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