Shear oscillations in neutron stars and the nuclear symmetry energy

Abstract

The shear and interface modes excited inside the neutron star due to the presence of elasticity depend on the properties of both the crust and core region. To examine how such eigenfrequencies depend on the neutron star properties, we solve the eigenvalue problem by adopting the relativistic Cowling approximation. Then, we confirm that the number of the interface modes excited in the star is generally equivalent to the number of the interface where the shear modulus discontinuously becomes zero, but we also find that the number of interface modes becomes smaller than that of the interface for the stellar model with lower or higher value of the slope parameter $L$. Furthermore, we derive the empirical relations for expressing the shear modes and one of the interface modes ($i_1$-mode in the text), which is the mode whose amplitude becomes dominant at the interface between the crust and envelopes and at the interface between the phases of slablike and cylindrical nuclei. At the end, we also show the possibility of identifying the higher QPO frequencies observed in GRB 200415A with the shear oscillations, as an alternative possibility instead of the torsional oscillations.