SIMBAD references

We discuss three specific modes of accretion disks around rotating magnetized neutron stars which may explain the separations of the kilohertz quasi-periodic oscillations (QPOs) seen in low-mass X-ray binaries. The existence of these modes requires that there be a maximum in the angular velocity of the accreting material, and that the fluid be in stable, nearly circular motion near this maximum rather than moving rapidly toward the star or out of the disk plane into funnel flows. It is currently not known whether these conditions occur, but we are exploring this with 3D magnetohydrodynamic simulations and will report the results elsewhere. The first mode is a corotation mode which is radially trapped in the vicinity of the maximum of the disk rotation rate and is unstable. The second mode, relevant to relatively slowly rotating stars, is a magnetically driven eccentric (m=1) oscillation of the disk excited at a Lindblad radius in the vicinity of the maximum of the disk rotation. The third mode, relevant to rapidly rotating stars, is a magnetically coupled eccentric (m=1) and an axisymmetric (m=0) radial disk perturbation which has an inner Lindblad radius also in the vicinity of the maximum of the disk rotation. We suggest that the first mode is associated with the upper QPO frequency ν_u, the second with the lower QPO frequency ν_l=ν_u-ν_*, and the third with the lower QPO frequency ν_l=ν_u-ν_*/2, where ν_* is the star's rotation rate.