Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI
Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI
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DOI：
10.3847/2041-8213/ac6584
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发表时间：
2022-05
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通讯作者：
A. Broderick;R. Gold;B. Georgiev;D. Pesce;P. Tiede;C. Ni;Kotaro Moriyama;K. Akiyama;A. Alberdi
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作者：
A. Broderick;R. Gold;B. Georgiev;D. Pesce;P. Tiede;C. Ni;Kotaro Moriyama;K. Akiyama;A. Alberdi
事件视界望远镜提供的非凡物理分辨率为两个已知黑洞M87*和Sgr a *在视界尺度上展开的天体物理现象打开了一扇窗。然而，随着分辨率的飞跃，出现了一系列新的实际问题。Sgr A*表现出日内变异性，这违背了地球孔径合成的基本假设，将传统的图像重建方法限制在短时间尺度和覆盖非常稀疏（u, v）的数据集上。我们提出了一套新的工具来检测和减轻这种可变性。我们开发了一个数据驱动的、模型不可知的程序来检测和表征日内变异性的空间结构。该方法针对大量模拟数据集进行校准，产生亮度波动的空间功率谱的经验估计。我们提出了一种新的贝叶斯噪声建模算法，该算法同时重建了平均图像和对其波动的统计度量，使用参数化形式来表示复杂能见度中不能由统计误差解释的多余方差。这些方法通过各种模拟数据进行了验证，包括适用于Sgr a *和M87*的广义相对论磁流体力学模拟。我们发现重建的源结构和可变性对底层图像模型的变化具有鲁棒性。我们将这些方法应用于2017年M87*的EHT观测，寻找EHT观测活动中变化的证据。所提出的减缓变率策略一般广泛适用于可变源的超长基线干涉测量观测，为此，它们提供了一种数据知情的平均程序和历元间图像一致性的自然表征。
The extraordinary physical resolution afforded by the Event Horizon Telescope has opened a window onto the astrophysical phenomena unfolding on horizon scales in two known black holes, M87* and Sgr A*. However, with this leap in resolution has come a new set of practical complications. Sgr A* exhibits intraday variability that violates the assumptions underlying Earth aperture synthesis, limiting traditional image reconstruction methods to short timescales and data sets with very sparse (u, v) coverage. We present a new set of tools to detect and mitigate this variability. We develop a data-driven, model-agnostic procedure to detect and characterize the spatial structure of intraday variability. This method is calibrated against a large set of mock data sets, producing an empirical estimator of the spatial power spectrum of the brightness fluctuations. We present a novel Bayesian noise modeling algorithm that simultaneously reconstructs an average image and statistical measure of the fluctuations about it using a parameterized form for the excess variance in the complex visibilities not otherwise explained by the statistical errors. These methods are validated using a variety of simulated data, including general relativistic magnetohydrodynamic simulations appropriate for Sgr A* and M87*. We find that the reconstructed source structure and variability are robust to changes in the underlying image model. We apply these methods to the 2017 EHT observations of M87*, finding evidence for variability across the EHT observing campaign. The variability mitigation strategies presented are widely applicable to very long baseline interferometry observations of variable sources generally, for which they provide a data-informed averaging procedure and natural characterization of inter-epoch image consistency.