Excitation of low-frequency QPOs in black-hole accretion flows

We present the results of global three-dimensional magneto-hydrodynamic simulations of black-hole accretion flows. We focus on the dependence of the numerical results on the gas temperature supplied from the outer region. General-relativistic effects were taken into account using the pseudo-Newtonian potential. We ignored radiative cooling of the accreting gas. The initial state was a torus threaded by a weak azimuthal magnetic field. We found that the mass-accretion rate and the mass-outflow rate strongly depend on the temperature of the initial torus. The ratio of the average Maxwell stress generated by the magneto-rotational instability (MRI) to the gas pressure, alpha equivalent to < B omega B phi/4 pi >/< P >, is alpha similar to 0.05 in a hot torus and alpha similar to 0.01 in a cool torus. In the cool model, a constant angular momentum inner torus is formed around 4-8 r(s), where r(s) is the Schwarzschild radius. 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 a circular shape and starts the next cycle. The power spectral density (PSD) of the time variation of the mass-accretion rate in the cool model has a low-frequency peak around 10 Hz when we assumed a 10M(circle dot) black hole. The mass outflow rate in the low temperature model also shows quasi-periodic oscillation.