Variability Timescale and Spectral Index of Sgr A* in the Near Infrared: Approximate Bayesian Computation Analysis of the Variability of the Closest Supermassive Black Hole
Variability Timescale and Spectral Index of Sgr A* in the Near Infrared: Approximate Bayesian Computation Analysis of the Variability of the Closest Supermassive Black Hole
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DOI：
10.3847/1538-4357/aace62
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发表时间：
2018-06
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影响因子：
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通讯作者：
G. Witzel;G. Martinez;J. Hora;S. Willner;M. Morris;C. Gammie;E. Becklin;M. Ashby;F. Baganoff
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作者：
G. Witzel;G. Martinez;J. Hora;S. Willner;M. Morris;C. Gammie;E. Becklin;M. Ashby;F. Baganoff
人马座A*（Sgr A*）是与银河系中心黑洞吸积有关的可变射电、近红外（NIR）和X射线源。我们对迄今为止最全面的Sgr A* 近红外光谱变异性数据集进行了分析：用斯皮策太空望远镜上的IRAC仪器在4.5 μm连续监测Sgr A* 的8个24小时历元，在甚大望远镜上从Naos Conica获得的2.18 μm数据的93个历元，以及在凯克天文台的NIRC 2相机获得的2.12 μm数据的30个历元，总共进行了94，929次测量。一种新的近似贝叶斯计算方法拟合的一阶结构函数提取信息超出目前的快速傅里叶变换（FFT）方法的功率谱密度（PSD）估计。结合所有三个观测站的数据，Sgr A* 的特征相干时标为分钟（90%可信区间）。PSD在低至8.5分钟（95%可信水平）的时间尺度上没有可检测的特征，这是无量纲自旋参数a = 0.92的ISCO轨道频率。在低通量密度阶段，在2.12和4.5 μm处同时测量的一条光变曲线给出光谱指数αs = 1.6 ± 0.1（）。该值意味着Sgr A* NIR颜色在较高通量密度阶段期间变得更蓝。组合数据集的通量密度的概率密度是最好的对数正态分布拟合。基于这些分布，Sgr A* 光谱能量分布与来自低于20 GHz的非热电子群通过NIR的同步辐射一致。
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We present an analysis of the most comprehensive NIR variability data set of Sgr A* to date: eight 24 hr epochs of continuous monitoring of Sgr A* at 4.5 μm with the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18 μm data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12 μm data from the NIRC2 camera at the Keck Observatory, in total 94,929 measurements. A new approximate Bayesian computation method for fitting the first-order structure function extracts information beyond current fast Fourier transformation (FFT) methods of power spectral density (PSD) estimation. With a combined fit of the data of all three observatories, the characteristic coherence timescale of Sgr A* is minutes (90% credible interval). The PSD has no detectable features on timescales down to 8.5 minutes (95% credible level), which is the ISCO orbital frequency for a dimensionless spin parameter a = 0.92. One light curve measured simultaneously at 2.12 and 4.5 μm during a low flux-density phase gave a spectral index αs = 1.6 ± 0.1 ( ). This value implies that the Sgr A* NIR color becomes bluer during higher flux-density phases. The probability densities of flux densities of the combined data sets are best fit by log-normal distributions. Based on these distributions, the Sgr A* spectral energy distribution is consistent with synchrotron radiation from a non-thermal electron population from below 20 GHz through the NIR.