||Magneto-elastic Oscillations and Magnetar QPOs
||459, 6th International Conference of Numerical Modeling of Space Plasma Flows (ASTRONUM 2011)
||Stergioulas, N.; Gabler, M.; Cerdá-Durán, P.; Font, J. A.; Müller, E.
||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 × 1013 G, where crustal shear modes dominate
the evolution; (b)
5 × 1013 G < B < 1015 G, where Alfvén QPOs are mainly
confined to the core of the star and the crustal shear modes are damped
very efficiently; and (c) B > 1015 G, where
magneto-elastic oscillations reach the surface and approach the behavior of
purely Alfvén 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 Alfvén-like, global, turning-point QPOs in models with dipolar magnetic
field strengths in the narrow range of 5 × 1015 G ≤ B ≤ 1.4 × 1016 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.