Multi-Frequency VLBI Observations of the Active Galaxy NGC 1052

Abstract

Active galactic nuclei (AGN) are among the most energetic sources in the universe, a large fraction of which are visible across the entire electromagnetic spectrum. Historically a zoo of different types of AGN were categorized based on a variety of observational properties, which can be explained by one unification scheme. A subset of these sources is characterized by relativistic outflows, called jets. The standard model assumes intrinsic symmetry between the jet and the counter-jet. Radio interferometric observations provide the highest achievable resolution which is key to understanding the physics driving AGN jets. The scope of this thesis is to investigate the physical processes responsible for the launching and collimation of relativistic jets. This is achieved with Very Long Baseline Interferometry (VLBI) at centimetre and millimetre wavelengths of the double-sided relativistic outflows within the active galaxy NGC 1052. At a distance of only 20 Mpc, linear scales down to a few hundred Schwarzschild radii can be imaged with mm-VLBI. The orientation of both jets close to the plane of the sky makes NGC 1052 an ideal target to study the symmetry-paradigm predicted by the unification scheme. The thesis is organized as follows. The first two chapters will give an introduction on our current understanding of launching, collimation, and emission processes of AGN and their jets as well as an overview on the technique of VLBI. In chapters 3 trough 5 I will present the analysis and results of a multi-frequency and multi-epoch study on NGC1052. Chapter 6 summarizes these findings and places them within the context of current AGN/jet scholarship. Additional information on the analysis is provided in tabular and graphical form in the appendices A and B. During my thesis work I developed a set of python scripts for calibration and analysis, which are presented in appendix C. In the following I give a short overview on the main results from this dissertation. Observations of NGC1052 at 22 GHz and 43 GHz over 4 years suggest an intrinsic asymmetry between both jets, which evolve east- and westwards in the plane of the sky. Based on a study of the outflow velocities, the eastern jet was found to be significantly faster than the western jet. Overall faster velocities were found compared to earlier estimates performed at lower frequencies. As the observing frequency increases regions are imaged at closer proximity to the jet spine. Therefore, these results point towards a transversal velocity gradient within both jets. The images from this study were used as input information for relativistic hydrodynamic simulations of the relativistic jets in NGC1052. The simulations favor a scenario in which a slightly over-pressured jet, resulting from a pressure-mismatch between the jet and the ambient medium at the nozzle, penetrates into a decreasing-pressure ambient medium. A molecular torus has been included in the simulations to account for thermal absorption. Based on the simulation results the torus particle number density is estimated within the range 0.7–1.0×10^22 cm^−2 . This numerical estimate is consistent with estimates from X-ray and radio observations. In addition, multi-frequency VLBI studies from 1.5 GHz to 86 GHz trace the absorbing effect of this torus, which covers large parts of the western, receding jet. It results in an emission gap between both jets whose size decreases with increasing frequency. Observations and simulations draw a consistent picture of the frequency-dependent thermal absorption of the non-thermal particles in the jet due to the optically thick structure. The torus only has a very small impact on the 43 GHz emission (and higher frequencies). Both jets are extremely straight and unresolved, however, there is a slight change in the western jet direction at about 2 milliarcseconds, which cannot be observed in the eastern jet. This kind of structure can only be explained by asymmetries, intrinsic to the jet or arising from interactions with the ambient medium.