长期不稳定性如何在超吸积流中增长？
How does a Secular Instability Grow in a Hyperaccretion Flow?
摘要：
质量吸积速率远高于爱丁顿速率的超吸积流在$Σ$ - $\dot{M}$平面上具有N形平衡曲线（$Σ$和$\dot{M}$分别是表面密度和质量吸积速率）。N形下支$Σ$上的吸积流为光学厚的平流主导吸积流（ADAF），上支上的吸积流为中微子主导吸积流（NHAF）。中间分支在$Σ$ - $\dot{M}$平面上具有负斜率，这意味着该分支上的流动长期不稳定。为了研究不稳定性如何影响流动结构和产生什么可观察的签名，我们研究了不稳定的超吸积流响应于可变质量注入率的时间演化，通过求解粘性吸积流的高度平均方程。当从下分支到上分支（或从上分支到下分支）发生过渡时，表面密度在该过渡区域周围迅速增加（减小），这引起局部增强的质量流（称为非稳态质量流）进入（离开）该区域。我们证实，非定常流可以创建一种扰动，它在整个磁盘上传播。然而，非定常质量流不足以在整个盘上诱导相干转变，除非质量注入率随时间变化。当注入速率连续变化时，中微子光度间歇性地变化，从而产生阶跃函数样的光变曲线，因为辐射效率在每次局部跃迁发生时不连续地变化。结合伽马射线暴，简要讨论了改变N形对伽玛射线暴辐射的影响和可能的观测结果。
英文摘要：
Hyperaccretion flows with mass accretion rates far above the Eddington rate have an N-shaped equilibrium curve on the $Σ$-$\dot{M}$ plane (with $Σ$ and $\dot{M}$ being surface density and mass accretion rate, respectively). The accretion flow on the lower $Σ$ branch of the N-shape is optically thick, advection-dominated accretion flow (ADAF) while that on the upper one is neutrino-dominated accretion flow (NDAF). The middle branch has a negative slope on the $Σ$-$\dot{M}$ plane, meaning that the flow on this branch is secularly unstable. To investigate how the instability affects the flow structure and what observable signatures are produced, we study the time evolution of the unstable hyperaccretion flow in response to variable mass injection rates by solving the height-averaged equations for viscous accretion flows. When a transition occurs from the lower branch to the upper branch (or from the upper branch to the lower branch), the surface density rapidly increases (decreases) around that transition region, which induces locally enhanced mass flow (referred to as non-steady mass flow) into (out of) that region. We confirm that the non-steady flow can create a kind of disturbance and that it propagates over the whole disk. However, the non-steady mass flow is not strong enough to induce coherent transition over the whole disk, unless the mass injection rate varies with time. When the injection rate continuously changes, the neutrino luminosity varies intermittently, thus producing step-function-like light curves, as the radiation efficiency discontinuously changes every time the local transition occurs. The effects of changing the N-shape and possible observational consequences on the gamma-ray burst emission are briefly discussed in relation to gamma-ray bursts.
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