Nuclear Symmetry Energies at Supra-saturation Densities Extracted from Observations of Neutron Stars and Gravitational Waves

Abstract

The high-density behavior of nuclear symmetry energy is the most uncertain part of the Equation of State (EOS) of dense neutron-rich nucleonic matter. It has significant ramifications in understanding properties of nuclear reactions induced by rare isotopes, neutron stars and gravitational waves from various sources. To infer the underlying EOS of dense neutron-rich nuclear matter from observables of neutron stars is the longstanding neutron star inverse-structure problem. To make progress in solving this problem, we advanced two inversion approaches: (1) the direct inversion in the three-dimensional high-density EOS parameter space and (2) Bayesian inference of six EOS parameters statistically. Neutron star observational data obtained so far, especially their radii from analyzing gravitational waves (e.g., via the tidal deformability) from GW170817 and X-rays from quiescent low-mass X-ray binaries can already help constrain significantly the nuclear EOS up to about twice the saturation density of nuclear matter. In particular, the nuclear symmetry energy at twice the saturation density of nuclear matter E sym(2ρ 0) is found to be MeV at 68% confidence level.