The Neutron Star Mass-Radius Relation and the Equation of State of Dense Matter

TL;DR

This paper constrains the neutron star mass-radius relation and dense matter equation of state using recent observations, showing that the radius of a 1.4 solar mass neutron star is between 10.4 and 12.9 km, independent of core composition assumptions.

Contribution

It provides the first robust constraints on neutron star radii that remain valid across different source types and core compositions, including quark matter scenarios.

Findings

Neutron star radius for 1.4 solar masses is between 10.4 and 12.9 km.

Constraints are valid even when removing extreme sources or assuming quark matter.

Results are consistent with heavy-ion collision data and neutron matter theories.

Abstract

The equation of state (EOS) of dense matter has been a long-sought goal of nuclear physics. Equations of state generate unique mass versus radius (M-R) relations for neutron stars, the ultra-dense remnants of stellar evolution. In this work, we determine the neutron star mass-radius relation and, based on recent observations of both transiently accreting and bursting sources, we show that the radius of a 1.4 solar mass neutron star lies between 10.4 and 12.9 km, independent of assumptions about the composition of the core. We show, for the first time, that these constraints remain valid upon removal from our sample of the most extreme transient sources or of the entire set of bursting sources; our constraints also apply even if deconfined quark matter exists in the neutron star core. Our results significantly constrain the dense matter EOS and are, furthermore, consistent with constraints from both heavy-ion collisions and theoretical studies of neutron matter. We predict a relatively weak dependence of the symmetry energy on the density and a value for the neutron skin thickness of lead which is less than 0.20 fm, results that are testable in forthcoming experiments.