We discuss distinctive features of luminous accretion disks shining at the Eddington luminosity in the context of galactic black-hole candidates (GBCs). We first note that the standard-disk picture is not applicable, although it is often postulated. Rather, the disk becomes advection-dominated while remaining optically thick (the so-called slim disk). The slim disk exhibits several noteworthy signatures: (1) The disk luminosity is insensitive to the mass-flow rates, Ṁ, and is always kept around the Eddington luminosity, LE, even if Ṁ greatly exceeds LE/c2. This reflects the fact that radiative cooling is no longer balanced by viscous heating and excess energy is carried by accreting matter to black holes. (2) The spectra of the slim disks are multi-color blackbody characterized by (i) a high maximum temperature, kTin ∼ a few keV, (ii) a small size of an emitting region, rin < 3 rg (with rg being Schwarzschild radius), due to substantial radiation coming out from inside 3 rg, and (iii) flatter spectra in the soft-X bands, vSv ∼ v0, because of a flatter effective temperature profile of the slim disk, Teff ∝ r−1/2 (in contrast with Teff ∝ r−3/4 in the standard disk). Thus, a small rin (≪ 3rg) does not necessarily mean the presence of a Kerr hole. Furthermore, (3) as Ṁ increases, Tin increases, while rin decreases as rin ∝ (Tin)−1 approximately. That is, the changes in rin derived from the fitting do not necessarily mean the changes in the physical boundary of the optically thick portions of the disk. Observational implications are discussed in relation to binary jet sources.

For a supercritical accretion regime, we propose a critical accretion disk, where the mass-accretion rate is regulated just at the critical rate with the help of wind mass-loss. We first derive a critical radius, inside of which the standard picture is violated, using the condition that the radiative force is balanced by the gravity in the vertical direction. The critical radius $r_\mathrm{cr}$ is found to be $r_\mathrm{cr} = (9\sqrt{3}\sigma_\mathrm{T} / 16\pi c m_\mathrm{p}) \dot{M}_\mathrm{input} = 1.95 (\dot{M}_\mathrm{input} / \dot{M}_\mathrm{crit}) r_\mathrm{g}$, where $\dot{M}_\mathrm{input}$ is the mass-accretion rate at the outer edge of the disk, $\dot{M}_\mathrm{crit}$ the critical accretion rate, and $r_\mathrm{g}$ the Schwarzschild radius of the central object. Outside of this critical radius, the disk is in a radiation-pressure dominated standard state, while inside this radius the disk is in a critical state, where the excess mass is expelled by wind and the accretion rate is kept to be just at the critical rate at any radius inside $r_\mathrm{cr}$. In such a critical accretion disk, the disk thickness is $H \sim (1/6\sqrt{3})r$ and the surface temperature is $\sigma T^4 \sim (2/3\sqrt{3}) L_\mathrm{E} / 4\pi r^2$, where $L_\mathrm{E}$ is the Eddington luminosity. The total disk luminosity becomes $L_\mathrm{disk} \sim (2/3\sqrt{3}) [\ln (r_\mathrm{cr}/r_\mathrm{in})+1] L_\mathrm{E}$, where $r_\mathrm{in}$ is the inner radius. We apply the present model to microquasars and narrow-line Seyfert 1 galaxies, which are supposed to be under supercritical accretion.

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We investigated the radiation fields of a self-similar slim disk and the behavior of wind particles, which are driven by the radiation pressure of a self-similar slim disk. When the accretion rate is of the order of a critical rate, the accretion disk must puff up in the vertical direction to form a so-called slim disk. In contrast to a standard alpha disk, this slim disk has two major features: i) the disk is geometrically (mildly) thick, and ii) the radial motion is comparable to the rotational motion (advection). These effects make the opening angle of the disk less than 180°, and the disk radiation fields are expected to enhance towards the center. However, we found that trajectories of wind particles are accelerated along the disk surface. This indicates that the shape of the disk strongly influence the motion of plasma particles. Furthermore, particles lose angular momentum by radiation drag, while gaining angular momentum from rotating radiation fields. Taking into consideration the Compton drag, the income and expenditure of angular momentum of wind particles is positive, and they tend to spread out in a radial direction.