@phdthesis{Truestedt2016,
  author    = {Tr{\"u}stedt, Jonas Elias},
  title     = {Long-wavelength radio observations of blazars with the Low-Frequency Array (LOFAR)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-144406},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {Aktive Galaxienkerne (AGN) geh{\"o}ren zu den hellsten Objekten in unserem Universum. Diese Galaxien werden als aktiv bezeichnet, da ihre Zentralregion heller ist als alle Sterne in einer Galaxie zusammen beitragen k{\"o}nnten. Das Zentrum besteht aus einem supermassiven schwarzen Loch, das von einer Akkretionsscheibe und weiter außerhalb von einem Torus aus Staub umgeben ist. Diese AGN k{\"o}nnen {\"u}ber das ganze elektromagnetische Spektrum verteilt gefunden werden, von Radiowellen {\"u}ber Wellenl{\"a}ngen im optischen und R{\"o}ntgenbereich bis hin zur \$\gamma\$-Strahlung. Allerdings sind nicht alle Objekte bei jeder Wellenl{\"a}nge detektierbar. In dieser Arbeit werden {\"u}berwiegend Blazare bei niedrigen Radiofrequenzen untersucht. Blazare geh{\"o}ren zu den radio-lauten AGN, welche {\"u}blicherweise stark kollimierte Jets senkrecht zur Akkretionsscheibe aussenden. Bei Blazaren sind diese Jets in die Richtung des Beobachters gerichtet und ihre Emissionen sind stark variabel. \\ AGN werden anhand ihres Erscheinungsbildes verschiedenen Untergruppen zugeordnet. Diese Untergruppen werden in einem vereinheitlichen AGN Modell zusammengef{\"u}hrt, welches besagt, dass diese Objekte sich nur in ihrer Luminosit{\"a}t und ihrem Winkel zur Sichtlinie unterscheiden. Blazare sind diejenigen Objekte, deren Jets in unsere Sichtrichtung zeigen, w{\"a}hrend die Objekte deren Jets eher senkrecht zur Sichtlinie orientiert sind als Radiogalaxien bezeichnet werden. Daraus folgt, dass Blazare die Gegenst{\"u}cke zu Radiogalaxien mit einem anderen Winkel zur Sichtlinie sind. Diese Beziehung soll unter anderem in dieser Arbeit untersucht werden. \\ Nach ihrer Entdeckung in den 1940er Jahren wurden die aktiven Galaxien bei allen zug{\"a}nglichen Wellenl{\"a}ngen untersucht. Durch die Entwicklung von Interferometern aus Radioteleskopen, welche eine erh{\"o}hte Aufl{\"o}sung bieten, konnten die Beobachtungen stark verbessert werden. In den letzten 20 Jahren wurden viele AGN regelm{\"a}ßig beobachtet. Dies erfolgte unter anderem durch Programme wie dem MOJAVE Programm, welches 274 AGNs regelm{\"a}ßig mithilfe der Technik der ``Very Long Baseline Interferometry" (VLBI) beobachtet. Durch diese Beobachtungen konnten Informationen zur Struktur und Entwicklung der AGN und Jets gesammelt werden. Allerdings sind die Prozesse zur Bildung von Jets und deren Kollimation noch nicht vollst{\"a}ndig bekannt. Durch relativistische Effekte ist es schwierig die eigentlichen Gr{\"o}ßen der Jets anstelle der scheinbaren zu messen. Um die intrinsische Energie von Jets zu messen, sollen die ausgedehnten Emissionsregionen untersucht werden, in denen die Jets enden und mit dem Intergalaktischen Medium interagieren. Beobachtungen bei niedrigen Radiofrequenzen sind empfindlicher um solche ausgedehnte, diffuse Emissionsregionen zu detektieren. \\ Seit Dezember 2012 ist ein neues Radioteleskop f{\"u}r niedrige Frequenzen in Betrieb, dessen Stationen aus Dipolantennen besteht. Die meisten dieser Stationen sind in den Niederlanden verteilt (38 Stationen) und werden durch 12 internationale Stationen in Deutschland, Frankreich, Schweden, Polen und England erg{\"a}nzt. Dieses Instrument tr{\"a}gt den Namen ``Low Frequency Array'' (LOFAR). LOFAR bietet die M{\"o}glichkeit bei Frequenzen von 30--250 MHz bei einer h{\"o}heren Aufl{\"o}sung als bisherige Radioteleskope zu beobachten (Winkelaufl{\"o}sungen unter 1 arcsec f{\"u}r das gesamte Netzwerk aus Teleskopen). \\ Diese Arbeit behandelt die Ergebnisse von Blazaruntersuchungen mithilfe von LOFAR-Beobachtungen. Daf{\"u}r wurden AGNs aus dem MOJAVE Programm verwendet um von den bisherigen Multiwellenl{\"a}ngen-Beobachtungen und Untersuchungen der Kinematik zu profitieren. Das ``Multifrequency Snapshot Sky Survey'' (MSSS) Projekt hat den gesamten Nordhimmel mit kurzen Beobachtungen abgerastert. Aus dem daraus resultierenden vorl{\"a}ufigen Katalog wurden die Flussdichten und Spektralindizes f{\"u}r MOJAVE-Blazare untersucht. In den kurzen Beobachtungen von MSSS