Corotational Instability, Magnetic Resonances and Global Inertial-Acoustic Oscillations in Magnetized Black-Hole Accretion Discs

Understand

Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic modes) trapped in the inner-most region of hydrodynamic accretion discs around black holes, are plausible candidates for high-frequency quasi-periodic oscillations (QPOs) observed in a number of accreting black-hole systems.

• These modes are subject to global instabilities due to wave absorption at the corotation resonance (where the wave pattern frequency $\omega/m$ equals the disc rotation rate $\Omega$), when the fluid vortensity, $\zeta=\kappa^2/(2\Omega\Sigma)$ (where $\kappa$ and $\Sigma$ are the radial epicyclic frequency and disc surface density, respectively), has a positive gradient.
• We investigate the effects of disc magnetic fields on the wave absorption at corotation and the related wave super-reflection of the corotation barrier, and on the overstability of disc p-modes.
• For discs with a pure toroidal field, the corotation resonance is split into two magnetic resonances, where the wave frequency in the corotating frame of the fluid, $\tomega=\omega-m\Omega$, matches the slow magnetosonic wave frequency.