A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows
A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows
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DOI：
10.3847/2041-8213/ac65eb
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发表时间：
2022-05
期刊：
影响因子：
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通讯作者：
B. Georgiev;D. Pesce;A. Broderick;G. Wong;Vedant Dhruv;M. Wielgus;C. Gammie;Chi-kwan Chan;
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作者：
B. Georgiev;D. Pesce;A. Broderick;G. Wong;Vedant Dhruv;M. Wielgus;C. Gammie;Chi-kwan Chan;
我们提出了一个表征超大质量黑洞周围水平尺度发射结构的时空变异性功率谱的框架，并将该框架应用于与事件视界望远镜（EHT）观测Sgr a *相关的广义相对论磁流体动力学（GRMHD）模拟库和相关的广义相对论射线跟踪图像。研究发现，变率功率谱在时间和空间尺度上均表现为红噪声过程，功率峰值出现在最长的时间尺度和最大的空间尺度上。当去除时间平均光源结构和光曲线空间积分变异性时，剩余功率谱表现出普遍的破幂律行为。在较小的空间频率上，剩余功率谱随空间频率的平方而上升，并与发射质心的方差成正比。在变异性功率的某个峰值之外，剩余功率谱下降为时间平均源结构的功率谱，这在模拟中是相似的；如果变异性来自于一个比时间平均源结构具有更陡峭的高频幂律指数的乘法随机场，那么这种行为可以自然地解释。我们简要地探讨了功率谱变异性研究约束GRMHD模拟相关物理参数的能力，这些参数可以被缩放以提供在光学薄区一系列系统中的黑洞预测。我们对EHT观测到的M87*和Sgr A*吸积流的行为提出了具体的期望。
We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT.