Measurements of Black Hole and Accretion Properties from Observations of Strong Field Gravitational Lensing

Abstract

Advancements in Very Long Baseline Interferometry (VLBI) have ushered in a new era of high-resolution imaging that now allow us to probe electromagnetic signatures around black holes on unprecedentedly short gravitational length scales. These include the first images of black hole shadows produced from observations of the near-horizon environments of the supermassive black holes Sgr A∗ and M87∗ by the Event Horizon Telescope (EHT). Accessing this information, however, requires the development of novel techniques that can infer the properties of sources from the visibility domain products of VLBI. In this thesis, we present developments towards techniques for extracting information about black holes and their accretion environments from VLBI observations of low luminosity active galactic nuclei (LLAGN). We first measure the geometric features of the images of M87∗ from data acquired during the 2018 EHT campaign, where we show that the appearance of the source is consistent with the Kerr hypothesis. We then outline a novel, automatic differentiable, general relativistic ray tracing code which we use to define a dual-cone emission model for LLAGN. We show that the dual-cone model can accurately reproduce synthetic observations of the averaged accretion flows present in general relativistic magnetohydrodynamic (GRMHD) simulations of M87∗ and Sgr A∗. We then fit the dual-cone model to the EHT’s 2017 observations of M87∗ to measure properties of the system’s accretion flow and central supermassive black hole. We then study the effects of intrinsic source variability on the inference capabilities of the dual-cone model by performing a multi-epoch fit to the EHT’s 2017 and 2018 observations of M87∗, and present paths towards mitigating these effects. Finally, we discuss the development of a Gaussian Markov random field model as a potential solution for variability mitigation when performing snapshot and multi-epoch measurements from observations of M87∗ and Sgr A∗