Neutron star mass-radius constraints using the high-frequency QPOs of GRB 200415A

Quasi-periodic oscillations (QPOs) observed in a giant flare of a strongly magnetized neutron star (magnetar), are carrying crucial information for extracting the neutron star properties. The aim of the study is to constrain the mass and radius of the neutron star model for GRB 200415A, by identifying the observed QPOs with the crustal torsional oscillations together with the experimental constraints on the nuclear matter properties. The frequencies of the crustal torsional oscillations are determined by solving the eigenvalue problem with the Cowling approximation, assuming a magnetic field of about $10^{15}$G. We find that the observed QPOs can be identified with several overtones of crustal oscillations, for carefully selected combinations of the nuclear saturation parameters. Thus, we can inversely constrain the neutron star mass and radius for GRB 200415A by comparing them to the values of nuclear saturation parameters obtained from terrestrial experiments. We impose further constraints on the neutron star mass and radius while the candidate neutron star models are consistent with the constraints obtained from other available astronomical and experimental observations.