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

The influence of Galactic wind upon the star formation histories of Local Group galaxies

We examine the possibility that ram pressure exerted by the galactic wind from the Galaxy could have stripped gas from the Local Group dwarf galaxies, thereby affecting their star formation histories. Whether gas stripping occurs or not depends on the relative magnitudes of two counteracting forces acting on gas in a dwarf galaxy: ram pressure force by the wind and the gravitational binding force by the dwarf galaxy itself. We suggest that the galactic wind could have stripped gas in a dwarf galaxy located within the distance of $R_{c}\simeq 120(r_{s}/1 {kpc})^{3/2} ({\cal E}_{b}/10^{50} {erg})^{-1/2}$ kpc (where $r_{s}$ is the surface radius and ${\cal E}_{b}$ is the total binding energy of the dwarf galaxy, respectively) from the Galaxy within a timescale of Gyr, thereby preventing star formation there. Our result based on this Galactic wind model explains the recent observation that dwarfs located close to the Galaxy experienced star formation only in the early phase of their lifetimes, whereas distant dwarfs are still undergoing star formation. The present star formation in the Large Magellanic Cloud can also be explained through our Galactic wind model.Comment: 7 pages LaTeX, no figures, to appear in MNRA

A Novel Jet Model: Magnetically Collimated, Radiation-Pressure Driven Jet

Relativistic jets from compact objects are ubiquitous phenomena in the Unvierse, but their driving mechanism has been an enigmatic issue over many decades. Two basic models have been extensively discussed: magnetohydrodynamic (MHD) jets and radiation-hydrodynamic (RHD) jets. Currently, the former is more widely accepted, since magnetic field is expected to provide both the acceleration and collimation mechanisms, whereas radiation field cannot collimate outflow. Here, we propose a new type of jets, radiation-magnetohydrodynamic (RMHD) jets, based on our global RMHD simulation of luminous accretion flow onto a black hole shining above the Eddington luminosity. The RMHD jet can be accelerated up to the relativistic speed by the radiation-pressure force and is collimated by the Lorentz force of a magnetic tower, inflated magnetic structure made by toroidal magnetic field lines accumulated around the black hole, though radiation energy greatly dominates over magnetic energy. This magnetic tower is collimated by a geometrically thick accretion flow supported by radiation-pressure force. This type of jet may explain relativistic jets from Galactic microquasars, appearing at high luminosities.Comment: 5 pages, 2 figures, accepted for publication in PAS

Spectral energy distribution of super-Eddington flows

Spectral properties of super-Eddington accretion flows are investigated by means of a parallel line-of-sight calculation. The subjacent model, taken from two-dimensional radiation hydrodynamic simulations by Ohsuga et al. (2005), consists of a disc accretion region and an extended atmosphere with high velocity outflows. The non-gray radiative transfer equation is solved, including relativistic effects, by applying the FLD approximation. The calculated spectrum is composed of a thermal, blackbody-like emission from the disc which depends sensitively on the inclination angle, and of high energy X-ray and gamma-ray emission from the atmosphere. We find mild beaming effects in the thermal radiation for small inclination angles. If we compare the face-on case with the edge-on case, the average photon energy is larger by a factor of ~1.7 due mainly to Doppler boosting, while the photon number density is larger by a factor of ~3.7 due mainly to anisotropic matter distribution around the central black hole. This gives an explanation for the observed X-ray temperatures of ULXs which are too high to be explained in the framework of intermediate-mass black holes. While the main features of the thermal spectral component are consistent with more detailed calculations of slim accretion discs, the atmosphere induces major changes in the high-energy part, which cannot be reproduced by existing models. In order to interpret observational data properly, simple approaches like the Eddington-Barbier approximation cannot be applied.Comment: 10 pages, 8 figures, accepted for publication in MNRA