Quasi-periodic X-ray brightness oscillations (QPOs) have been observed in many
X-ray binaries including neutron star (NS) and black hole (BH) candidates. After
the discoveries of twin peak QPOs in both NS and BH systems, many interpretations
of QPOs in the context of disk oscillations have been developed. Within the
framework of resonant disk oscillation models, frequencies of QPOs are
regulated by the mass and the spin of the central object. This is why QPOs
are believed to be a powerful tool to investigate a nature of accretion disk in
the vicinity of black holes and neutron stars. In spite of many
three-dimensional (3-D) magnetohydrodynamic (MHD) simulations on
accretion flows plunging into black holes having been performed by many
groups, MHD studies on disk oscillations have been poorly understood.
In my talk, we review our findings on QPOs in 3-D MHD simulations of
accretion disks and discuss the origin of such oscillations. We also introduce
on our recent findings of variable emissions in MHD accretion disks. |