Radiative Spectra from Disk Corona and Inner Hot Flow in Black Hole X-ray Binaries

To understand the origin of hard X-ray emissions from black hole X-ray binaries during their low/hard states, we calculate the X-ray spectra of black-hole accretion flow for the following three configurations of hot and cool media: (a) an inner hot flow and a cool outer disk (inner hot flow model), (b) a cool disk sandwiched by disk coronae (disk corona model), and (c) the combination of those two (hybrid model). The basic features we require for successful models are (i) significant hard X-ray emission whose luminosity exceeds that of soft X-rays, (ii) high hard X-ray luminosities in the range of (0.4 - 30) times 10^{37} erg s^{-1}, and (iii) the existence of two power-law components in the hard X-ray band with the photon indices of Gamma_s ~ 2 > Gamma_h, where Gamma_s and Gamma_h are the photon indices of the softer (<10 keV) and the harder (>10 keV) power-law components, respectively. Contribution by non-thermal electrons nor time-dependent evolution are not considered. We find that Models (a) and (b) can be ruled out, since the spectra are always dominated by the soft component, and since only one power-law component, at most, can be reproduced. Only Model (c) can account for sufficiently strong hard X-ray emissions, as well as the existence of the two power-law components, for a large ratio of the accretion rate in the corona to that in the thin disk. The outer disk corona (where the Compton y-parameter is smaller, y < 1) produces the softer power-law component with photon index of Gamma_s ~ 2, whereas the inner hot flow (where y gtrsim 1) generates the harder component with Gamma_h < 2. This model can also account for the observed relationship between the photon index and the reflection fraction.Comment: 13 pages, 13 figures, accepted for publication in PAS

Where is a Marginally Stable Last Circular Orbit in Super-Critical Accretion Flow?

Impressed by the widespread misunderstanding of the issue, we return to the old question of the location of the inner edge of accretion disk around black hole. We recall the fundamental results obtained in the 1970's and 1980's by Warsaw and Kyoto research groups that proved, in particular, that the inner edge does not coincide with the location of the innermost stable Keplerian circular orbit. We give some novel illustrations of this particular point and of some other fundamental results obtained by Warsaw and Kyoto groups. To investigate the flow dynamics of the inner edge of accretion disk, we carefully solve the structure of the transonic flow and plot the effective potential profile based on the angular-momentum distribution calculated numerically. We show that the flow does not have a potential minimum for accretion rates, {\dot M} > 10 L_E/c^2 (with L_E being the Eddington luminosity and $c$ being the speed of light). This property is realized even in relatively small viscosity parameters (i.e., \alpha ~ 0.01), because of the effect of pressure gradient. In conclusion, the argument based on the last circular orbit of a test particle cannot give a correct inner boundary of the super-critical flow and the inner edge should be determined in connection with radiation efficiency. The same argument can apply to optically thin ADAF. The interpretation of the observed QPO frequencies should be re-considered, since the assumption of Kepler rotation velocity can grossly over- or underestimate the disk rotation velocity, depending on the magnitude of viscosity.Comment: 7 pages, 3 figures, accepted for PAS

Evolution of Hydromagnetic Disturbances in Low Ionized Cosmic Plasmas

We consider the propagation of hydromagnetic waves generated by a compact turbulent source in low ionized plasmas, applying the Lighthill theory. We assume the plasma to be isothermal, and adopt a uniform, stationary medium thread by ordered magnetic fields as an initial condition. Then, the distinct properties of the hydromagnetic waves originating from a source oscillating with a fixed frequency are studied in the linear regime. As is well known, in low ionized plasmas, the generated waves dissipate due to ion-neutral damping. In this paper, the dependence of the dissipation rate on the frequency of the oscillating source is investigated. The larger the frequency becomes, the more substantial is the wave dissipation. Implications of our results on the energy source in molecular clouds are also discussed. Interestingly, since the outflow lobes associated with young stellar objects act as compact turbulent sources, hydromagnetic waves are generated by them. From our order-estimations, about 70% of the energy of the outflow itself propagates as waves or turbulences, while the remaining 30% dissipates and heats the neutrals via ion-neutral damping. Then, we confirm that the outflows are significant energy sources in molecular clouds in the context of the Lighthill theory.Comment: 17 pages LaTeX, 3 PostScript figures, accepted, PASJ (Vol. 51, No. 3, pp. 337 - 344, 1999

Binary Black Hole Accretion Flows in Merged Galactic Nuclei

We study the accretion flows from the circumbinary disks onto the supermassive binary black holes in a subparsec scale of the galactic center, using a smoothed particles hydrodynamics (SPH) code. Simulation models are presented in four cases of a circular binary with equal and unequal masses, and of an eccentric binary with equal and unequal masses. We find that the circumblack-hole disks are formed around each black holes regardless of simulation parameters. There are two-step mechanisms to cause an accretion flow from the circumbinary disk onto supermassive binary black holes: First, the tidally induced elongation of the circumbinary disk triggers mass inflow towards two closest points on the circumbinary disk from the black holes. Then, the gas is increasingly accumulated on these two points owing to the gravitational attraction of black holes. Second, when the gas can pass across the maximum loci of the effective binary potential, it starts to overflow via their two points and freely infalls to each black hole. In circular binaries, the gas continues to be supplied from the circumbinary disk (i.e. the gap between the circumbinary disk and the binary black hole is always closed.) In eccentric binaries, the mass supply undergoes the periodic on/off transitions during one orbital period because of the variation of periodic potential. The gap starts to close after the apastron and to open again after the next periastron passage. Due to this gap closing/opening cycles, the mass-capture rates are eventually strongly phase dependent. This could provide observable diagnosis for the presence of supermassive binary black holes in merged galactic nuclei.Comment: 16 pages, 27 figures, 2 tables, accepted for publication in PASJ. "High Resolution Version is Available at "http://www2.yukawa.kyoto-u.ac.jp/~kimitake/bbhs.html" Three observational references are added. Grammatical errors and typos are correcte

Modified Slim-Disk Model Based on Radiation-Hydrodynamic Simulation Data: The Conflict Between Outflow and Photon Trapping

Photon trapping and outflow are two key physics associated with the supercritical accretion flow. We investigate the conflict between these two processes based on two-dimensional radiation-hydrodynamic (RHD) simulation data and construct a simplified (radially) one-dimensional model. Mass loss due to outflow, which is not considered in the slim-disk model, will reduce surface density of the flow, and if very significant, it will totally suppress photon trapping effects. If the photon trapping is very significant, conversely, outflow will be suppressed because radiation pressure force will be reduced. To see what actually occurs, we examine the RHD simulation data and evaluate the accretion rate and outflow rate as functions of radius. We find that the former monotonically decreases, while the latter increases, as the radius decreases. However, the former is kept constant at small radii, inside several Schwarzschild radii, since the outflow is suppressed by the photon trapping effects. To understand the conflict between the photon trapping and outflow in a simpler way, we model the radial distribution of the accretion rate from the simulation data and build up a new (radially) one-dimensional model, which is similar to the slim-disk model but incorporates the mass loss effects due to the outflow. We find that the surface density (and, hence, the optical depth) is much reduced even inside the trapping radius, compared with the case without outflow, whereas the effective temperature distribution hardly changes. That is, the emergent spectra do not sensitively depend on the amount of mass outflow. We conclude that the slim-disk approach is valid for interpreting observations, even if the outflow is taken into account.Comment: 15 pages, 5 figures, accepted for publication in PAS