Neutron skins and neutron stars in the multi-messenger era

TL;DR

This paper uses gravitational-wave data from neutron star mergers to constrain the equation of state of dense matter, linking neutron skin thickness measurements to neutron star radii and the properties of nuclear symmetry energy.

Contribution

It provides new constraints on neutron star radii and the symmetry energy by combining gravitational-wave observations with nuclear physics models.

Findings

Upper limit on neutron star radius: < 13.76 km for 1.4 solar masses

Upper limit on neutron skin thickness of ${}^{208}$Pb: < 0.25 fm

Constraints suggest potential phase transition in neutron star interiors

Abstract

The historical first detection of a binary neutron star merger by the LIGO-Virgo collaboration [B. P. Abbott et al. Phys. Rev. Lett. 119, 161101 (2017)] is providing fundamental new insights into the astrophysical site for the $r$-process and on the nature of dense matter. A set of realistic models of the equation of state (EOS) that yield an accurate description of the properties of finite nuclei, support neutron stars of two solar masses, and provide a Lorentz covariant extrapolation to dense matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Given the sensitivity of the gravitational-wave signal to the underlying EOS, limits on the tidal polarizability inferred from the observation translate into constraints on the neutron-star radius. Based on these constraints, models that predict a stiff symmetry energy, and thus large stellar radii, can be ruled out. Indeed, we deduce an upper limit on the radius of a $1.4\,M_{\odot}$ neutron star of $R_{\star}^{1.4}\!<\!13.76\,{\rm km}$. Given the sensitivity of the neutron-skin thickness of ${}^{208}$Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of about $R_{\rm skin}^{208}\!\lesssim\!0.25\,{\rm fm}$. However, if the upcoming PREX-II experiment measures a significantly thicker skin, this may be evidence of a softening of the symmetry energy at high densities---likely indicative of a phase transition in the interior of neutron stars.