Abstract
Constraints on neutron star masses and radii now come from a variety of sources: theoretical and experimental nuclear physics, astrophysical observations including pulsar timing, thermal and bursting X-ray sources, and gravitational waves, and the assumptions inherent to general relativity and causality of the equation of state. These measurements and assumptions also result in restrictions on the dense matter equation of state. The two most important structural parameters of neutron stars are their typical radii, which impacts intermediate densities in the range of one to two times the nuclear saturation density, and the maximum mass, which impacts the densities beyond about three times the saturation density. Especially intriguing has been the multi-messenger event GW170817, the first observed binary neutron star merger, which provided direct estimates of both stellar masses and radii as well as an upper bound to the maximum mass.
Highlights
The study of neutron stars represents our best chance to study matter under conditions of high density, extreme isospin asymmetry, and relatively cold temperatures which cannot be examined through heavy ion collisions
Neutron star matter is in strong- and weak-interaction equilibrium, which for densities larger than the nuclear saturation density, ns ' 0.16 fm−3, which is the normal density found inside atomic nuclei, results in very neutron-rich compositions in which the neutron/proton ratio nn /n p is 10 to 20
Measurements of the radii of neutron stars are excellent probes of neutron star matter from ns − 3ns, as the pressure of such matter in this range is highly correlated with the radius [3], while the maximum mass neutron star mass probes densities in the range from 3ns − 5ns
Summary
The study of neutron stars represents our best chance to study matter under conditions of high density, extreme isospin asymmetry, and relatively cold temperatures which cannot be examined through heavy ion collisions. [1] for a review and references) is available through nuclear structure studies of neutron-rich nuclei such as mass, neutron skin, and giant monopole and dipole resonances that can probe cold matter up to ns , but under relatively symmetric conditions; see Section 4. Recent advances in theoretical neutron matter studies [2] complement experiments and probe extremely neutron-rich matter, but are expansions and limited to densities below about 2ns. Lower and upper bounds on neutron star radii are found through the assumptions of general relativity and causality [4], as discussed in Sections 2 and 5.
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