On the nature of quasi-periodic oscillations in the tail of soft gamma repeater giant flares

A model is presented for the quasi-periodic component of magnetar emission during the tail phase of giant flares. The model invokes modulation of the particle number density in the magnetosphere. The magneto spheric currents are modulated by torsional motion of the surface, and we calculate that the amplitude of neutron star (NS) surface oscillation should be ∼1% of the NS radius in order to produce the observed features in the power spectrum. Using an axisymmetric analytical model for structure of the magnetosphere of an oscillating NS, we calculate the angular distribution of the optical depth to the resonant Compton scattering. The anisotropy of the optical depth may be why quasi-periodic oscillations are observed only at particular rotational phases.