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

Abstract

Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.

Empirical neutron star mass formula based on experimental observables

“…11, for the neutron star model predicted with the empirical formula of ∆R Pb n , we only plot the stellar model whose central density is up to threefold the saturation density. In the same figure, we show the constraints on the neutron star mass and radius obtained from the various astronomical observations; the gravitational wave observations in the GW170817 event, i.e., the 1.4M neutron star radius is less than 13.6 km [8], whose constraint may become more severe by combining with the multimessenger observations and nuclear theory [71,72]; the x-ray observations via NICER for PSR J0030+0451 [16,17] and MSP J0740+6620 [18,19]; the observations of x-ray burst through the theoretical models [73]; and the identification of the magnetar quasi-periodic oscillations observed in GRB 200415A with the crustal torsional oscillations [74]. As the theoretical constraint, the top-left region can be excluded from the causality [75].…”