@phdthesis{Paul2010, author = {Paul, Surajit}, title = {Evolution of shocks and turbulence in major galaxy-cluster mergers}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-47266}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {Mergers between rich clusters of galaxies represent the most violent events in the Universe. The merger events initiate a complex chain of processes that leads to the dissipation of the collisional energy. This phase of violent relaxation is accompanied by turbulence and shock waves as well as non-thermal particle acceleration. This thesis aims at the interpretation of multi-wavelength observations of the merging cluster of galaxies Abell 3376 in the framework of a theoretical model of the involved effects. Observations with the Very Large Array radio interferometer were carried out and analyzed to clarify the morphology of the non-thermal particle distribution in Abell 3376, in particular about the shocked regions. The dissipation in the hot intra-cluster gas was studied using archival X-ray observations with ROSAT and XMM. Results were compared with constrained numerical simulations of the evolution of the merger process in the framework of cosmological structure formation. For this purpose, the ENZO-Code was employed for the computation of the gas dynamics and self-gravity of the colliding mass distribution. The non-thermal properties of the intra-cluster gas could be indirectly inferred from the local Mach number and the strength of the turbulence.}, subject = {Galaxienhaufen}, language = {en} } @phdthesis{Rueger2011, author = {R{\"u}ger, Michael}, title = {Ein zeitabh{\"a}ngiges, selbstkonsistentes hadronisch-leptonisches Strahlungsmodell zur Modellierung der Multiwellenl{\"a}ngenemission von Blazaren}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56955}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Diese Arbeit besch{\"a}ftigt sich mit Strahlungsprozessen in Blazaren. Bei den Blazaren handelt es sich um eine Unterkategorie der aktiven Galaxienkerne, bei denen die Jetachse in Richtung des Beobachters zeigt. Charakteristisch f{\"u}r die Blazare ist ein Multifrequenzspektrum der Photonen, welches sich vom Radiobereich bis hin zur Gamma-Strahlung mit TeV-Energien erstreckt. Insbesondere der Gamma-Bereich r{\"u}ckt aktuell in den Fokus der Betrachtung mit Experimenten wie zum Beispiel FERMI und MAGIC. Ziel dieser Arbeit ist die Modellierung der auftretenden Strahlungsprozesse und die Beschreibung der Multifrequenzspektren der Blazare mit Hilfe eines hadronisch-leptonischen Modells. Grundlage hierf{\"u}r ist ein selbstkonsistentes Synchrotron-Selbst-Compton-Modell (SSC), welches zur Beschreibung des Spektrums der Quelle 1 ES 1218+30.4 verwendet wird. Dabei wird die Parameterwahl unterst{\"u}tzt durch eine Absch{\"a}tzung der Masse des zentralen schwarzen Loches. Das hier behandelte SSC-Modell wird dahingehend untersucht, wie es sich unter Ver{\"a}nderung der Modellparameter verh{\"a}lt. Dabei werden Abh{\"a}ngigkeiten des Photonenspektrums von {\"A}nderungsfaktoren der Parameter abgeleitet. Außerdem werden diese Abh{\"a}ngigkeiten in Relation gesetzt und aus dieser Betrachtung ergibt sich die Schlussfolgerung, dass unter der Voraussetzung eines festen Spektralindex der Elektronenverteilung die Wahl eines Parametersatzes zur Modellierung eines Photonenspektrums eindeutig ist. Zur Einf{\"u}hrung eines zeitabh{\"a}ngigen, hadronischen Modells wird das SSCModell um die Anwesenheit nichtthermischer Protonen erweitert. Dadurch kann Proton-Synchrotron-Strahlung einen Beitrag im Gamma-Bereich leisten. Außerdem werden durch Proton-Photon-Wechselwirkung Pionen erzeugt. Aus deren Zerfall werden zusammen mit der Paarbildung aus Photon-Photon-Absorption sekund{\"a}re Elektronen und Positronen produziert, die wiederum zum Hochenergiespektrum beitragen. Neben den Pionen werden bei der Proton-Photon- Wechselwirkung außerdem noch Neutrinos und Neutronen erzeugt, die einen direkten Einblick in die Emissionsregion erlauben. Das hier vorgestellte hadronische Modell wird auf die Quelle 3C 279 angewandt. F{\"u}r diese Quelle reicht mit der Detektion im VHE-Bereich der SSCAnsatz nicht aus, um das Photonenspektrum zu beschreiben. Mit dem vorgelegten Modell gelingt die Beschreibung des Spektrums in den SSC-kritischen Bereichen sehr gut. Insbesondere k{\"o}nnen verschiedene Flusszust{\"a}nde modelliert und allein durch Ver{\"a}nderung der Maximalenergien von Protonen und Elektronen ineinander {\"u}berf{\"u}hrt werden. Diese einfache M{\"o}glichkeit der Modellierung der Variabilit{\"a}t der Quelle unterstreicht die Wahl des hadronischen Ansatzes. Somit wird hier ein sehr gutes Werkzeug zur Untersuchung der Emissionsprozesse in Blazaren geliefert. Dar{\"u}ber hinaus ist mit der Absch{\"a}tzung des Neutrino-Flusses zwar die Detektion von 3C 279 als Punktquelle mit IceCube unwahrscheinlich, jedoch liefert das Modell generell die M{\"o}glichkeit im Kontext des Multimessenger-Ansatzes Antworten zu liefern. Im gleichen Kontext wird auch der Beitrag zur kosmischen Strahlung durch entweichende Neutronen untersucht.