sind nur die Stationen in den Niederlanden verwendet worden, wodurch Aufl{\"o}sung und Sensitivit{\"a}t begrenzt sind. F{\"u}r die Erstellung des vorl{\"a}ufigen Kataloges wurde die Aufl{\"o}sung auf \$\sim\$120 arcsec beschr{\"a}nkt. Ein weiterer Vorteil der MOJAVE Objekte ist die regelm{\"a}ßige Beobachtung der AGN mit dem ``Owens Vally Radio Observatory'' zur Erstellung von Lichtkurven bei 15 GHz. Dadurch ist es m{\"o}glich nahezu zeitgleiche Flussdichtemessungen bei 15 GHz zu den entsprechenden MSSS-Beobachtungen zu bekommen. Da diese Beobachtungen zu {\"a}hnlichen Zeitpunkten durchgef{\"u}hrt wurden sind diese Flussdichten weniger von der Variabilit{\"a}t der Blazare beeinflusst. Die Spektralindizes berechnet aus den Flussdichten von MSSS und OVRO k{\"o}nnen verwendet werden um den Anteil an ausgedehnter Emission der AGNs abzusch{\"a}tzen. \\ Im Vergleich der Flussdichten aus dem MSSS Katalog mit den Beobachtungen von OVRO f{\"a}llt auf, dass die Flussdichten bei niedrigen Frequenzen tendenziell h{\"o}her sind, was durch den h{\"o}heren Anteil an ausgedehnter Struktur zu erwarten ist. Die Spektralindexverteilung zwischen MSSS und OVRO zeigt ihren h{\"o}chsten Wert bei \$\sim-0.2\$. In der Verteilung existieren Objekte mit steilerem Spektralindex durch den h{\"o}heren Anteil von ausgedehnter Emission in der Gesamtflussdichte, doch {\"u}ber die H{\"a}lfte der untersuchten Objekte besitzt flache Spektralindizes. Die flachen Spektralindizes bedeuten, dass die Emissionen dieser Objekte gr{\"o}ßtenteils von relativistischen Effekten beeinflusst sind, die schon aus Beobachtungen bei GHz-Frequenzen bekannt sind. \\ Durch neue Auswertung der MSSS Beobachtungsdaten konnten Bilder bei einer verbesserten Aufl{\"o}sung von \$\sim\$20--30 arcsec erstellt werden, wodurch bei einigen Blazaren ausgedehnte Struktur detektiert werden konnte. Diese h{\"o}her aufgel{\"o}sten Bilder sind allerdings nicht komplett kalibriert und k{\"o}nnen somit nur f{\"u}r strukturelle Informationen verwendet werden. Die {\"U}berarbeitung der Beobachtungsdaten konnte f{\"u}r 93 Objekte f{\"u}r ein Frequenzband durchgef{\"u}hrt werden. F{\"u}r 45 der 93 Objekte konnten sogar alle vorhandenen Frequenzb{\"a}nder {\"u}berarbeitet werden und dadurch gemittelte Bilder erstellt werden. Diese Bilder werden in dieser Arbeit vorgestellt. Die resultierenden Bilder mit verbesserter Aufl{\"o}sung wurden verwendet um Objekte auszuw{\"a}hlen, die mit allen LOFAR-Stationen beobachtet und auf ausgedehnte Struktur untersucht werden k{\"o}nnen. \\ Im zweiten Teil der Arbeit werden die Ergebnisse von internationalen LOFAR Beobachtungen von vier Blazaren pr{\"a}sentiert. Da sich die Auswertung und Kalibration von internationalen LOFAR Beobachtungen noch in der Entwicklung befindet, wurde ein Schwerpunkt auf die Kalibration und deren Beschreibung gelegt. Die Kalibration kann zwar noch verbessert werden, aber die Bilder aus der angewandten Kalibration erreichen eine Aufl{\"o}sung von unter 1 arcsec. Die Struktur der untersuchten vier Blazare entspricht den Erwartungen f{\"u}r Radiogalaxien unter einem anderen Sichtwinkel. Durch die gemessenen Flussdichten der ausgedehnten Struktur aus den Helligkeitsverteilungen konnte die Luminosit{\"a}t der ausgedehnten Emissionen berechnet werden. Im Vergleich mit den Luminosit{\"a}ten, die von Radiogalaxien bekannt sind, entsprechen auch diese Werte den Erwartungen des vereinheitlichten AGN Modells. \\ Durch die in dieser Arbeit vorgestellte Kalibration k{\"o}nnen noch mehr Blazare mit LOFAR inklusive den internationalen Stationen beobachtet werden und somit Bilder der Struktur bei {\"a}hnlicher Aufl{\"o}sung erstellt werden. Durch eine erh{\"o}hte Anzahl von untersuchten Blazaren k{\"o}nnten anschließend auch statistisch signifikante Ergebnisse erzielt werden.\\},
  subject      = {Blazar},
  language  = {en}
}
@phdthesis{Lewandowska2015,
  author    = {Lewandowska, Natalia Ewelina},
