Quasi-periodic oscillations, trapped inertial waves and strong toroidal magnetic fields in relativistic accretion discs

Abstract

The excitation of trapped inertial waves (r-modes) by warps and eccentricities in the inner regions of a black hole accretion disc may explain the high-frequency quasi-periodic oscillations (HFQPOs) observed in the emission of Galactic X-ray binaries. However, it has been suggested that strong vertical magnetic fields push the oscillations' trapping region toward the innermost stable circular orbit (ISCO), where conditions could be unfavourable for their excitation. This paper explores the effects of large-scale magnetic fields that exhibit \textit{both} toroidal and vertical components, through local and global linear analyses. We find that a strong toroidal magnetic field can reduce the detrimental effects of a vertical field: in fact, the isolation of the trapping region from the ISCO may be restored by toroidal magnetic fields approaching thermal strengths. The toroidal field couples the r-modes to the disc's magneto-acoustic response and inflates the effective pressure within the oscillations. As a consequence, the restoring force associated with the vertical magnetic field's tension is reduced. Given the analytical and numerical evidence that accretion discs threaded by poloidal magnetic field lines develop a strong toroidal component, our result provides further evidence that the detrimental effects of magnetic fields on trapped inertial modes are not as great as previously thought.

16 pages, 6 figures, MNRAS accepted

References in corpus (10)

• X-ray Properties of Black-Hole Binaries
• Modelling the behaviour of accretion flows in X-ray binaries
• Global Evolution of an Accretion Disk with Net Vertical Field: Coronal Accretion, Flux Transport, and Disk Winds
• On the different flavours of Lense-Thirring precession around accreting stellar mass black holes
• On an excitation mechanism for trapped inertial waves in discs around black holes
• Diskoseismology and QPOs Confront Black Hole Spin
• Weak nonlinear coupling between epicyclic modes in slender tori
• Disk-Wind Connection During the Heartbeats of GRS 1915+105
• Warp and eccentricity propagation in discs around black holes
• Quasi-periodic oscillations and the global modes of relativistic, MHD accretion discs

Cited by in corpus (6)

• HFQPOs and discoseismic mode excitation in eccentric, relativistic discs. II. Magnetohydrodynamic simulations
• HFQPOs and discoseismic mode excitation in eccentric, relativistic discs. I. Hydrodynamic simulations
• Focusing of nonlinear eccentric waves in astrophysical discs
• A Possible Origin of kHZ QPOs in Low-Mass X-ray Binaries
• Focusing of nonlinear eccentric waves in astrophysical discs. II. Excitation and damping of tightly-wound waves
• A Local Model for the Spherical Collapse/Expansion Problem