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arXiv 1003.0966astro-ph.HEastro-ph.GA
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黑洞周围磁化盘的模拟:黑洞自旋、盘厚度和磁场几何的影响

Simulations of Magnetized Disks Around Black Holes: Effects of Black Hole Spin, Disk Thickness, and Magnetic Field Geometry

Robert F. Penna, Jonathan C. McKinney, Ramesh Narayan, Alexander Tchekhovskoy, Rebecca Shafee, Jeffrey E. McClintock

摘要:

由Novikov和Thorne(NT)于1973年提出的黑洞周围极薄吸积盘的标准广义相对论模型假设剪切应力在最内层稳定圆轨道(ISCO)半径处消失,而在ISCO之外,剪切应力由有效湍流粘性产生。然而,天体物理学的吸积盘并不是非常薄,在ISCO处剪切应力是否必然消失还不确定,而被认为驱动盘中湍流的磁场可能包含大尺度结构,其行为不像简单的局部标量粘度。我们描述了三维广义相对论磁流体动力学模拟的吸积盘周围的黑洞的自旋参数的范围内,我们使用的模拟来评估NT模型的有效性。我们的基准初始磁场由多个(交替极性)极向场回路,其形状是大致各向同性的磁盘,以匹配各向同性的湍流预期在极向平面。对于长宽比为|H/R| ~ 0.07,我们发现吸积比角动量相对于NT模型减少了2.9%,ISCO内部的过剩光度为3.5%。在自旋黑洞的情况下,偏差也是同样的量级。此外,偏差随着减小而减小|H/R|.因此我们得出结论,在热/高软谱态下,x射线双星中磁化薄吸积盘应该可以用NT模型很好地描述,特别是当光度低于30%爱丁顿时,我们预计吸积盘的厚度很小|H/R| <~ 0.05。我们还讨论了应力和亮度内的ISCO依赖于假设的初始磁场几何形状。(节略)

英文摘要:

The standard general relativistic model of a razor-thin accretion disk around a black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at the radius of the innermost stable circular orbit (ISCO) and that, outside the ISCO, the shear stress is produced by an effective turbulent viscosity. However, astrophysical accretion disks are not razor-thin, it is uncertain whether the shear stress necessarily vanishes at the ISCO, and the magnetic field, which is thought to drive turbulence in disks, may contain large-scale structures that do not behave like a simple local scalar viscosity. We describe three-dimensional general relativistic magnetohydrodynamic simulations of accretion disks around black holes with a range of spin parameters, and we use the simulations to assess the validity of the NT model. Our fiducial initial magnetic field consists of multiple (alternating polarity) poloidal field loops whose shape is roughly isotropic in the disk in order to match the isotropic turbulence expected in the poloidal plane. For a thin disk with an aspect ratio |h/r| ~ 0.07 around a non-spinning black hole, we find a decrease in the accreted specific angular momentum of 2.9% relative to the NT model and an excess luminosity from inside the ISCO of 3.5%. The deviations in the case of spinning black holes are of the same order. In addition, the deviations decrease with decreasing |h/r|. We therefore conclude that magnetized thin accretion disks in x-ray binaries in the thermal/high-soft spectral state ought to be well-described by the NT model, especially at luminosities below 30% of Eddington where we expect a very small disk thickness |h/r| <~ 0.05. We also discuss how the stress and the luminosity inside the ISCO depend on the assumed initial magnetic field geometry. (abridged)

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