The study of quasi-periodic oscillations from soft gamma repeaters /

Hyperflares from Soft Gamma Repeaters have revealed quasi-periodic oscillations, suggested to be torsional modes of the neutron star crust produced during starquakes. We study how a magnetic field affects these modes. We make a plane-parallel model of the neutron star crust with a vertical and non-vertical field and solve for the eigenvalues and eigenfunctions. In the vertical field case we find a discrete set of modes comparable to the observed frequencies giving evidence for this theory. We find that the lower order modes do not depend on B while the higher order modes do when the field strength is low or extremely high. We make a simple analytic model of a non-vertical field in a closed box representing the neutron star crust. The mode spectrum in this model is continuous rather than discrete, raising the puzzle of why only specific modes are excited during the hyperflare