}, subject = {Blazar}, language = {de} } @phdthesis{Berger2009, author = {Berger, Karsten}, title = {Discovery and Characterization of the first Low-Peaked and Intermediate-Peaked BL Lacertae Objects in the Very High Energy Gamma-Ray Regime}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37431}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {20 years after the discovery of the Crab Nebula as a source of very high energy gamma-rays, the number of sources newly discovered above 100 GeV using ground-based Cherenkov telescopes has considerably grown, at the time of writing of this thesis to a total of 81. The sources are of different types, including galactic sources such as supernova remnants, pulsars, binary systems, or so-far unidentified accelerators and extragalactic sources such as blazars and radio galaxies. The goal of this thesis work was to search for gamma-ray emission from a particular type of blazars previously undetected at very high gamma-ray energies, by using the MAGIC telescope. Those blazars previously detected were all of the same type, the so-called high-peaked BL Lacertae objects. The sources emit purely non-thermal emission, and exhibit a peak in their radio-to-X-ray spectral energy distribution at X-ray energies. The entire blazar population extends from these rare, low-luminosity BL Lacertae objects with peaks at X-ray energies to the much more numerous, high-luminosity infrared-peaked radio quasars. Indeed, the low-peaked sources dominate the source counts obtained from space-borne observations at gamma-ray energies up to 10 GeV. Their spectra observed at lower gamma-ray energies show power-law extensions to higher energies, although theoretical models suggest them to turn over at energies below 100 GeV. This opened the quest for MAGIC as the Cherenkov telescope with the currently lowest energy threshold. In the framework of this thesis, the search was focused on the prominent sources BL Lac, W Comae and S5 0716+714, respectively. Two of the sources were unambiguously discovered at very high energy gamma-rays with the MAGIC telescope, based on the analysis of a total of about 150 hours worth of data collected between 2005 and 2008. The analysis of this very large data set required novel techniques for treating the effects of twilight conditions on the data quality. This was successfully achieved and resulted in a vastly improved performance of the MAGIC telescope in monitoring campaigns. The detections of low-peaked and intermediate-peaked BL Lac objects are in line with theoretical expectations, but push the models based on electron shock acceleration and inverse-Compton cooling to their limits. The short variability time scales of the order of one day observed at very high energies show that the gamma-rays originate rather close to the putative supermassive black holes in the centers of blazars, corresponding to less than 1000 Schwarzschild radii when taking into account relativistic bulk motion.}, subject = {Aktiver galaktischer Kern}, language = {en} } @phdthesis{Glawion2014, author = {Glawion, Dorit}, title = {Contemporaneous Multi-Wavelength Observations of the Gamma-Ray Emitting Active Galaxy IC 310 - New Clues on Particle Acceleration in Extragalactic Jets}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-113866}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {In this thesis, the broad band emission, especially in the gamma-ray and radio band, of the active galaxy IC 310 located in the Perseus cluster of galaxies was investigated. The main experimental methods were Cherenkov astronomy using the MAGIC telescopes and high resolution very long baseline interferometry (VLBI) at radio frequencies (MOJAVE, EVN). Additionally, data of the object in different energy bands were studied and a multi-wavelength campaign has been organized and conducted. During the campaign, an exceptional bright gamma-ray flare at TeV energies was found with the MAGIC telescopes. The results were compared to theoretical acceleration and emission models for explaining the high energy radiation of