Symmetry energy and neutron star properties constrained by chiral effective field theory calculations

Abstract

We investigate the nuclear symmetry energy and neutron star properties using a Bayesian analysis based on constraints from different chiral effective field theory calculations using new energy density functionals that allow for large variations at high densities. Constraints at high densities are included from observations of GW170817 and from NICER. In particular, we show that both NICER analyses lead to very similar posterior results for the symmetry energy and neutron star properties when folded into our equation-of-state framework. Using the posteriors, we provide results for the symmetry energy and the slope parameter, as well as for the proton fraction, the speed of sound, and the central density in neutron stars. Moreover, we explore correlations of neutron star radii with the pressure and the speed of sound in neutron stars. Our 95\% credibility ranges for the symmetry energy $S_v$, the slope parameter $L$, and the radius of a 1.4$\msun$ neutron star, $R_{1.4}$, are $S_v=(30.6\text{--}33.9)$\,MeV, $L=(43.7\text{--}70.0)$\,MeV, and $R_{1.4}=(11.6\text{--}13.2)$\,km. Our analysis for the proton fraction shows that larger and/or heavier neutron stars are more likely to cool rapidly via the direct Urca process. Within our equation-of-state framework a maximum mass of neutron stars $M_{\rm max}>2.1\msun$ indicates that the speed of sound needs to exceed the conformal limit.