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Abstract

The properties of neutron stars are determined by the nature of the matter
that they contain. These properties can be constrained by measurements of the
star's size. We obtain the most stringent constraints on neutron-star radii to
date by combining multimessenger observations of the binary neutron-star merger
GW170817 with nuclear theory that best accounts for density-dependent
uncertainties in the equation of state. We construct equations of state
constrained by chiral effective field theory and marginalize over these using
the gravitational-wave observations. Combining this with the electromagnetic
observations of the merger remnant that imply the presence of a short-lived
hyper-massive neutron star, we find that the radius of a $1.4M_\odot$ neutron
star is $R_{1.4M_{\odot}} = 11.0^{+0.9}_{-0.6}~{\rm km}$ ($90\%$ credible
interval). This constraint has important implications for dense-matter physics
and for astrophysics.