Universität zu Köln

Röder, Jan ORCID: 0000-0002-2426-927X (2024). Dynamics and radiative processes of relativistic magnetized plasmas in active galactic nuclei. PhD thesis, Universität zu Köln.

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Abstract

The heart of every galaxy in the universe is home to an awesome monster: a supermassive black hole (SMBH). Reaching masses of millions to billions of times that of our sun, they are among the most powerful sources of energy in the universe. They are surrounded by hot, magnetized plasma, some of which has the unfortunate fate to be accreted and swallowed by the black hole. A small fraction of it is, however, saved by magnetic fields– and is launched thousands of light years into interstellar space, in the form of highly collimated relativistic jets. Such an SMBH-powered system is known as an active galactic nucleus (AGN). Even though AGN have been an active subject of research for almost a century, many aspects of their inner workings remain obscured. What is the nature of the SMBH residing at the center of the AGN? What is the mechanism to launch relativistic jets? How are the emission processes in different parts of the electromagnetic spectrum connected? What is the nature of the commonly observed moving features in relativistic jets, and how general is our established description of AGN? This thesis addresses a number of these questions, in an attempt to incrementally advance the field, having converged into three publications. These are preceded by four introductory chapters describing the methodology of observational radio astronomy, the anatomy of AGN, relevant radiative processes, as well as the basics of numerical simulations. With ongoing advances in observational techniques, direct images of SMBHs can be obtained to extreme resolutions. The first work presented in this thesis is concerned with the interpretation of present and future observational results: Probing the space time and accretion model for the Galactic Center: Comparison of Kerr and dilaton black hole shadows (Röder et al., 2023, A&A vol. 671, A143), investigates the influence of models for the accretion mechanism, emission processes, and the background spacetime, on our ability to determine the nature of an observed black hole. To this end, a Kerr- and a non-Kerr black hole were studied in different accretion environments, as well as under the aspect of non-thermal emission from the jet sheath. Specifically, two 3D general-relativistic magneto-hydrodynamic (GRMHD) simulations were carried out for each background spacetime, with different initial magnetic field configurations. These four simulations were then post-processed using a general-relativistic radiative transfer (GRRT) code to obtain synthetic images of the black holes. In the process, the electron temperature was parametrized based on plasma quantities, and in addition to the standard thermal energy distribution function, a kappa distribution was applied in the jet sheath. The accretion model and the electron temperature parametrization were found to have a much larger impact on the image morphology compared to the choice of the background spacetime or the electron distribution. Moving to larger spatial scales, Up around the bend: A multiwavelength view of the quasar 3C345 (Röder et al., 2024, A&A vol. 684, A211) explores the connection between the ejection of moving features into the radio jet of the quasar 3C345 with the observed gamma-ray emission. The abundance of monitoring data at both radio frequencies and in the gamma-ray regime allows for an in-depth analysis of this connection. Very long baseline interferometry (VLBI) observations at ten epochs and three frequencies were complemented with archival radio (VLBI) data and the Fermi/LAT gamma ray light curve. The results confirm conclusions drawn in previous studies, namely, that often times a spike in the gamma ray light curve corresponds to the ejection of a new moving feature into the jet, or rather its passing through a standing feature. Violent interactions within the downstream jet appear to also be linked to increased gamma activity. Finally, observations of AGN with the EHT allow for a more general view on AGN at extreme resolution. In Testing models of ultracompact jets with the EHT (Röder et al., to be submitted to A&A), relativistic jets were analyzed in the 2–230 GHz range of observing frequencies, estimating the magnetic field strength and interpreting the brightness temperature evolution in the jet. Along with the two main science targets, M87 and SgrA*, a number of AGN were observed by the EHT in 2017; many data sets are too sparse to be imaged, and model fitting routines were employed to extract information on the (polarized) source structure. The brightness temperatures of the VLBI cores at 230GHz were complemented with measurements at lower frequencies, confirming a monotonously decreasing trend previously observed up to 86GHz. Within the scope of these publications, this thesis, even if incrementally, advances each of the fields the addressed questions touch on. The works presented here provide a fertile ground for future follow-up work in numerical simulations, VLBI data analysis, and gamma ray observations. The conjunction of observations in and out of the radio regime and numerical simulations was, is, and will be an immensely powerful tool to explore the dynamics of AGN– from the direct vicinity of the black hole, all the way to kilo-parsec scales in interstellar space.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language Dynamik und Strahlungsprozesse von relativistischen magnetisierten Plasmas in aktiven Galaxienkernen German
Creators:	Creators Email ORCID ORCID Put Code Röder, Jan jroeder.astro@gmail.com orcid.org/0000-0002-2426-927X UNSPECIFIED
URN:	urn:nbn:de:hbz:38-736755
Date:	2024
Language:	English
Faculty:	External institution
Divisions:	Externe Einrichtungen
Subjects:	Natural sciences and mathematics Physics
Uncontrolled Keywords:	Keywords Language active galactic nuclei English black holes English relativistic jets English numerical simulations English very long baseline interferometry English
Date of oral exam:	10 September 2024
Referee:	Name Academic Title Zensus, J. Anton Professor Eckart, Andreas Professor Schneider, Karl Professor Würschig, Ana Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/73675