2:3 Twin Quasi-Periodic Oscillations in Magnetohydrodynamic Accretion Flows

We have studied the radial and vertical oscillations in three-dimensional magnetohydrodynamic (MHD) accretion flows around black holes. General-relativistic effects were taken into account by using a pseudo-Newtonian potential. We found that the structure of MHD flows is changed at 3.8 \,rₛ ≤ r ≤ 6.3 \,rₛ, and that two pairs of quasi-periodic oscillations (twin QPOs) are excited in that region. The time evolution of the power spectrum density (PSD) indicates that these twin QPOs are most likely to be produced by a resonance between the Keplerian frequency, νK, and the epicyclic frequency, νκ. The PSD shows that the lower peak frequency, νₗ, corresponds to, νK, while the upper peak frequency, νᵤ, corresponds to νK + νκ. The ratio of two peak frequencies is close to $2:3$. The results provide the first direct evidence for the excitation of resonant disk oscillation in MHD accretion flows.