Magneto-elastic Oscillations and Magnetar QPOs

The origin of the quasi-periodic oscillations (QPOs) observed in the giant flares of soft gamma-ray repeaters (SGRs) remains uncertain. Current models explore the idea that long-term quasi-periodic oscillations are trapped at the turning points of the continuum of torsional magneto-elastic oscillations in the magnetar's interior. After reviewing recent work in this field, we describe our latest efforts using two-dimensional, general-relativistic, magneto-hydrodynamical simulations, coupled to evolutions of shear waves in the solid crust, in order to explore the viability of this model when a purely dipolar magnetic field is assumed. We demonstrate the existence of three different regimes (a) B < 5 x 10(13) G, where crustal shear modes dominate the evolution; (b) 5 x 10(13) G < B < 10(15) G, where Alfven QPOs are mainly confined to the core of the star and the crustal shear modes are damped very efficiently; and (c) B > 10(15) G, where magneto-elastic oscillations reach the surface and approach the behavior of purely Alfven QPOs. Our results do not leave much room for a crustal-mode interpretation of observed QPOs in SGR giant flares, in the case of a purely dipolar magnetic field. On the other hand, the observed QPOs could originate from Alfven-like, global, turning-point QPOs in models with dipolar magnetic field strengths in the narrow range of 5 x 10(15) G less than or similar to B less than or similar to 1.4 x 10(16) G. To agree with estimates for magnetic field strengths in known magnetars, a more complicated magnetic field structure or superfluidity of the neutrons and superconductivity of the protons should be taken into account.