Why Is Supercritical Disk Accretion Feasible?

Although the occurrence of steady supercritical disk accretion onto a black hole has been speculated about since the 1970s, it has not been accurately verified so far. For the first time, we previously demonstrated it through two-dimensional, long-term radiation-hydrodynamic simulations. To clarify why this accretion is possible, we quantitatively investigate the dynamics of a simulated supercritical accretion flow with a mass accretion rate of ∼ 10LE/c 2 (with LE and c being, respectively, the Eddington luminosity and the speed of light). We confirm two important mechanisms underlying supercritical disk accretion flow, as previously claimed, one of which is the radiation anisotropy arising from the anisotropic density distribution of very optically thick material. We qualitatively show that despite a very large radiation energy density, E0 > ∼ 10LE/4πr c (with r being the distance from the black hole), the radiative flux F0 ∼ cE0/τ could be small due to a large optical depth, typically τ ∼ 10, in the disk. Another mechanism is photon trapping, quantified by vE0, where v is the flow velocity. With a large |v| and E0, this term significantly reduces the radiative flux and even makes it negative (inward) at r < 70rS , where rS is the Schwarzschild radius. Due to the combination of these effects, the radiative force in the direction along the disk plane is largely attenuated so that the gravitational force barely exceeds the sum of the radiative force and the centrifugal force. As a result, matter can slowly fall onto the central black hole mainly along the disk plane with velocity much less than the free-fall velocity, even though the disk luminosity exceeds the Eddington luminosity. Along the disk rotation axis, in contrast, the strong radiative force drives strong gas outflows. Subject headings: accretion: accretion disks — black hole physics — hydrodynamics — radiative transfer