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Global 3D Simulations of Magnetic Boundary Layers and High-frequency QPOs

Numerical simulations of accretion to stars with very small magnetospheres show that they often accrete in the unstable regime and that a new type of equatorial streams form connecting a star and the disk  ( Romanova  &  Kulkarni 2008, MNRAS submitted, arXiV0812.0241 ). The ordered streams  form which rotate with angular velocity approximately equal to the angular velocity of the inner disk and thus produce the high-frequency QPOs. Variation of the accretion rate leads to the drift of the QPO frequency.

Top panels: one of density levels and selected field lines are shown. Bottom panel shows equatorial slice of density.  Pictures are shown in coordinate system rotating with the star. Time is in units of 1/16 of rotation period at the radius r=1.

Animation 1 (avi, 10.1 Mbt). Ordered rotation of 3D streams. Animation shows ordered rotation of equatorial streams (in star's frame).

Animation 2 (avi, 1.8  Mbt). Slice of density in the equatorial plane. Animation shows variation of density distribution in the equatorial plane (in star's frame). Density varies between the highest (red)  to the lowest (blue). Red line separates the magnetosphere where the magnetic energy-density dominates. 

 Animation 3 (avi, 1.8 Mbt). Rotation of hot spots. Animation shows hot spots at the surface of the star (in  star's frame). The red color shows  the maximum in energy flux, blue - the minimum.