Astrophysical Constraints on the Symmetry Energy and the Neutron Skin of $^{208}$Pb with Minimal Modeling Assumptions

TL;DR

This study uses astrophysical observations and minimal modeling assumptions to constrain the nuclear symmetry energy and neutron skin thickness of lead-208, providing results consistent with experimental data and theoretical models.

Contribution

It introduces a nonparametric Gaussian process approach to constrain nuclear matter properties from astrophysical data with minimal assumptions.

Findings

Astrophysical data favor smaller neutron skin and symmetry energy slope L.

Combined data yields S0=33.0+/-2.0 MeV, L=53+/-15 MeV, R_skin=0.17+/-0.04 fm.

Results are consistent with PREX-II and chiral effective field theory constraints.

Abstract

The symmetry energy and its density dependence are crucial inputs for many nuclear physics and astrophysics applications, as they determine properties ranging from the neutron-skin thickness of nuclei to the crust thickness and the radius of neutron stars. Recently, PREX-II reported a value of $0.283 \pm 0.071$ fm for the neutron-skin thickness of $^{208}$Pb, implying a slope parameter $L = 106 \pm 37$ MeV, larger than most ranges obtained from microscopic calculations and other nuclear experiments. We use a nonparametric equation of state representation based on Gaussian processes to constrain the symmetry energy $S_0$, $L$, and $R_\mathrm{skin}^{^{208}\mathrm{Pb}}$ directly from observations of neutron stars with minimal modeling assumptions. The resulting astrophysical constraints from heavy pulsar masses, LIGO/Virgo, and NICER clearly favor smaller values of the neutron skin and $L$, as well as negative symmetry incompressibilities. Combining astrophysical data with PREX-II and chiral effective field theory constraints yields $S_0 = 33.0^{+2.0}_{-1.8}$ MeV, $L=53^{+14}_{-15}$ MeV, and $R_\mathrm{skin}^{^{208}\mathrm{Pb}}=0.17^{+0.04}_{-0.04}$ fm.