RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF THE FORMATION OF HOT ACCRETION DISK CORONAE
RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF THE FORMATION OF HOT ACCRETION DISK CORONAE
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DOI：
10.1088/0004-637x/784/2/169
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发表时间：
2014-02
期刊：
影响因子：
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通讯作者：
Y. Jiang 姜;J. Stone;S. Davis
中科院分区：
文献类型：
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作者：
Y. Jiang 姜;J. Stone;S. Davis
利用三维辐射磁流体动力学（MHD）模拟方法，探讨了在吸积盘光球层上方形成磁压支撑的高温日冕的新机制。通过平衡磁盘表面的辐射冷却与由于磁旋转不稳定性（MRI）驱动的湍流耗散而导致的加热，自洽地计算磁盘的热特性。正如在以前的工作中所指出的，我们发现单位质量的耗散率随着中平面以上的高度而急剧增加，这与α盘模型形成鲜明对比，α盘模型假设这个量是常数。因此，我们发现在低表面密度（因此是浅光球层）的模拟中，光球层上方耗散的能量比例是显著的（在我们的最低表面密度模型中约为3.4%），并且这一比例随着表面密度的降低而增加。当吸积能量的一个显着的分数是在光学薄光球耗散，气体温度大幅增加，形成高温，磁压支持的日冕。在磁盘电晕的体积平均温度是10倍以上的磁盘中面。此外，日冕中的气体温度与气体密度呈强反相关，这意味着MRI湍流形成的日冕是斑块状的。这种吸积盘日冕的形成机制有助于解释活动星系核的光谱指数与光度之间的关系以及某些活动星系核的软X射线过剩现象。它也可能与X射线双星的光谱状态变化有关。
A new mechanism to form a magnetic pressure supported, high temperature corona above the photosphere of an accretion disk is explored using three dimensional radiation magnetohydrodynamic (MHD) simulations. The thermal properties of the disk are calculated self-consistently by balancing radiative cooling through the surfaces of the disk with heating due to dissipation of turbulence driven by magneto-rotational instability (MRI). As has been noted in previous work, we find the dissipation rate per unit mass increases dramatically with height above the mid-plane, in stark contrast to the α-disk model which assumes this quantity is a constant. Thus, we find that in simulations with a low surface density (and therefore a shallow photosphere), the fraction of energy dissipated above the photosphere is significant (about 3.4% in our lowest surface density model), and this fraction increases as surface density decreases. When a significant fraction of the accretion energy is dissipated in the optically thin photosphere, the gas temperature increases substantially and a high temperature, magnetic pressure supported corona is formed. The volume-averaged temperature in the disk corona is more than 10 times larger than at the disk mid-plane. Moreover, gas temperature in the corona is strongly anti-correlated with gas density, which implies the corona formed by MRI turbulence is patchy. This mechanism to form an accretion disk corona may help explain the observed relation between the spectral index and luminosity from active galactic nucleus (AGNs), and the soft X-ray excess from some AGNs. It may also be relevant to spectral state changes in X-ray binaries.