Masses, Radii, and the Equation of State of Neutron Stars

TLDR

Recent advances in instrumentation and computation have rapidly increased precise radio pulsar timing, yielding many neutron‑star mass measurements. The paper reviews current knowledge of neutron‑star masses and radii. New mass measurements reveal a broader distribution extending to 1.9–2.0 M⊙, while improved radius determinations (10–11.5 km) and laboratory constraints now significantly constrain the equation of state of dense matter.

Abstract

We summarize our current knowledge of neutron-star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of radio pulsars in binaries in the past few years, leading to a large number of mass measurements. These discoveries show that the neutron-star mass distribution is much wider than previously thought, with three known pulsars now firmly in the 1.9–2.0-M ⊙ mass range. For radii, large, high-quality data sets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 10–11.5-km range and shrinking their uncertainties, owing to a better understanding of the sources of systematic errors. The combination of the massive-neutron-star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk properties of cold nuclear matter at densities higher than that of the atomic nucleus, a major unsolved problem in modern physics.