Wave Excitation in Disks Around Rotating Magnetic Stars

TL;DR

This paper investigates how periodic magnetic forces from rotating stars excite bending waves in surrounding disks, potentially causing observable variabilities like quasi-periodic oscillations in systems such as neutron star binaries.

Contribution

It provides a hydrodynamical model of wave excitation, propagation, and dissipation in magnetized accretion disks, highlighting the role of resonances in wave dynamics.

Findings

Bending waves are excited at Lindblad/vertical resonances.

Waves can reach significant amplitudes and modulate system flux.

Resonant torque is negligible compared to accretion torque.

Abstract

The accretion disk around a rotating magnetic star (neutron star, white dwarf or T Tauri star) is subjected to periodic vertical magnetic forces from the star, with the forcing frequency equal to the stellar spin frequency or twice the spin frequency. This gives rise bending waves in the disk that may influence the variabilities of the system. We study the excitation, propagation and dissipation of these waves using a hydrodynamical model coupled with a generic model description of the magnetic forces. The $m=1$ bending waves are excited at the Lindblad/vertical resonance, and propagate either to larger radii or inward toward the corotation resonance where dissipation takes place. While the resonant torque is negligible compared to the accretion torque, the wave nevertheless may reach appreciable amplitude and can cause or modulate flux variabilities from the system. We discuss applications of our result to the observed quasi-periodic oscillations from various systems, in particular neutron star low-mass X-ray binaries.