Probing Accretion Turbulence in the Galactic Centre with EHT Polarimetry
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Date
2019-01-09
Authors
Ni, Chunchong
Journal Title
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Publisher
University of Waterloo
Abstract
We explore the origin and the influence of the interstellar scattering on the observation of Sgr A*, and the method to mitigate this scattering via Event Horizon Telescope (EHT) polarimetry. Interstellar scattering is due to the existence of inhomogeneous plasma screens between the Earth and Sgr A*. At radio wavelengths, this scattering adds and removes small structures in the images. In the EHT observation, the scattering contaminates the image by moving power from long baselines to short baselines in a fashion that may be described by a linear transformation characterized by the Scattering Shift Kernel. However, for credible interstellar magnetic field strengths, this transformation is insensitive to polarization. Therefore, it is possible to distinguish intrinsic and scattered structures via the image power spectra constructed in different polarization components.
Via numerical experiments, we demonstrate a method for reconstructing intrinsic structure information. We do this for two cases: a toy model in which we show that this method accurately reproduces the characteristics of controlled image fluctuations, and general relativistic magnetohydrodynamic simulation images. Specifically, we show that the ratio of the power spectra obtained independently for different polarization components is independent of the scattering screen given the current observational limitations of the EHT. Therefore, these power spectra ratios provide a window directly into the magnetohydrodynamic turbulence believed to drive accretion onto black holes.