The nuclear equation of state

Recent observations from cooling neutron stars and of photospheric radius expansions in X-ray bursters are used to simultaneously estimate their masses and radii. Although the observational uncertainties for these sources are considerable, as a group they snugly constrain the mass-radius relation and the equation of state. The results of a Bayesian analysis using a parametrized equation of state are discussed. It is shown that the low-density equation of state is consistent with that obtained from recent estimates for pure neutron matter. Also, the deduced nuclear incompressibility is surprisingly compatible with nuclear systematics and experiment. The density dependence of the nuclear symmetry energy is predicted to be relatively small, leading to correspondingly small values for the predicted neutron skin thickness of lead and for the radii of 1.4 M⊙ stars. The high-density equation of state stiffens, however, and the neutron star maximum mass, to 90% confidence, is predicted to be greater than 1.85 solar masses.