Magneto-elastic oscillations and the damping of crustal shear modes in magnetars

ABSTRACT In a realistic model of magneto‐elastic oscillations in magnetars, we find that crustal shear oscillations, often invoked as an explanation of quasi‐periodic oscillations (QPOs) seen after giant flares in soft gamma‐ray repeaters (SGRs), are damped by resonant absorption on time‐scales of at most 0.2 s, for a lower limit on the dipole magnetic field strength of 5 × 1013 G. At higher magnetic field strengths (typical in magnetars) the damping time‐scale is even shorter, as anticipated by earlier toy models. We have investigated a range of equations of state and masses and if magnetars are dominated by a dipole magnetic field, our findings exclude torsional shear oscillations of the crust from explaining the observed low‐frequency QPOs. In contrast, we find that the Alfvén QPO model is a viable explanation of observed QPOs, if the dipole magnetic field strength exceeds a minimum strength of about several times 1014–1015 G. Then, Alfvén QPOs are no longer confined to the fluid core, but completely dominate in the crust region and have a maximum amplitude at the surface of the star.