Astrophysical Constraints on the Symmetry Energy and the Neutron Skin of 208Pb 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 0.071 fm for the neutron-skin thickness of 208Pb, implying a slope parameter L = 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 Rskin^208Pb 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 S0 = 33.0+2.0-1.8 MeV, L=53+14-15 MeV, and Rskin^208Pb=0.17+0.04-0.04 fm.