  title     = {A Correlation Study of Radio Giant Pulses and Very High Energy Photons from the Crab Pulsar},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123533},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Pulsars (in short for Pulsating Stars) are magnetized, fast rotating neutron stars. The basic picture of a pulsar describes it as a neutron star which has a rotation axis that is not aligned with its magnetic field axis. The emission is assumed to be generated near the magnetic poles of the neutron star and emitted along the open magnetic field lines. Consequently, the corresponding beam of photons is emitted along the magnetic field line axis. The non-alignment of both, the rotation and the magnetic field axis, results in the effect that the emission of the pulsar is only seen if its beam points towards the observer. The emission from a pulsar is therefore perceived as being pulsed although its generation is not. This rather simple geometrical model is commonly referred to as Lighthouse Model and has been widely accepted. However, it does not deliver an explanation of the precise mechanisms behind the emission from pulsars (see below for more details). Nowadays more than 2000 pulsars are known. They are observed at various wavelengths. Multiwavelength studies have shown that some pulsars are visible only at certain wavelengths while the emission from others can be observed throughout large parts of the electromagnetic spectrum. An example of the latter case is the Crab pulsar which is also the main object of interest in this thesis. Originating from a supernova explosion observed in 1054 A.D. and discovered in 1968, the Crab pulsar has been the central subject of numerous studies. Its pulsed emission is visible throughout the whole electromagnetic spectrum which makes it a key figure in understanding the possible mechanisms of multiwavelength emission from pulsars. The Crab pulsar is also well known for its radio emission strongly varying on long as well as on short time scales. While long time scale behaviour from a pulsar is usually examined through the use of its average profile (a profile resulting from averaging of a large number of individual pulses resulting from single rotations), short time scale behaviour is examined via its single pulses. The short time scale anomalous behaviour of its radio emission is commonly referred to as Giant Pulses and represents the central topic of this thesis. While current theoretical approaches place the origin of the radio emission from a pulsar like the Crab near its magnetic poles (Polar Cap Model) as already indicated by the Lighthouse model, its emission at higher frequencies, especially its gamma-ray emission, is assumed to originate further away in the geometrical region surrounding a pulsar which is commonly referred to as a pulsar magnetosphere (Outer Gap Model). Consequently, the respective emission regions are usually assumed not to be connected. However, past observational results from the Crab pulsar represent a contradiction to this assumption. Radio giant pulses from the Crab pulsar have been observed to emit large amounts of energy on very short time scales implying small emission regions on the surface of the pulsar. Such energetic events might also leave a trace in the gamma-ray emission of the Crab pulsar. The aim of this thesis is to search for this connection in the form of a correlation study between radio giant pulses and gamma-photons from the Crab pulsar. To make such a study possible, a multiwavelength observational campaign was organized for which radio observations were independently applied for, coordinated and carried out with the Effelsberg radio telescope and the Westerbork Synthesis Radio Telescope and gamma-ray observations with the Major Atmospheric Imaging Cherenkov telescopes. The corresponding radio and gamma-ray data sets were reduced and the correlation analysis thereafter consisted of three different approaches: 1) The search for a clustering in the differences of the times of arrival of radio giant pulses and gamma-photons; 2) The search for a linear correlation between radio giant pulses and gamma-photons using the Pearson correlation approach; 3) A search for an increase of the gamma-ray flux around occurring radio giant pulses. In the last part of the correlation study an increase of the number of gamma-photons centered on a radio giant pulse by about 17\% (in contrast with the number of gamma-photons when no radio giant pulse occurs in the same time window) was discovered. This finding suggests that a new theoretical approach for the emission of young pulsars like the Crab pulsar, is necessary.},
