YITP WORKSHOP ON
Quasi-Periodic Oscillations and Time Variabilities of Accretion Flows
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ABSTRACT

NAME
INSTITUTE
TITLE OF THE TALK
M. Machida
NAOJ, Japan
High frequency QPOs excited by low-frequency disk oscillation
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.


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