Estimation of the Neutron Star Mass and Radius of FRB 20240114A by Identification of Crustal Oscillations

Abstract By identifying quasiperiodic oscillations (QPOs) reported in FRB 20240114A (from the Five-hundred-meter Aperture Spherical Telescope) with neutron star (NS) crustal torsional oscillations, together with experimental constraints on the incompressibility K 0 of symmetric nuclear matter at saturation density, we constrain the mass and radius of an extragalactic NS at redshift z ≈ 0.13. Identifying the low-order QPO frequencies as fundamental oscillations, and frequencies of 567.7 or 655.5 Hz (rest frame) as first overtone candidates, implies NS mass ranges of 1.00–1.55 M ⊙ or 1.17–1.76 M ⊙ , respectively. The radius is also constrained, with a self-consistent value of around 13 km, consistent with the calculation of the NS structure within the low-mass/low-central density regime. Simultaneously, we also constrain another nuclear saturation parameter, namely, the density dependence of the nuclear symmetry energy at saturation density (i.e., the slope parameter), L , and determine it to be L = 59.5–96.8 MeV with ∼10% systematic uncertainty, which is broadly consistent with previous constraints on L obtained from experiments and astronomical observations. Thus, a mapping of fast radio burst (FRB) QPOs to crustal torsional modes seems reasonable. This interpretation will be tested with the discovery of additional QPOs in upcoming FRB surveys.