  subject      = {Pulsar},
  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}
}
@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}
}
@phdthesis{Ruegamer2012,
  author    = {R{\"u}gamer, Stefan},
  title     = {Multi-Wavelength Observations of the high-peaked BL Lacertae objects 1ES 1011+496 and 1ES 2344+514},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-77846},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {BL Lacertae objects belong to the most luminous sources in the Universe. They represent a subclass of active galactic nuclei with a spectrum that is dominated by non-thermal emission, extending from radio wavelengths to tera electronvolt (TeV) energies. The emission is strongly variable on time scales of years down to minutes, and arises from relativistic jets pointing at small angles to the line of sight of the observer, which is the reason for naming them "blazars". Blazars are the dominant extragalactic source class in the radio, microwave and gamma-ray regime, are prime candidates for the origin of the Cosmic Rays and excellent laboratories to study black hole and jet physics as well as relativistic effects. Despite more than 20 years of observational efforts, the physical mechanisms driving their emission are not yet fully understood. So far, studies of their broad-band continuum emission were mostly concentrated on bright, flaring states. However, for a better understanding of the central engine powering the jets, the bias from flux-limited observations of the past must be overcome and their long-term average continuum spectral energy distributions (SEDs) must be determined. This work presents the first simultaneous multi-wavelength campaigns from the radio to the TeV regime of two high-frequency peaked BL Lacertae objects known to emit at TeV energies. The first source, 1ES 1011+496, was observed between February and May 2008, the second one, 1ES 2344+514, between September 2008 and February 2009. The extensive observational campaigns were organised independently from an external trigger for the presence of a flaring state. Since the duty cycle of major flux outbursts is known to be rather low, the campaigns were expected to yield SEDs representative of the long-term average emission. Central for this thesis is the analysis of data obtained with the MAGIC Cherenkov telescope, measuring energy spectra and light curves from ~0.1 to ~10 TeV. For the remaining instruments, observation time was proposed and additional data was organised by collaboration with the instrument teams by the author of this work. Such data was obtained mostly in a fully reduced state. Individual light curves are investigated as well as combined in a search for inter-band correlations. The data of both sources reveal a notable lack of a correlation between the emission at radio and optical wavelengths, indicating that the radio and short-wavelength emission arise in different regions of the jet. Quasi-simultaneous SEDs of two different flux states are observationally determined and described by a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. First approaches to model the SEDs by means of a Chi2 minimisation technique are briefly discussed. The SEDs and the resulting model parameters, characterising the physical conditions in the emission regions, are compared to archival data. Though the models can describe the data well, for 1ES 1011+496 the model parameters indicate that in addition to the synchrotron and inverse-Compton emission of relativistic electrons, emission due to accelerated protons seems to be required. The SEDs of 1ES 2344+514 reveal one of the lowest activity states ever detected from the source. Despite that, the model parameters are not indicative of a distinct quiescent state, which may be caused by the degeneracy of the different parameters in one-zone models. Moreover, indications accumulate that the radiation can not be attributed to a single emission region. The results disfavour some of the current blazar classification schemes and the so-called "blazar sequence", emphasising the need for a more realistic explanation of the systematics of the blazar SEDs in terms of fundamental parameters.},
  subject      = {Blazar},
  language  = {en}
}
@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}
}