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 timescales of at most 0.2s , for a lower limit on the dipole magnetic field strength of 5 \times 10 ^ { 13 } G. At higher magnetic field strengths ( typical in magnetars ) the damping timescale 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 10 ^ { 14 } G to 10 ^ { 15 } 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 .