[论文解读] A Model for Anisotropic Interstellar Scattering and its Application to Sgr A*

Dimitrios Psaltis, Michael D. Johnson+4 more|arXiv (Cornell University)|May 3, 2018

Astrophysical Phenomena and Observations被引用 12

一句话总结

本文提出了一种基于磁流体动力学湍流与有限内尺度及漂移磁场的物理驱动模型，用于描述Sgr A*方向的各向异性星际散射，表明在1.3 mm波长下，散射核明显非高斯，且对磁场漂移的依赖性较弱；而折射亚结构强烈依赖于模型，这对事件视界望远镜对Sgr A*的成像至关重要。

ABSTRACT

Scattering in the ionized interstellar medium is commonly observed to be anisotropic, with theories of magnetohydrodynamic (MHD) turbulence explaining the anisotropy through a preferred magnetic field direction throughout the scattering regions. In particular, the line of sight to the Galactic Center supermassive black hole, Sgr A*, exhibits strong and anisotropic scattering, which dominates its observed size at wavelengths of a few millimeters and longer. Therefore, inferences of the intrinsic structure of \sgra\ at these wavelengths are sensitive to the assumed scattering model. In addition, extrapolations of the scattering model from long wavelengths, at which its parameters are usually estimated, to 1.3 mm, where the Event Horizon Telescope (EHT) seeks to image Sgr A* on Schwarzschild-radius scales, are also sensitive to the assumed scattering model. Past studies of Sgr A* have relied on simple Gaussian models for the scattering kernel that effectively presume an inner scale of turbulence far greater than the diffractive scale; this assumption is likely violated for Sgr A* at 1.3 mm. We develop a physically motivated model for anisotropic scattering, using a simplified model for MHD turbulence with a finite inner scale and a wandering transverse magnetic field direction. We explore several explicit analytic models for this wandering and derive the expected observational properties --- scatter broadening and refractive scintillation --- for each. For expected values of the inner scale, the scattering kernel for all models is markedly non-Gaussian at 1.3 mm but is straightforward to calculate and depends only weakly on the assumed model for the wandering of the magnetic field direction. On the other hand, in all models, the refractive substructure depends strongly on the wandering model and may be an important consideration in imaging Sgr A* with the EHT.

研究动机与目标

• 开发一种物理解释的Sgr A*方向各向异性星际散射模型，考虑湍流中的有限内尺度。
• 评估1.3 mm波段的散射特性（对事件视界望远镜EHT观测至关重要）如何依赖于所假设的散射模型。
• 评估折射闪烁与散射展宽对密度涨落功率谱角向依赖性的敏感度。
• 通过量化散射效应对源本征结构的影响，为解释EHT数据提供一个框架。

提出的方法

• 模型采用简化的磁流体动力学湍谱，包含有限内尺度，并引入空间漂移的横向磁场。
• 推导了三种显式解析模型用于磁场漂移：冯·米塞斯分布、偶极子分布和方盒分布。
• 通过密度涨落功率谱的相位涨落集合平均计算散射核。
• 利用可见度振幅方差对折射闪烁进行建模，其对角向功率谱结构敏感。
• 采用佩德近似（Padé近似）拟合各向异性程度、主/次轴展宽及折射可见度方差的模型系数。
• 通过将预测的波长依赖性散射与现有观测及EHT约束进行比较，对模型进行验证。

实验结果

研究问题

• RQ1 星际湍流中的有限内尺度如何影响Sgr A*在1.3 mm波长下的散射核形状？
• RQ2 散射图像中的折射亚结构在多大程度上依赖于磁场漂移的角分布？
• RQ3 预测的散射展宽特性对密度涨落功率谱角向假设的敏感度如何？
• RQ4 1.3 mm波长下的散射核能否准确建模为高斯分布，还是由于有限内尺度而显著非高斯？
• RQ5 散射各向异性对事件视界望远镜解析黑洞阴影有何影响？

主要发现

• 所有具有有限内尺度的模型在1.3 mm波长下，其散射核均显著非高斯，与普遍假设的高斯散射核相矛盾。
• 散射展宽图像特性仅对湍流功率谱角向依赖性的细节弱敏感，因此在不同模型间具有鲁棒性。
• 折射闪烁振幅强烈依赖于磁场漂移的模型，某些模型预测垂直于主轴方向的功率可忽略不计。
• 各向异性程度随波长减小而增加，其与湍流幂律指数α及内尺度rin相关。
• 对于rin的预期取值，散射核保持可计算且行为良好，但折射亚结构可能显著影响EHT图像重建。
• 该模型为解释Sgr A*的EHT观测并校正视界尺度图像上的散射效应提供了物理解释一致的框架。