Three Disk Oscillation Modes of Rotating Magnetized Neutron Stars

Abstract

We discuss three specific modes of accretion disks around rotating magnetized neutron stars which may explain the separations of the kilo Hertz quasi periodic oscillations (QPO) 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 is in stable, nearly circular motion near this maximum rather than moving rapidly towards the star or out of the disk plane into funnel flows. It is presently not known if 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, ${\nu }_u$, the second with the lower QPO frequency, ${\nu }_{\ell }={\nu }_u-{\nu }_{\ast }$, and the third with the lower QPO frequency, ${\nu }_{\ell }={\nu }_u-{\nu }_{\ast }/2$, where ${\nu }_{\ast }$ is the star's rotation rate.

6 pages, 2 figures

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• Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations
• Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations
• Energy Dependent Time Delays of kHz Oscillations due to Thermal Comptonization
• QPO emission from moving hot spots on the surface of neutron stars: a model
• Discovery of Drifting High-frequency QPOs in Global Simulations of Magnetic Boundary Layers
• Stability of the Magnetopause of Disk-Accreting Rotating Stars
• Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars
• MHD Simulations of Magnetospheric Accretion, Ejection and Plasma-field Interaction