Magnetised neutron star crusts and torsional shear modes of magnetars

Abstract

We discuss outer and inner crusts of neutron stars in strong magnetic fields. Here, we demonstrate the effect of Landau quantization of electrons on the ground state properties of matter in outer and inner crusts in magnetars. This effect leads to the enhancement of the electron number density in strong magnetic fields with respect to the zero field case. For the outer crust, we adopt the magnetic Baym-Pethick-Sutherland model and obtain the sequence of nuclei and equation of state (EoS). The properties of nuclei in the inner crust in the presence of strong magnetic fields are investigated using the Thomas-Fermi model. The coexistence of two phases of nuclear matter-liquid and gas, is assumed in this case. The proton number density in the Wigner-Seitz cell is affected in strong magnetic fields through the charge neutrality. We perform this calculation using the Skyrme nucleon-nucleon interaction with different parameterisations. We find nuclei with larger mass and atomic numbers in the inner crust in the presence of strong magnetic fields than those of the zero field case for all those parameter sets. Further we investigate torsional shear mode frequencies using the results of magnetised neutron star crusts and compare those with observations.