Detailed examination of astrophysical constraints on the symmetry energy and the neutron skin of 208Pb with minimal modeling assumptions

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 ±0.071 fm for the neutron-skin thickness of ²⁰⁸Pb, R_skin^208Pb, implying a symmetry-energy slope parameter L of 106 ±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 S₀, L , and R_skin^208Pb 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 (χ EFT ) and PREX-II constraints yields S₀=33 .0_−1.8^+2.0 MeV, L =53₋₁₅⁺¹⁴ MeV, and R_skin^208Pb=0 .17_−0.04^+0.04 fm. We also examine the consistency of several individual χ EFT calculations with astrophysical observations and terrestrial experiments. We find that there is only mild tension between χ EFT , astrophysical data, and PREX-II's R_skin^208Pb measurement (p value =12.3 % ) and that there is excellent agreement between χ EFT , astrophysical data, and other nuclear experiments.