active galactic nuclei. Many open questions regarding the particle acceleration to very high energies in the jets of active galactic nuclei, the particle content of the jets, or how the jets are launched, were addressed in this thesis by investigating the variability of IC 310 in the very high energy band. It is argued that IC310 was originally mis-classified as a head-tail radio galaxy. Instead, it shows a variability behavior in the radio, X-ray, and gamma-ray band similar to the one found for blazars. These are active galactic nuclei that are characterized by flux variability in all observed energy bands and at all observed time scales. They are viewed at a small angle between the jet axis and the line-of-sight. Thus, strong relativistic beaming influences the variability properties of blazars. Observations of IC 310 with the European VLBI Network helped to find limits for the angle between the jet axis and the line-of-sight, namely 10 deg - 20 deg. This places IC 310 at the borderline between radio galaxies (larger angles) and blazars (smaller angles). During the gamma-ray outburst detected at the beginning of the multi-wavelength campaign, flux variability as short as minutes was measured. The spectrum during the flare can be described by a simple power-law function over two orders of magnitude in energy up to ~10 TeV. Compared to previous observations, no significant variability of the spectral shape was found. Together with the constraint on the viewing angle, this challenges the currently accepted models for particle acceleration at shock waves in the jets. Alternative models, such as stars moving through the jets, mini-jets in the jet caused, e.g., by reconnection events, or gap acceleration in a pulsar-like magnetosphere around the black hole were investigated. It was found that only the latter can explain all observational findings, which at least suggests that it could even be worthwhile to reconsider published investigations of AGN with this new knowledge in mind. The first multi-wavelength campaign was successfully been conducted in 2012/2013, including ground-based as well as space-based telescopes in the radio, optical, ultraviolet, X-ray, and gamma-ray energy range. No pronounced variability was found after the TeV flare in any energy band. The X-ray data showed a slightly harder spectrum when the emission was brighter. The long-term radio light curve indicated a flickering flux variability, but no strong hint for a new jet component was found from VLBI images of the radio jet. In any case, further analysis of the existing multi-wavelength data as well as complimentary measurements could provide further exciting insights, e.g., about the broad band spectral energy distribution. Overall, it can be stated that IC 310 is a key object for research of active galactic nuclei in the high-energy band due to its proximity and its peculiar properties regarding flux variability and spectral behavior. Such objects are ideally suited for studying particle acceleration, jet formation, and other physical effects and models which are far from being fully understood.}, subject = {Aktiver galaktischer Kern}, language = {en} } @misc{Wendel2022, type = {Master Thesis}, author = {Wendel, Christoph}, title = {Bestimmung des hochenergetischen Spektrums des Crab-Pulsars anhand eines Outer Gap-Modells}, doi = {10.25972/OPUS-25719}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-257191}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Im Rahmen eines selbst-konsistenten Outer-Gap-Modells der Pulsar-Magnetosph{\"a}re wurde die elektromagnetische sehr hochenergetische Strahlung des Crab-Pulsars simuliert. Dies wurde parallel anhand zweier verschiedener F{\"a}lle getan, die sich in den angenommenen Gleichungen f{\"u}r die elektrische Feldst{\"a}rke und f{\"u}r den Kr{\"u}mmungsradius der magnetischen Feldlinien unterscheiden. Die Kinetik der geladenen Teilchen bei ihrer Propagation durch die Outer Gap wurde unter Einbeziehung von Kr{\"u}mmungsstrahlung, inverser Compton-Streuung und Triple Paarbildung betrachtet. Das theoretisch simulierte Spektrum wird mit von Fermi-LAT und von den MAGIC Teleskopen gemessenen Daten verglichen.}, subject = {Neutronenstern}, language = {de} } @phdthesis{Maier2008, author = {Maier, Andreas}, title = {Adaptively Refined Large-Eddy Simulations of Galaxy Clusters}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-32274}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {It is aim of this work to develop, implement, and apply a new numerical scheme for modeling turbulent, multiphase astrophysical flows such as galaxy cluster cores and star forming regions. The method combines the capabilities of adaptive mesh refinement (AMR) and large-eddy simulations (LES) to capture localized features and to represent unresolved turbulence, respectively; it will be referred to as Fluid mEchanics with Adaptively Refined Large-Eddy SimulationS or FEARLESS.