Strongly magnetized accretion discs: structure and accretion from global magnetohydrodynamic simulations
Strongly magnetized accretion discs: structure and accretion from global magnetohydrodynamic simulations
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DOI：
10.1093/mnras/stz3572
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发表时间：
2019-07
影响因子：
4.8
通讯作者：
B. Mishra;M. Begelman;P. Armitage;J. Simon
中科院分区：
文献类型：
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作者：
B. Mishra;M. Begelman;P. Armitage;J. Simon
我们利用全球磁流体力学模拟研究了在牛顿极限下垂直磁场对几何薄（H/r≈0.05）吸积盘结构的影响。我们考虑初始的中间面气磁压力比$\beta _0 = 1000,\, 300$和100，跨越弱和强磁化吸积体制之间的过渡。我们发现磁压力对圆盘的垂直结构都很重要，在两种磁化最强的模型中，吸积发生在$z$/R≈0.2处。圆盘的中间平面显示的是流出而不是吸积。通过表层的吸积主要是由相干大尺度磁场产生的应力驱动，而不是由湍流应力驱动。从我们的模拟中测量的等效粘度参数显示出与剪切箱模拟相似的初始β0依赖性，尽管即使在最强磁场的情况下，圆盘中间平面也不是磁压主导。风是存在的，但不是圆盘演化的主要驱动力。在我们的模拟（有限）持续时间内，我们发现证据表明，净通量在能够稳定地维持强磁化盘的水平上达到准稳定状态。我们认为，在观测到的系统中，几何上薄的吸积盘可能通常存在于磁性“升高”状态，其特征是非零但适度的垂直磁通量，这对x射线双星和活动星系核的盘现象学具有潜在的重要意义。
We use global magnetohydrodynamic simulations to study the influence of net vertical magnetic fields on the structure of geometrically thin (H/r ≈ 0.05) accretion discs in the Newtonian limit. We consider initial mid-plane gas to magnetic pressure ratios $\beta _0 = 1000,\, 300$, and 100, spanning the transition between weakly and strongly magnetized accretion regimes. We find that magnetic pressure is important for the discs’ vertical structure in all three cases, with accretion occurring at $z$/R ≈ 0.2 in the two most strongly magnetized models. The disc mid-plane shows outflow rather than accretion. Accretion through the surface layers is driven mainly by stress due to coherent large-scale magnetic field rather than by turbulent stress. Equivalent viscosity parameters measured from our simulations show similar dependencies on initial β0 to those seen in shearing box simulations, though the disc mid-plane is not magnetic pressure dominated even for the strongest magnetic field case. Winds are present but are not the dominant driver of disc evolution. Over the (limited) duration of our simulations, we find evidence that the net flux attains a quasi-steady state at levels that can stably maintain a strongly magnetized disc. We suggest that geometrically thin accretion discs in observed systems may commonly exist in a magnetically ‘elevated’ state, characterized by non-zero but modest vertical magnetic fluxes, with potentially important implications for disc phenomenology in X-ray binaries and active galactic nuclei.