Three Disk Oscillation Modes of Rotating Magnetized Neutron Stars

TL;DR

This paper proposes three specific disk oscillation modes around rotating magnetized neutron stars to explain observed QPO separations, supported by theoretical analysis and ongoing 3D MHD simulations.

Contribution

It introduces three novel disk oscillation modes linked to neutron star rotation and magnetic fields, offering explanations for QPO frequency separations.

Findings

Three disk oscillation modes identified and characterized.

Proposed associations between modes and QPO frequencies.

Ongoing 3D MHD simulations to test conditions for mode existence.

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, $\nu_\ell =\nu_u-\nu_*$, and the third with the lower QPO frequency, $\nu_\ell=\nu_u-\nu_*/2$, where $\nu_*$ is the star's rotation rate.