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

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 $S_0=33.0_{-1.8}^{+2.0}$ MeV, $L=53_{-15}^{+14}$ MeV, and $R_{\mathrm{skin}}^{{}^{208}\mathrm{Pb}}=0.17_{-0.04}^{+0.04}$ fm.

11 pages, 6 figures, 2 tables