Gravitational waves emitted by binary neutron-star inspirals carry information on components’ masses and tidal deformabilities, but not directly radii, which are measured by electromagnetic observations of neutron stars (NSs). To improve the multimessenger astronomy studies of neutron stars, an expression for neutron-star radii as a function of gravitational-wave only data would be advantageous, as it would allow us to compare information from two different channels. In order to do so, a symbolic regression method, pysr, is trained on Tolman-Oppenheimer-Volkoff solutions to piecewise polytropic equation of state (EOS) input to discover an approximate symbolic expression for the neutron-star radius as a function of gravitational-wave measurements only. The approximation is tested on piecewise polytropic EOS NS data, as well as on NS sequences based on selected realistic (nonpolytropic) dense-matter theory EOSs, achieving consistent agreement between the ground truth values and the symbolic approximation for a broad range of NS parameters covering current astrophysical observations, with average radii differences of few hundred meters. Additionally, the approximation is applied to the GW170817 gravitational-wave mass and tidal deformability posteriors, and compared to reported inferred radius distributions.

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• Electromagnetic radiation astronomy
• Gravitational waves
• Nuclear matter in neutron stars
• Machine learning