Leveraging Polarization in the Era of Submillimeter VLBI
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Date
2024-06-14
Authors
Ni, Chunchong
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
With the advancement of technology, global very long baseline interferometry (VLBI) observations at millimeter wavelengths become possible. The Event Horizon Telescope (EHT) is the first such experiment, which makes observing accretion disk and jet launching regions near supermassive black holes and active galactic nuclei (AGN) possible, including polarimetry observations.
Centaurus A (Cen A) is a nearby radio-loud AGN, with large jet structures of angular size measured in degrees. It was observed by the EHT, whose first total intensity image shows a fork-shaped edge brightening jet structure. Chapter 2 applies Bayesian imaging method to the Cen A data. We first construct the total intensity image of Cen A, which we directly compare with the previous publication. Second, the Bayesian method produces the first polarization studies of Cen A jet. Both the total intensity imaging and the polarization mapping feature a full image posteriors with access to the image uncertainty. This proves to be essential in the case of Cen A, where the data is very challenging for various reasons. With polarization image posterior of Cen A, we are able to study different regions of the jet separately, eventually producing a robust estimate of a collection of important physics quantities, including magnetic field strength, the electron number density and the jet velocity.
In Chapter 3, we explore the origin and influence of the interstellar scattering on observations of Sgr A*, and propose a novel method to mitigate this scattering via EHT and next-generation EHT (ngEHT) polarimetry in the future. In EHT and other radio astronomical observations of Sgr A*, scattering contaminates the image with external small-scale structures, essentially preventing further studies of the turbulence in the accretion disk. However, for credible interstellar magnetic field strengths, the scattering is proved to be 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 structural information from the scattered power spectrum. We demonstrate that this is feasible through a series of numerical experiments with general relativistic magnetohydrodynamic (GRMHD) simulation images. Specifically, we show that the ratio of the power spectra, obtained independently for different polarization components, is independent of the scattering screen. Therefore, these power spectra ratios provide a window directly into the MHD turbulence believed to drive accretion onto black holes.
Description
Keywords
radio astronomy, very long baseline interferometry, EHT, polarimetry, AGN, black holes, relativistic jets, interstellar scattering