Universität zu Köln

Benke, Petra ORCID: 0009-0006-4186-9978 (2024). Multiwavelength variability in active galactic nuclei jets and other VLBI studies. PhD thesis, Universität zu Köln.

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Abstract

Active galactic nuclei (AGN) jets are one of the most fascinating objects in the Universe. Since their discovery by Curtis about a hundred years ago, we uncovered many of their secrets, but fundamental mechanisms related to the nature of their launching, evolution and multiwavelength variability are still not perfectly understood. In this dissertation work, I present my research on very long baseline interferometry (VLBI) observations of AGN jets, investigating the above-mentioned of topics. After the introductory chapters describing phenomena related to AGN (Ch. 3), I describe the observational method, VLBI (Ch. 4), which enables radio astronomers to reach unprecedented spatial resolutions to study the central parsec region of AGN. AGN emit radiation throughout the entire electromagnetic spectrum, and the correlation of the activity between the different wavebands is a widely investigated topic. This includes temporal variations in brightness and polarization, ejection of new jet features in connection to elevated flux density states or detection of high-energy astroparticles associated with a given object, as well as common characteristics of astroparticle-emitter sources. In this dissertation, I research the link between the radio and the gamma-ray activity in AGN jets using VLBI imaging techniques. In Chapter 5 I present the first images from the 2.3-GHz monitoring of the TANAMI Collaboration targeting the TeV-emitter AGN of the southern sky. The goal of the study is to prove the viability of such a program, as many of the objects are extremely faint, and more importantly, to investigate the characteristics of TeV-detected objects. When analyzing this biased sample of twenty-four TeV-emitter AGN, I found that the core brightness temperatures shows an inverse relation with synchrotron peak frequency, and a direct relationship with the 0.1-100 GeV gamma-ray flux density. In addition, the median and average core brightness temperature of the sample is a magnitude lower than the value observed for samples dominated by low- and intermediate-synchrotron-peaked blazars. These results suggest that Doppler boosting is more prevalent in low-synchrotron-peaked objects than in high-synchrotron-peaked ones. While blazars are the most common objects populating the gamma-ray sky, only a handful of them were detected at high energies above z>2.5, and overarching, contemporaneous multiwavelength studies are rarely focused on these high-redshift objects. For the first time to our knowledge, I present a multi-frequency VLBI follow-up analysis of the z=4.31 blazar, TXS 1508+572 in Chapter 6. I aim to track the morphological changes, spectral index and opacity variations which could be associated with the preceding gamma-ray flare observed in 2022 February. I find significant changes in brightness and morphology in the jet region, a feature which shows superluminal apparent motion, and was most likely ejected during 2016-2019. The position of the core with respect to the jet apex stays consistent through the observed time period of ~1 yr. I associated the gamma-ray flare to the jet feature, and propose that the flare resulted from an interaction between the jet feature and the new plasma that was injected in this region. As galaxy evolution in the ΛCDM Universe happens in a hierarchical manner via mergers, their central supermassive black holes form binaries (SMBHBs). SMBHBs tell us about the merger history of galaxies, the growth of black holes, as well as AGN activity related to mergers. PSO J334.2028+1.4075 (hereafter PSO J334) was one such candidate, whose periodic optical flux density variability triggered several multiwavelength observations to confirm its nature. The only remaining indication of a SMBHB residing in the center of this AGN was a 39° misalignment between its parsec- and kiloparsec-scale jet. In Chapter 7 I discuss my observations taken with the Karl G. Jansky Very Large Array that resolve the kiloparsec-scale jet of this PSO J334. The 6.2-GHz map shows a two-sided, lobe-dominated quasar with an asymmetric jet structure. The jets are remarkably straight until they bend, probably due to an interaction with the ambient medium, suggested by the magnetic field structure. I find no indication of jet precession, showing that PSO J334 harbors only a single black hole in its center. In Chapter 8 I investigate the evolution of a recently ejected jet feature in the radio galaxy 3C 84, in order to learn more about jet launching and collimation. For this endeavor, I use 22-GHz data from RadioAstron, which takes VLBI to its extreme by utilizing space baselines, thus providing an unprecedented angular resolution, only matched by the Event Horizon Telescope. Comparing two RadioAstron observations, one from 2013, and the one presented in this thesis from 2016, I confirm the presence of an edge-brightened jet and counter-jet, but also find significant changes in jet morphology. These include a change in the collimation profile from a quasi-cylindrical to a parabolic shape, indicative of the decreasing pressure of the mini-cocoon inflated by the jet that cannot collimate the jet as efficiently as when it was closer to the core. This finding highlights one of the important external collimation mechanisms in AGN jets. The works presented in this dissertation were made in anticipation of future telescope arrays and space missions, such as the Square Kilometer Array, that will revolutionize AGN surveys, especially of southern-sky sources, the Cherenkov Telescope Array which promises increased sensitivity at TeV energies, and the Laser Interferometer Space Antenna, which will open a new window to the gravitational Universe and hunt for supermassive black hole binaries. That so many upcoming instruments are focused on AGN science, tells us that these objects still present us with a plethora of open questions, many of which may be uncovered with these next generation telescopes.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language UNSPECIFIED English
Creators:	Creators Email ORCID ORCID Put Code Benke, Petra pbenke@mpifr-bonn.mpg.de orcid.org/0009-0006-4186-9978 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-736628
Date:	2024
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Natural sciences and mathematics Physics
Uncontrolled Keywords:	Keywords Language Experimental physics English Astrophysics English Black holes English Jet physics English
Date of oral exam:	9 September 2024
Referee:	Name Academic Title Eckart, Andreas Prof. Dr. Zensus, J. Anton Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/73662