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

Understand

The symmetry energy and its density dependence are pivotal 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 $0.283\pm0.071$ fm for the neutron-skin thickness of $^{208}$Pb, $R_{\rm skin}^{^{208}\text{Pb}}$, implying a symmetry-energy slope parameter $L$ of $106\pm37$ 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 $R_{\rm skin}^{^{208}\text{Pb}}$ directly from observations of neutron stars with minimal modeling assumptions.
• The resulting astrophysical constraints from heavy pulsar masses, LIGO/Virgo, and NICER favor smaller values of the neutron skin and $L$, as well as negative symmetry incompressibilities.