A Detailed Examination of 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 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 $0.283\pm 0.071$ fm for the neutron-skin thickness of ${}^{208}$Pb, $R_{\mathrm{skin}}^{{}^{208}\text{Pb}}$, implying a symmetry-energy slope parameter $L$ of $106\pm 37$ 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 $R_{\mathrm{skin}}^{{}^{208}\text{Pb}}$ 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. Combining astrophysical data with chiral effective field theory ($\ddot{I}‡$EFT) and PREX-II constraints yields $S_0=33.0_{-1.8}^{+2.0}$ MeV, $L=53_{-15}^{+14}$ MeV, and $R_{\mathrm{skin}}^{{}^{208}\text{Pb}}=0.17_{-0.04}^{+0.04}$ fm. We also examine the consistency of several individual $\ddot{I}‡$EFT calculations with astrophysical observations and terrestrial experiments. We find that there is only mild tension between $\ddot{I}‡$EFT, astrophysical data, and PREX-II's $R_{\mathrm{skin}}^{{}^{208}\mathrm{Pb}}$ measurement ($p$-value $=12.3\%$) and that there is excellent agreement between $\ddot{I}‡$EFT, astrophysical data, and other nuclear experiments.

18 pages, 12 figures, 1 table