A Model for Anisotropic Interstellar Scattering and its Application to Sgr A
A Model for Anisotropic Interstellar Scattering and its Application to Sgr A
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各向异性星际散射模型及其在Sgr A中的应用
DOI：
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发表时间：
2018
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通讯作者：
G. Bower
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作者：
D. Psaltis;Michael Johnson;R. Narayan;L. Medeiros;L. Blackburn;G. Bower
电离星际介质中的散射通常被观察到是各向异性的，磁流体动力学（MHD）湍流理论通过整个散射区域的首选磁场方向来解释各向异性。特别是，银河系中心超大质量黑洞Sgr A*的视线表现出强烈的各向异性散射，这种散射在几毫米或更长的波长范围内占据主导地位。因此，对sgra在这些波长的本征结构的推断对假设的散射模型很敏感。此外，散射模型的外推也对假设的散射模型很敏感，从长波（通常估计其参数）到1.3 mm，事件视界望远镜（EHT）试图在史瓦西半径尺度上对Sgr A*成像。过去对Sgr A*的研究依赖于散射核的简单高斯模型，该模型有效地假设湍流的内部尺度远大于衍射尺度；这个假设很可能被Sgr A*的1.3 mm所违背。我们建立了一个物理驱动的各向异性散射模型，使用一个简化的MHD湍流模型，具有有限的内尺度和一个流浪的横向磁场方向。我们探索了这种漫游的几个显式分析模型，并为每个模型推导出预期的观测特性——散射展宽和折射闪烁。对于内尺度的期望值，所有模型的散射核在1.3 mm处明显是非高斯的，但计算起来很简单，并且仅弱地依赖于磁场方向漂移的假设模型。另一方面，在所有模型中，折射子结构强烈依赖于流浪模型，这可能是用EHT成像Sgr A*的重要考虑因素。
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.