}, subject = {Turbulenz}, language = {en} } @article{ChatterjeeChaelTiedeetal.2023, author = {Chatterjee, Koushik and Chael, Andrew and Tiede, Paul and Mizuno, Yosuke and Emami, Razieh and Fromm, Christian and Ricarte, Angelo and Blackburn, Lindy and Roelofs, Freek and Johnson, Michael D. and Doeleman, Sheperd S. and Arras, Philipp and Fuentes, Antonio and Knollm{\"u}ller, Jakob and Kosogorov, Nikita and Lindahl, Greg and M{\"u}ller, Hendrik and Patel, Nimesh and Raymond, Alexander and Traianou, Efthalia and Vega, Justin}, title = {Accretion flow morphology in numerical simulations of black holes from the ngEHT model library: the impact of radiation physics}, series = {Galaxies}, volume = {11}, journal = {Galaxies}, number = {2}, issn = {2075-4434}, doi = {10.3390/galaxies11020038}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-304084}, year = {2023}, abstract = {In the past few years, the Event Horizon Telescope (EHT) has provided the first-ever event horizon-scale images of the supermassive black holes (BHs) M87* and Sagittarius A* (Sgr A*). The next-generation EHT project is an extension of the EHT array that promises larger angular resolution and higher sensitivity to the dim, extended flux around the central ring-like structure, possibly connecting the accretion flow and the jet. The ngEHT Analysis Challenges aim to understand the science extractability from synthetic images and movies to inform the ngEHT array design and analysis algorithm development. In this work, we compare the accretion flow structure and dynamics in numerical fluid simulations that specifically target M87* and Sgr A*, and were used to construct the source models in the challenge set. We consider (1) a steady-state axisymmetric radiatively inefficient accretion flow model with a time-dependent shearing hotspot, (2) two time-dependent single fluid general relativistic magnetohydrodynamic (GRMHD) simulations from the H-AMR code, (3) a two-temperature GRMHD simulation from the BHAC code, and (4) a two-temperature radiative GRMHD simulation from the KORAL code. We find that the different models exhibit remarkably similar temporal and spatial properties, except for the electron temperature, since radiative losses substantially cool down electrons near the BH and the jet sheath, signaling the importance of radiative cooling even for slowly accreting BHs such as M87*. We restrict ourselves to standard torus accretion flows, and leave larger explorations of alternate accretion models to future work.}, language = {en} } @phdthesis{Schulz2016, author = {Schulz, Robert Frank}, title = {A radio view of high-energy emitting AGNs}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-137358}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {The most energetic versions of active galactic nuclei (AGNs) feature two highly-relativistic plasma outflows, so-called jets, that are created in the vicinity of the central supermassive black hole and evolve in opposite directions. In blazars, which dominate the extragalactic gamma-ray sky, the jets are aligned close to the observer's line of sight leading to strong relativistic beaming effects of the jet emission. Radio observations especially using very long baseline interferometry (VLBI) provide the best way to gain direct information on the intrinsic properties of jets down to sub-parsec scales, close to their formation region. In this thesis, I focus on the properties of three AGNs, IC 310, PKS 2004-447, and 3C 111 that belong to the small non-blazar population of gamma-ray-loud AGNs. In these kinds of AGNs, the jets are less strongly aligned with respect to the observer than in blazars. I study them in detail with a variety of radio astronomical instruments with respect to their high-energy emission and in the context of the large samples in the monitoring programmes MOJAVE and TANAMI. My analysis of radio interferometric observations and flux density monitoring data reveal very different characteristics of the jet emission in these sources. The work presented in this thesis illustrates the diversity of the radio properties of gamma-ray-loud AGNs that do not belong to the dominating class of blazars.}, subject = {Aktiver galaktischer Kern}, language = {en} }