磁化黑洞吸积盘中的共转不稳定性、磁共振和整体惯性声振荡
Corotational Instability, Magnetic Resonances and Global Inertial-Acoustic Oscillations in Magnetized Black-Hole Accretion Discs
摘要：
低阶非轴对称p模（也称为惯性声学模）被捕获在黑洞周围流体动力吸积盘的最内部区域中，是在许多吸积黑洞系统中观察到的高频准周期振荡（QPO）的可能候选者。当流体涡度$ζ=κ^2/(2ΩΣ)$（其中$κ$和$Σ$分别是径向周转频率和圆盘表面密度）具有正梯度时，由于在共转共振（其中波型频率$ω/m$等于圆盘旋转速率$Ω$）处的波吸收，这些模式受到全局不稳定性的影响。我们研究了圆盘磁场对共转时的波吸收和相关的共转势垒的波超反射的影响，以及对圆盘p模超稳定性的影响。对于具有纯环向场的盘，共转共振被分成两个磁共振，其中流体的共转坐标系中的波频率$\tomega=ω-mΩ$与慢磁声波频率相匹配。显著的波能量/角动量吸收发生在两个磁共振处，但具有相反的符号。这两种磁共振的组合效应是降低超反射和过稳p模的增长率。我们表明，即使是亚热的环形场可以抑制流体动力学（B=0）p-模式的过稳定性。对于吸积盘混合（环形和垂直）磁场，两个额外的阿尔芬共振出现，其中$\tomega$匹配当地阿尔芬波频率。它们进一步降低了p模的生长速率。我们的研究结果表明，为了使非轴对称的p-模式是一个可行的候选人所观察到的高频QPO，光盘磁场必须明显亚热，或其他模式的激发机制在工作。
英文摘要：
Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic modes) trapped in the inner-most region of hydrodynamic accretion discs around black holes, are plausible candidates for high-frequency quasi-periodic oscillations (QPOs) observed in a number of accreting black-hole systems. These modes are subject to global instabilities due to wave absorption at the corotation resonance (where the wave pattern frequency $ω/m$ equals the disc rotation rate $Ω$), when the fluid vortensity, $ζ=κ^2/(2ΩΣ)$ (where $κ$ and $Σ$ are the radial epicyclic frequency and disc surface density, respectively), has a positive gradient. We investigate the effects of disc magnetic fields on the wave absorption at corotation and the related wave super-reflection of the corotation barrier, and on the overstability of disc p-modes. For discs with a pure toroidal field, the corotation resonance is split into two magnetic resonances, where the wave frequency in the corotating frame of the fluid, $\tomega=ω-mΩ$, matches the slow magnetosonic wave frequency. Significant wave energy/angular momentum absorption occurs at both magnetic resonances, but with opposite signs. The combined effect of the two magnetic resonances is to reduce the super-reflection and the growth rate of the overstable p-modes. We show that even a subthermal toroidal field may suppress the overstability of hydrodynamic (B=0) p-modes. For accretion discs with mixed (toroidal and vertical) magnetic fields, two additional Alfven resonances appear, where $\tomega$ matches the local Alfven wave frequency. They further reduce the growth rate of p-modes. Our results suggest that in order for the non-axisymmetric p-modes to be a viable candidate for the observed high-frequency QPOs, the disc magnetic field must be appreciably subthermal, or other mode excitation mechanisms are at work.
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