Properties of Neutron Stars

I review attempts made to determine the properties of neutron stars. I focus on constraints on the maximum mass that a neutron star can have, and on attempts to measure neutron-star radii. So far, there appears to be only one neutron star for which there is strong evidence that its mass is above the canonical 1.4 M ⊙ , viz., Vela X-1, for which a mass close to 1.9 M ⊙ is found. Prospects for progress appear brightest for studies of systems in which the neutron star should have accreted substantial amounts of matter. While for individual systems the evidence that neutron stars can have high masses is weak, the ensemble appears to show that masses around 1.6 M ⊙ are possible. For the radius determination, most attempts have focussed on neutron stars in low-mass X-ray binaries in which accretion has temporarily shut down. These neutron stars are easiest to model, since they should have pure Hydrogen atmospheres and low magnetic fields. To obtain accurate radii, however, requires precise distances and very high quality data. The physics of matter at ultra-high density is not only of interest on its own accord, but also because of its astronomical implications: to understand the core collapse of massive stars, the supernova phenomenon, and the existence and properties of neutron stars, knowledge of the physics, as summarised in the equation of state (EOS), is required. As is clear from other contributions to these proceedings, quantum-chromodynamics calculations are not yet developed well enough to determine the densities at which, e.g., meson condensation and the transition between the hadron and quark-gluon phases occur. At densities slightly higher than nuclear and at high temperatures, the model predictions can be compared with the results of heavy-nuclei collision experiments. The substantial progress on this front is discussed elsewhere in these proceedings. For higher densities and low temperatures, however, no terrestrial experiments seem possible; the models can be compared only with neutron-star parameters. Recent