Neutron star equation of state

Abstract

Experimental information concerning the equation of state in neutron stars is lacking, because of the necessary extrapolations in both density and neutron excess from the nearly symmetric nuclear matter observed in nuclei. However, the combination of new developments in the theory of neutron star structure and in astronomical observations provides important constraints. From a theoretical perspective, it is argued that the extrapolation in neutron excess is more crucial for neutron star structure than is the density extrapolation. For example, the radius of neutron stars is primarily a function of the pressure of matter in the vicinity of nuclear matter density, which is essentially determined by the isospin properties of dense matter. In the absence of extreme softening in the dense matter equation of state, a measurement of the radius of a neutron star more accurate than about 1 km will usefully constrain the equation of state. In addition, the moment of inertial and the binding energy of neutron stars are nearly universal functions of the star’s compactness. The potential constraints that can be deduced from observations of thermal emission from young neutron stars, neutrinos from newly born neutron stars, Quasi-Periodic Oscillations from X-ray emitting neutron stars in binaries, and glitches from pulsars are discussed.