Looking for the underlying cause of black hole X-ray variability in GRMHD simulations
Looking for the underlying cause of black hole X-ray variability in GRMHD simulations
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DOI：
10.1093/mnras/staa1808
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发表时间：
2020-06
影响因子：
4.8
通讯作者：
D. Bollimpalli;D. Bollimpalli;Ra’ad D Mahmoud;C. Done;P. C. Fragile;P. C. Fragile;W. Klu'zniak
中科院分区：
文献类型：
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作者：
D. Bollimpalli;D. Bollimpalli;Ra’ad D Mahmoud;C. Done;P. C. Fragile;P. C. Fragile;W. Klu'zniak
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。观测到的光曲线表现出通量的对数正态分布和线性均方根-通量关系等特征，表明潜在的变率过程是随机的。它也被认为是吸积的内在特征。这种可变性被建模为质量吸积速率向内传播的波动，尽管驱动这种波动的物理过程仍然令人困惑。在这项工作中，我们分析了五个异常长时间的广义相对论磁流体动力学（GRMHD）模拟光学薄，几何厚，黑洞吸积流，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粘性频率以下向内传播波动的证据，当比较较小的半径和较大的半径时，具有强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，并且与频率无关。我们的模拟也支持这样的观点，即˙M的波动以乘法方式增加，因为模拟显示出线性的均方根-质量通量关系，以及它们的质量通量的对数正态分布。当将模拟的质量通量与假设的发射率剖面相结合时，我们还发现与观测到的功率谱和时间滞后的广泛一致，包括时间滞后的频率依赖性的恢复。
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.