Long-term observations have shown that black hole X-ray binaries exhibit strong, aperiodic variability on time-scales of a few milliseconds to seconds. The observed light curves display various characteristic features like a lognormal distribution of flux and a linear rms–flux relation, which indicate that the underlying variability process is stochastic in nature. It is also thought to be intrinsic to accretion. This variability has been modelled as inward propagating fluctuations of mass accretion rate, although the physical process driving the fluctuations remains puzzling. In this work, we analyse five exceptionally long-duration general relativistic magnetohydrodynamic (GRMHD) simulations of optically thin, geometrically thick, black hole accretion flows to look for hints of propagating fluctuations in the simulation data. We find that the accretion profiles from these simulations do show evidence for inward propagating fluctuations below the viscous frequency by featuring strong radial coherence and positive time lags when comparing smaller to larger radii, although these time lags are generally shorter than the viscous time-scale and are frequency-independent. Our simulations also support the notion that the fluctuations in ˙ M build up in a multiplicative manner, as the simulations exhibit linear rms–mass flux relations, as well as lognormal distributions of their mass fluxes. When combining the mass fluxes from the simulations with an assumed emissivity profile, we additionally find broad agreement with observed power spectra and time lags, including a recovery of the frequency dependency of the time lags.

中文翻译：

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在 GRMHD 模拟中寻找黑洞 X 射线变异的根本原因

长期观测表明，黑洞 X 射线双星在几毫秒到几秒的时间尺度上表现出强烈的非周期性变化。观察到的光变曲线显示出各种特征，如通量的对数正态分布和线性均方根-通量关系，这表明潜在的变异过程本质上是随机的。它也被认为是吸积的内在因素。这种可变性已被建模为质量吸积率的向内传播波动，尽管驱动波动的物理过程仍然令人费解。在这项工作中，我们分析了光学薄、几何厚的黑洞吸积流的五个超长持续时间一般相对论磁流体动力学 (GRMHD) 模拟，以寻找模拟数据中传播波动的线索。我们发现，这些模拟的吸积剖面确实显示了在粘性频率以下向内传播波动的证据，当比较较小和较大的半径时，具有很强的径向相干性和正时间滞后，尽管这些时间滞后通常比粘性时间尺度短，并且是频率无关的。我们的模拟还支持 ˙ M 的波动以乘法方式建立的概念，因为模拟表现出线性均方根 - 质量通量关系，以及质量通量的对数正态分布。当将来自模拟的质量通量与假设的发射率分布相结合时，我们还发现与观察到的功率谱和时滞的广泛一致，包括时滞的频率依赖性的恢复。