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 numerical results on the gas temperature Tout supplied from the outer region. General relativistic effects are taken into account using the pseudo-Newtonian potential. We ignore the radiative cooling of the accreting gas. The initial state is a torus whose density maximum is at 35rs or 50rs from the gravitating center, where rs is the Schwarzschild radius. The torus is initially threaded by a weak azimuthal magnetic field. We found that 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 magnetorotational instability (MRI) to gas pressure, α ≡ 〈B̟Bφ/4π〉/〈P 〉 is α ∼ 0.05 in the hot torus (Tout ∼ 9.5×10 K at 50rs) and α∼ 0.01 in the cool torus (Tout ∼ 1.1×10 K at 35rs). In the cool model, a constant angular momentum inner torus is formed around 4− 8rs. This inner torus deforms itself from a circle to a crescent quasiperiodically. 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. Power spectral density (PSD) of the time variation of the mass accretion rate in the cool model has a low frequency peak around 10Hz when we assumed a 10M⊙ black hole. The PSD of the hot model is flat in 1− 30Hz. The slope of the PSD in the cool model is steeper than that in the hot model in 30−100Hz. The mass outflow rate in the low temperature model also shows quasi-periodic oscillation. Intermittent