为什么超临界磁盘增长是可行的？
Why Is Supercritical Disk Accretion Feasible?
摘要：
虽然自20世纪70年代以来，人们一直在猜测黑洞上发生稳定的超临界盘吸积，但迄今为止还没有得到准确的证实。这是第一次，我们以前通过二维，长期的辐射流体动力学模拟证明了这一点。为了阐明为什么这种吸积是可能的，我们定量地研究了一个模拟的超临界吸积流的动力学，其质量吸积率为~10^2 L_E/c^2（L_E和c分别是爱丁顿光度和光速）。我们确认两个重要的机制超临界磁盘吸积流，如前所述，其中之一是辐射各向异性所产生的各向异性的密度分布非常光学厚的材料。我们定性地表明，尽管辐射能量密度很大，E_0> 10^2 L_E/（4 π r^2 c）（其中r是到黑洞的距离），但由于圆盘中的光学深度很大，通常τ为10^3，辐射通量F_0 cE_0/τ可能很小。另一种机制是光子捕获，用vE_0量化，其中v是流速。当|v|和E_0较大时，这一项显著地降低了辐射通量，甚至在r<70 r_S（其中r_S是史瓦西半径）时使辐射通量为负（向内）。由于这些效应的组合，沿圆盘平面方向的辐射力大大衰减，使得重力几乎不超过辐射力和离心力的总和。因此，物质可以主要沿着盘平面以远小于自由落体速度的速度缓慢地落在中心黑洞上，即使盘的光度超过爱丁顿光度。相反，沿圆盘旋转轴，强辐射力驱动强气体流出。
英文摘要：
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 ~10^2 L_E/c^2 (with L_E 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, E_0>10^2L_E/(4 pi r^2 c) (with r being the distance from the black hole), the radiative flux F_0 cE_0/tau could be small due to a large optical depth, typically tau 10^3, in the disk. Another mechanism is photon trapping, quantified by vE_0, where v is the flow velocity. With a large |v| and E_0, this term significantly reduces the radiative flux and even makes it negative (inward) at r<70r_S, where r_S 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.
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