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The violent giant flares of magnetars excite QPOs which persist for hundredsof seconds, as seen in the X-ray tail following the initial burst. Recentstudies, based on single-fluid barotropic magnetar models, have suggested thatthe lower-frequency QPOs correspond to magneto-elastic oscillations of thestar. The higher frequencies, however, in particular the strong 625 Hz peak,have proved harder to explain, except as high mode multipoles. In this work westudy the time evolutions of non-axisymmetric oscillations of two-fluidNewtonian magnetars with no crust. We consider models with superfluid neutronsand normal protons, and poloidal and toroidal background field configurations.We show that multi-fluid physics (composition-gradient stratification,entrainment) tends to increase Alfven mode frequencies significantly from theirvalues in a single-fluid barotropic model. The higher-frequency magnetar QPOsmay then be naturally interpreted as Alfven oscillations of the multi-fluidstellar core. The lower-frequency QPOs are less easily explained within ourpurely fluid core model, but we discuss the possibility that these are crustalmodes.

Stratification, superfluidity and magnetar QPOs