  Magneto-elastic oscillations of neutron stars are believed to explain
observed quasi-periodic oscillations (QPOs) in the decaying tail of the giant
flares of highly magnetized neutron stars (magnetars). Strong efforts of the
theoretical modelling from different groups have increased our understanding of
this phenomenon significantly. Here, we discuss some constraints on the matter
in neutron stars that arise if the interpretation of the observations in terms
of superfluid, magneto-elastic oscillations is correct. To explain the observed
modulation of the light curve of the giant flare, we describe a model that
allows the QPOs to couple to the stellar exterior through the magnetic field.
In this magnetosphere, the shaking magnetic field induces currents that provide
scattering targets for resonant cyclotron scattering of photons, which is
calculated with a Monte-Carlo approach and coupled to a code that calculates
the momentum distribution of the charge carriers as a one-dimensional
accelerator problem. We show first results of a simplified, but self-consistent
momentum distribution, i.e. a waterbag distribution, and of the corresponding
spectra.
