2018/05/03 by Dimitrios Psaltis, Michael Johnson, Michael D. Johnson +11 · 1 citation
Engineering · Physics and Astronomy · #Astronomical Observations and Instrumentation #Astrophysical Phenomena and Observations #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #Pulsars and Gravitational Waves Research #astro-ph.HE
paper · pdf · doi:10.48550/arxiv.1805.01242
submitted for publication the Astrophysical Journal
arxiv created 2018/05/03 · openalex publication_date 2018/05/03 · arxiv updated 2018/05/04 · openalex created_date 2018/05/07 · openalex updated_date 2026/07/28
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.