Multi-messenger Neutron Star Astrophysics and Determining the Composition of the Neutron Star Core

Using a model for the equation of state and composition of dense matter and the magnitude of singlet proton superconductivity and triplet neutron superfluidity, we perform the first simultaneous fit of neutron star masses and radii with neutron star luminosities and ages determined from observations of isolated neutron stars. We also simultaneously fit the charge radii and binding energies of heavy nuclei and find agreement with the tidal deformability obtained in GW 170817. We find that the threshold density for the direct Urca process lies between the central density of 1.7 and 2 solar mass neutron stars, but that 2 solar mass stars are unlikely to cool principally by the direct Urca process because of the suppression by neutron triplet superfluidity. This is the first quantitative constraint on the composition of matter above the saturation density.