Properties of neutron stars

Abstract

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 Msun, viz., Vela X-1, for which a mass close to 1.9 Msun 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 Msun 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.