Nikolaos Stergioulas (Aristotle University of Thessaloniki, Greece)

Magneto-elastic oscillations and magnetar QPOs

We have developed a model of magneto-elastic oscillations in magnetars with dipole magnetic fields, using
general-relativistic nonlinear simulations in ideal MHD. Our model includes a self-consistent evolution of
torsional oscillations in both the magnetized core and crust and we find strong resonant absorption of crustal
oscillations by the Alfv\'en continuum, even for magnetic fields much weaker than those expected in
magnetars. Our results do not leave much room for crustal oscillations to be responsible for the observed
QPOs in the aftermath of giant flares in SGRs. In contrast, such QPOs could be explained by magnetoelastic
oscillations surviving at the turning points or edges of the continuum (as suggested by Levin) and we
find that this is possible if the magnetic field of a purely dipolar configuration is stronger than a threshold of
order 1015 G. Then, the magneto-elastic oscillations have a dominant magnetic character and reach the
surface of the star, modulating the footpoints of the exterior magnetic field. We will discuss our current effort
to include the magnetosphere modulation in our simulation code, with the aim of arriving at a complete
model for explaining the observed magnetar QPOs.