We present the results of global three dimensional magneto-hydrodynamic
simulations of black hole accretion flows.
We focus on the excitation mechanism of low and high frequency
QPOs observed during the state transitions.
Machida & Matsumoto (2007) showed that
a constant angular momentum inner torus is formed around $4-8 r_s$
when the gas temperature supplied from the outer region decreases.
This inner torus deforms itself from a circle to a crescent
quasi-periodically.
During this deformation, the mass accretion rate, the magnetic energy
and the Maxwell stress increase.
As the magnetic energy is released,
the inner torus returns to the circular shape
and starts the next cycle.
Time variation of mass accretion rate strongly correlates with
this deformation of the inner torus.
Power spectral density (PSD) of the time variation of the
mass accretion rate has a low frequency peak around $4-8 {\rm Hz}$
when we assumed 10$M_{\odot}$ black hole.
Moreover, high frequency peak around 60Hz appears in PSD.
The amplitude of the high frequency oscillation, however,
is small and its lifetime is short
because the inner torus is swallowed into the central black hole.
In this talk, we would like to present the results of simulations
including radiative cooling, which suppresses the heating
and subsequent infall of the inner torus.
We also show that when the low-frequency oscillation is
continuously excited the high-frequency QPOs are amplified
for a long time. |