Understanding supermassive black hole accretion and jet formation through extremely high angular resolution VLBI observations

This thesis investigates the structure, dynamics, and magnetic field properties of relativistic jets in active galactic nuclei (AGN) through high-resolution Very Long Baseline Interferometry (VLBI) observations and advanced imaging methods. To this end, it combines three complementary studies that collectively address the challenges of reconstructing and interpreting these objects from different perspectives. First, pushing the limits of current black hole imaging towards the recovery of the first movies of these objects, we contribute to the development of dynamic imaging methods for black holes by validating a regularized maximum likelihood (RML) framework. This algorithm, as part of a joint effort in one of the most ambitious projects within the Event Horizon Telescope Collaboration (EHTC), is focused on a multi-modality approach via a Multi-objective Particle Swarm Optimization (MO-PSO ) algorithm. This approach efficiently enables the exploration of the multimodal posterior distribution, and recovers dynamic, full Stokes reconstructions of variable sources, such as Sagittarius A* (SgrA∗), the supermassive black hole at the centre of our galaxy. We use simulated synthetic data that present the same properties as EHTC data from SgrA* in April 11 2017. These data are used to create different synthetic models varying in morphology and increasing dynamic complexity. This way, the ability to robustly reconstruct these models is carefully validated following different metrics. Moreover, we show that while many solutions can fit the sparse VLBI data, only a subset are physically plausible. Second, we present a multi-frequency polarization study of the quasar 3C 273, using Very Long Baseline Array (VLBA) observations at six different frequencies (5, 8, 15, 22, 43 and 86 GHz) to produce total intensity, linear polarization, and rotation measure (RM) maps. The results reveal a robust transverse RM gradient across the jet. This provides strong evidence for an ordered helical magnetic field structure, likely playing a key role in jet collimation. A comparison with earlier epochs shows temporal variations in the RM, suggesting a dynamic evolution in the jet sheath’s magnetized plasma environment. Second, we revisit the parsec-scale magnetic field structure of the quasar 3C 273 through a new polarization study using the VLBA at six frequencies (5, 8, 15, 22, 43, and 86 GHz). Our study produces detailed maps of total intensity, linear polarization, and RM. We confirm the presence of a robust transverse RM gradient across the jet, supporting the existence of a large-scale helical magnetic field, likely playing a key role in jet collimation. By comparing with earlier epochs, we detect temporal variations in both the RM magnitude and gradient structure, indicating dynamical evolution in the jet’s magnetized plasma environment. These findings provide new information on the evolution of the jet’s magnetic field structure. Lastly, we report space VLBI observations of the blazar 3C 279 at 22 GHz using the space radio telescope from the RadioAstron program, achieving an angular resolution of ∼ 26μas with ground-space baselines up to 8 Gλ. The reconstructed image in total flux density and polarization shows electric vector position angles mostly aligned with the jet axis, consistent with the presence of a predominantly toroidal magnetic field. The fine-scale jet structure is traced up to ∼ 180 parsec from the core, showing no signs of any filamentary structure previously observed in an earlier RadioAstron data study of the source, most likely due to the sparsity of the uv-coverage, and coinciding with a notable decrease in total jet intensity. The analysis of this follow-up work includes a synthetic data test and measuring the brightness temperature of the radio core, yielding an estimate of the intrinsic magnetic field strength of ∼ 0.2 G, consistent with the jet being in equipartition. Together, these studies provide new insights into the dynamics, magnetic field configurations, temporal and structure evolution, and imaging challenges of relativistic jets and supermassive black holes, contributing to the methodology and observational studies necessary to achieve a better understanding of these fascinating, yet still mysterious objects.