Formation of Magnetically Supported Disks during Hard-to-Soft Transitions in Black Hole Accretion Flows
Formation of Magnetically Supported Disks during Hard-to-Soft Transitions in Black Hole Accretion Flows
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DOI：
10.1093/pasj/58.1.193
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发表时间：
2005-11
影响因子：
2.3
通讯作者：
M. Machida;K. Nakamura;R. Matsumoto
中科院分区：
文献类型：
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作者：
M. Machida;K. Nakamura;R. Matsumoto
我们通过假设轫致辐射冷却，对光学薄热黑洞吸积流中的冷却不稳定性进行了三维全局电阻磁流体动力学模拟。使用伪牛顿势来模拟广义相对论效应。当吸积盘的密度变得足够大时，冷却不稳定性就会增加。我们发现，随着不稳定性的增加，吸积流从光学上薄的、热的、气压支持的状态（低/硬状态）变为冷却的、磁支持的准稳态。在此转变期间，磁压力超过气体压力，因为圆盘在垂直方向收缩，同时几乎保持环形磁通量。由于磁盘的进一步垂直收缩被磁压力抑制，因此冷磁盘保持光学薄、光谱硬的状态。磁支撑盘存在的时间尺度比热时间尺度长得多，并且与吸积时间尺度相当。我们假设环形磁通量守恒，通过分析方法检查了磁性支撑磁盘的稳定性，并发现它具有热稳定性和长期稳定性。我们的发现可以解释为什么候选黑洞即使在其光度超过冷却不稳定开始的阈值时仍保持发光的硬状态。
We carried out three-dimensional global resistive magnetohydrodynamic simulations of the cooling instability in optically thin hot black hole accretion flows by assuming bremsstrahlung cooling. General relativistic effects are simulated using the pseudo-Newtonian potential. The cooling instability grows when the density of the accretion disk becomes sufficiently large. We found that as the instability grows the accretion flow changes from an optically thin, hot, gas pressure-supportedstate (low/hard state) to acooler, magnetically supported, quasi-steady state. During this transition, the magnetic pressure exceeds the gas pressure because the disk shrinks in the vertical direction while almost conserving the toroidal magnetic flux. Since further vertical contraction of the disk is suppressed by magnetic pressure, the cool disk stays in an optically thin, spectrally hard state. The magnetically supported disk exists for a time scale much longer than the thermal time scale, and comparable to the accretion time scale. We examined the stability of the magnetically supported disk analytically, assuming that the toroidal magnetic flux is conserved, and found it to be thermally and secularly stable. Our findings may explain why black hole candidates stay in luminous, hard state even when their luminosity exceeds the threshold for the onset of the cooling instability.