Excitation of Low-Frequency QPOs in Black-Hole Accretion Flows

Abstract

Abstract 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 \equiv \langle B_{\varpi} B_{\varphi}/4 \pi \rangle / \langle P \rangle $, is $\alpha \sim$0.05 in a hot torus and $\alpha \sim$0.01 in a cool torus. In the cool model, a constant angular momentum inner torus is formed around 4-8$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 10Hz when we assumed a 10$M_{\odot}$ black hole. The mass outflow rate in the low temperature model also shows quasi-periodic oscillation.