Selective Excitation of Stellar Oscillations of a Magnetar with an Externally Coupled Tangled Magnetic Field

Magnetars are neutron stars with strong magnetic fields (10$^{\mathrm{15}}$ Gauss) that form tangled configurations to stabilize. Some of them produce giant flares that exhibit quasi-periodic oscillations which have been attributed to stellar oscillations that modulate the emission. A tangled magnetic field supports normal modes that agree with observed quasi-periodic oscillations. We show that for a tangled field that extends outside of the magnetar, a local deposition of energy leads to selective mode excitation.