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 magneto-rotational instability (MRI) to gas pressure is alpha ~0.05 in the hot torus and alpha ~ 0.01 in the cool torus. 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 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. 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 solar mass 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 outflows are created in both models. The outflow speed is $0.01c - 0.05c$.Comment: 22 pages, 17 figures, accepted for publication in PASJ (PASJ,60,pp.613-626). Replaced to high resolution versio

Time Evolution of Relativistic Force-Free Fields Connecting a Neutron Star and its Disk

We study the magnetic interaction between a neutron star and its disk by solving the time-dependent relativistic force-free equations. At the initial state, we assume that the dipole magnetic field of the neutron star connects the neutron star and its equatorial disk, which deeply enters into the magnetosphere of the neutron star. Magnetic fields are assumed to be frozen to the star and the disk. The rotation of the neutron star and the disk is imposed as boundary conditions. We apply Harten-Lax-van Leer (HLL) method to simulate the evolution of the star-disk system. We carry out simulations for (1) a disk inside the corotation radius, in which the disk rotates faster than the star, and (2) a disk outside the corotation radius, in which the neutron star rotates faster than the disk. Numerical results indicate that for both models, the magnetic field lines connecting the disk and the star inflate as they are twisted by the differential rotation between the disk and the star. When the twist angle exceeds pi radian, the magnetic field lines expand with speed close to the light speed. This mechanism can be the origin of relativistic outflows observed in binaries containing a neutron star.Comment: 10 pages, 6figures, accepted for publication in PAS

Global Structure of Optically Thin, Magnetically Supported, Two-Temperature, Black Hole Accretion Disks

We present global solutions of optically thin, two-temperature black hole accretion disks incorporating magnetic fields. We assume that the {\pi}{\phi}-component of the Maxwell stress is proportional to the total pressure, and prescribe the radial dependence of the magnetic flux advection rate in order to complete the set of basic equations. We obtained magnetically supported (low-{\beta}) disk solutions, whose luminosity exceeds the maximum luminosity for an advection-dominated accretion flow (ADAF), L > 0.4 {\alpha}^2 L_Edd, where L_Edd is the Eddington luminosity. The accretion flow is composed of the outer ADAF, a luminous hot accretion flow (LHAF) inside the transition layer from the outer ADAF to the low-{\beta} disk, the low-{\beta} disk, and the inner ADAF. The low-{\beta} disk region becomes wider as the mass-accretion rate increases further. In the low-{\beta} disk, the magnetic heating balances the radiative cooling, and the electron temperature decreases from ~ 10^9.5 K to ~ 10^8 K as the luminosity increases. These results are consistent with the anti-correlation between the energy cutoff in X-ray spectra (hence the electron temperature) and the luminosity when L > 0.1 L_Edd, observed in the bright/hard state during the bright hard-to-soft transitions of transient outbursts in galactic black hole candidates.Comment: 27 pages, 15 figures, accepted for Publications of Astronomical Society of Japa

Recurrent Outbursts and Jet Ejections Expected in Swift J1644+57: Limit-Cycle Activities in a Supermassive Black Hole

The tidal disruption event by a supermassive black hole in Swift J1644+57 can trigger limit-cycle oscillations between a supercritically accreting X-ray bright state and a subcritically accreting X-ray dim state. Time evolution of the debris gas around a black hole with mass M=10^{6} {\MO} is studied by performing axisymmetric, two-dimensional radiation hydrodynamic simulations. We assumed the $\alpha$-prescription of viscosity, in which the viscous stress is proportional to the total pressure. The mass supply rate from the outer boundary is assumed to be ${\dot M}_{\rm supply}=100L_{\rm Edd}/c^2$, where $L_{\rm Edd}$ is the Eddington luminosity, and $c$ is the light speed. Since the mass accretion rate decreases inward by outflows driven by radiation pressure, the state transition from a supercritically accreting slim disk state to a subcritically accreting Shakura-Sunyaev disk starts from the inner disk and propagates outward in a timescale of a day. The sudden drop of the X-ray flux observed in Swift J1644+57 in August 2012 can be explained by this transition. As long as ${\dot M}_{\rm supply}$ exceeds the threshold for the existence of a radiation pressure dominant disk, accumulation of the accreting gas in the subcritically accreting region triggers the transition from a gas pressure dominant Shakura-Sunyaev disk to a slim disk. This transition takes place at $t {\sim}~50/({\alpha}/0.1)$ days after the X-ray darkening. We expect that if $\alpha > 0.01$, X-ray emission with luminosity $\gtrsim 10^{44}$ ${\rm erg}{\cdot}{\rm s}^{-1}$ and jet ejection will revive in Swift J1644+57 in 2013--2014.Comment: 6 pages, 4 figures, accepted for publication in PASJ Letter