Contribution

It provides the first systematic Bayesian analysis linking nuclear pasta properties with neutron star QPOs, incorporating a wide range of nuclear physics constraints.

Findings

Nuclear pasta appearance is mainly controlled by the symmetry-energy slope parameter L.

Transition density from spherical nuclei to rods is tightly constrained at 0.0588 fm^{-3}.

Predicted QPO frequencies correlate strongly with the curvature of the symmetry energy at sub-saturation density.

Abstract

We investigate the impact of nuclear pasta on crustal structure and torsional oscillations using a Bayesian ensemble of unified neutron-star equations of state based on relativistic mean-field models constrained by nuclear experiments, empirical saturation properties, chiral effective field theory, and multimessenger observations. For each posterior sample, we compute the pasta sequence within a compressible liquid-drop model and quantify the onset density, thickness, and mass fraction of the pasta layers. We show that the appearance and extent of nuclear pasta are primarily controlled by the symmetry-energy slope parameter $L$. While spherical and rod-like pasta configurations are present for all equations of state, only a small fraction of the posterior supports slab, tube, or bubble geometries. The transition from spherical nuclei to rods is tightly constrained to occur at a density of $\rho_{\rm sr} = 0.0588^{+0.0045}_{-0.0065}\,\mathrm{fm^{-3}}$. We further predict that the nuclear pasta layer occupies a relative radial thickness of $\Delta R_{\rm pasta}/\Delta R_{\rm c} = 0.140^{+0.025}_{-0.036}$ and contributes a relative mass fraction of $\Delta M_{\rm pasta}/\Delta M_{\rm c} = 0.475^{+0.071}_{-0.113}$. Using the resulting crust models, we present the first quasi-periodic oscillations (QPOs) analysis based on a Bayesian posterior ensemble of neutron-star equations of state and systematically assess their compatibility with observed low-frequency quasi-periodic oscillations. We find that the predicted QPO frequencies are strongly correlated with the curvature of the symmetry energy evaluated at sub-saturation density, $K_{\rm sym}(\rho_0/2)$, and that uncertainties in the equation of state translate into a range of angular indices $\ell$ consistent with the observed frequencies.