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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 $0.283\pm0.071$ fm for
the neutron-skin thickness of $^{208}$Pb, $R_{\rm skin}^{^{208}\text{Pb}}$,
implying a symmetry-energy slope parameter $L$ of $106\pm37$ 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 $R_{\rm
skin}^{^{208}\text{Pb}}$ 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 ($\chi$EFT) and PREX-II
constraints yields $S_0 = 33.0^{+2.0}_{-1.8}$ MeV, $L=53^{+14}_{-15}$ MeV, and
$R_{\rm skin}^{^{208}\text{Pb}} = 0.17^{+0.04}_{-0.04}$ fm. We also examine the
consistency of several individual $\chi$EFT calculations with astrophysical
observations and terrestrial experiments. We find that there is only mild
tension between $\chi$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 $\chi$EFT, astrophysical data, and
other nuclear experiments.