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

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 Pb-208, 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 S-0, L, and (RPbskin)-Pb-208 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-skin(208Pb) = 0.17(-0.04) (+0.04) fm.

Automated summary

The symmetry energy and its density dependence play crucial roles in nuclear and astrophysics, influencing properties ranging from the neutron-skin thickness of nuclei to the radius of neutron stars. Observations of neutron stars have allowed for constraints on the symmetry energy S-0, slope parameter L, and the neutron skin thickness of Pb-208, indicating a preference for smaller values of neutron skin and L.