High-Resolution Imaging of Relativistic Jets and Supermassive Black Holes

Resumen

This thesis focuses on the observational signatures encoded in the radio emission produced by accretion processes onto supermassive black holes (SMBH), located at the core of active galactic nuclei (AGN), and the highly collimated relativistic jets that emanate from their surroundings. Theoretical models predict that AGN jets are launched from the vicinity of these extreme, compact objects through energy extraction via large-scale helical magnetic fields (Blandford & Znajek, 1977; Blandford & Payne, 1982). We tackle this scenario by direct imaging of the main actors through high-angular-resolution very long baseline interferometric (VLBI) observations at millimeter and centimeter wavelengths (e.g., Event Horizon Telescope Collaboration et al., 2019a; Gómez et al., 2022). In addition to VLBI observations, we also study the properties of AGN jets at the parsec scale through relativistic magnetohydrodynamical (RMHD) simulations and their corresponding synchrotron emission (e.g., Gómez et al., 1997). Therefore, the work developed in this thesis covers a wide range of spatial (and temporal) scales, from the polarimetric properties and microarcsecond internal structure of AGN jets to the dynamic, event-horizon-scale radiation produced by the hot plasma accreting onto SMBHs. In Chapters 2 and 3, corresponding to the publications Fuentes et al. (2018, 2021), we study the influence of the helical magnetic field in the jet dynamics and emission at the parsec scale. To this aim, we analyze the polarimetric synchrotron radiation expected from several RMHD simulations of stationary overpressured magnetized relativistic jet models. These models are characterized by their dominant type of energy, namely, internal, kinetic, or magnetic. We find that the properties of recollimation shocks, formed by the pressure mismatch between the jet and the ambient medium, are mainly governed by the magnetosonic Mach number and the specific internal energy. Associated to these shocks, the radio maps obtained from the RMHD jet models feature a series of bright “knots”, typically reported in VLBI observations of AGN jets and particularly strong in the case of models dominated by the internal energy. We test several configurations of the threaded helical magnetic field and study the linearly polarized emission from the jet models. We recover a bimodal distribution of the polarization angle, especially for small viewing angles and magnetic fields dominated by their toroidal component. For larger viewing angles and poloidal magnetic field components, the polarization angle remains perpendicular to the jet propagation direction. Nonetheless, we find small rotations near the bright knots, a signature that can be used to identify recollimation shocks in VLBI observations of blazar jets. In Chapter 4, corresponding to the publication Fuentes et al. (submitted 2022), we focus on the internal and innermost structure of relativistic jets by extending the global VLBI network to space. Thus, we observe the archetypal blazar 3C 279 at 22 GHz (or 1.3 cm) with RadioAstron, a space-ground interferometer capable of providing microarcsecond angular resolutions at centimeter wavelengths. Supported by 23 radio telescopes on Earth, we report fringe detections of the source up to a projected baseline distance of 8 Earth diameters. Aided by novel image reconstruction algorithms, the highly eccentric orbit of the spacecraft allows us to resolve the transversal structure of the jet and reveal several filaments forming a helical shape. The origin of these filaments is likely related to the triggering of Kelvin-Helmholtz plasma instabilities in a kinetically dominated flow. Taking into account the image properties reconstructed, we estimate a flow Lorentz factor of 13 in a jet threaded by a helical magnetic field rotating clockwise, as seen in the direction of flow motion. Moreover, the brighter regions found in the jet, originated by a differential Doppler boosting within the filaments, should propagate down the jet with a pattern speed equal to the velocity of the instability. Based on this, we propose a novel model in which the jet variability observed in 3C 279, and possibly in other blazar sources, results from the propagation of plasma instabilities, as opposed to the standard shock-in-jet model usually invoked (Marscher & Gear, 1985). Finally, in Chapter 5, corresponding to the publication Event Horizon Telescope Collaboration et al. (2022c), we present the first movie reconstructions and dynamic characterization of a supermassive black hole accreting matter, the mechanism responsible for the formation of relativistic jets. Specifically, we employ dynamic imaging and modeling techniques to analyze the spatially-resolved intraday variability of Sagittarius A , the SMBH located at the Galactic Center. To this aim, we explore the data collected with the Event Horizon Telescope during the 2017 campaign, focusing on a small time window with the best (u; v)-coverage on April 6 and 7. We train our methods on a suite of synthetic data sets, including state-of-the-art black hole simulations. To quantify our ability to successfully reconstruct the dynamics of a given model, we compute the average position angle (PA). We find that we are able to recover the ground-truth PA in some cases, but we fail in others. On April 6, most dynamic imaging and modeling results agree on a quasi-static PA over the time window. On April 7, dynamic imaging and modeling results align when using strong spatial priors and show an evolution in the PA of 140 . However, we also see several other PA trends in the dynamic imaging results, including a PA evolution in the opposite direction and modes where the PA is static on both days. While this analysis supposes a promising starting point, the sparse coverage of the 2017 EHT array limits our ability to conclusively determine the PA evolution of Sagittarius A and our results should be interpreted with caution.