@phdthesis{HoehneMoench2010,
  author    = {H{\"o}hne-M{\"o}nch, Daniel},
  title     = {Steady-state emission of blazars at very high energies},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-53700},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {One key scientific program of the MAGIC telescope project is the discovery and detection of blazars. They constitute the most prominent extragalactic source class in the very high energy (VHE) Gamma-ray regime with 29 out of 34 known objects (as of April 2010). Therefore a major part of the available observation time was spent in the last years on high-frequency peaked blazars. The selection criteria were chosen to increase the detection probability. As the X-ray flux is believed to be correlated to the VHE Gamma-ray flux, only X-ray selected sources with a flux F(X) > 2 μJy at 1 keV were considered. To avoid strong attenuation of the Gamma-rays in the extragalactic infrared background, the redshift was restricted to values between z < 0.15 and z < 0.4, depending on the declination of the objects. The latter determines the zenith distance during culmination which should not exceed 30° (for z < 0.4) and 45° (for z < 0.15), respectively. Between August 2005 and April 2009, a sample of 24 X-ray selected high-frequency peaked blazars has been observed with the MAGIC telescope. Three of them were detected including 1ES 1218+304 being the first high-frequency peaked BL Lacertae object (HBL) to be discovered with MAGIC in VHE Gamma-rays. One previously detected object was not confirmed as VHE emitter in this campaign by MAGIC. A set of 20 blazars previously not detected will be treated more closely in this work. In this campaign, during almost four years ~ 450 hrs or ~ 22\% of the available observation time for extragalactic objects were dedicated to investigate the baseline emission of blazars and their broadband spectral properties in this emission state. For the sample of 20 objects in a redshift range of 0.018 < z < 0.361 integral flux upper limits in the VHE range on the 99.7\% confidence level (corresponding to 3 standard deviations) were calculated resulting in values between 2.9\% and 14.7\% of the integral flux of the Crab Nebula. As the distribution of significances of the individual objects shows a clear shift to positive values, a stacking method was applied to the sample. For the whole set of 20 objects, an excess of Gamma-rays was found with a significance of 4.5 standard deviations in 349.5 hours of effective exposure time. For the first time a signal stacking in the VHE regime turned out to be successful. The measured integral flux from the cumulative signal corresponds to 1.4\% of the Crab Nebula flux above 150 GeV with a spectral index α = -3.15±0.57. None of the objects showed any significant variability during the observation time and therefore the detected signal can be interpreted as the baseline emission of these objects. For the individual objects lower limits on the broad-band spectral indices αX-Gamma between the X-ray range at 1 keV and the VHE Gamma-ray regime at 200 GeV were calculated. The majority of objects show a spectral behaviour as expected from the source class of HBLs: The energy output in the VHE regime is in general lower than in X-rays. For the stacked blazar sample the broad-band spectral index was calculated to αX-Gamma = 1.09, confirming the result found for the individual objects. Another evidence for the revelation of the baseline emission is the broad-band spectral energy distribution (SED) comprising archival as well as contemporaneous multi-wavelength data from the radio to the VHE band. The SEDs of known VHE Gamma-ray sources in low flux states matches well the SED of the stacked blazar sample.},
  subject      = {MAGIC-Teleskop},
  language  = {en}
}
@phdthesis{Wendel2022,
  author    = {Wendel, Christoph},
  title     = {Spectral Imprints from Electromagnetic Cascades in Blazar Jets},
  doi       = {10.25972/OPUS-29007},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-290076},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {The extragalactic gamma-ray sky is dominated by blazars, active galactic nuclei (AGN) with a relativistic jet that is closely aligned with the line of sight. Galaxies develop an active nucleus if the central supermassive black hole (BH) accretes large amounts of ambient matter and magnetic flux. The inflowing mass accumulates around the plane perpendicular to the accretion flow's angular momentum. The flow is heated through viscous friction and part of the released energy is radiated as blackbody or non-thermal radiation, with luminosities that can dominate the accumulated stellar luminosity of the host galaxy. A fraction of the accretion flow luminosity is reprocessed in a surrounding field of ionised gas clouds. These clouds, revolving around the central BH, emit Doppler-broadened atomic emission lines. The region where these broad-line-emitting clouds are located is called broad-line region (BLR). About one in ten AGN forms an outflow of radiation and relativistic particles, called a relativistic jet. According to the Blandford-Znajek mechanism, this is facilitated through electromagnetic processes in the magnetosphere of a spinning BH. The latter induces a magnetospheric poloidal current circuit, generating a decelerating torque on the BH and inducing a toroidal magnetic field. Consequently, rotational energy of the BH is converted to Poynting flux streaming away mainly along the rotational axis and starting the jet. One possibility for particle acceleration near the jet base is realised by magnetospheric vacuum gaps, regions temporarily devoid of plasma, such that an intermittent electric field arises parallel to the magnetic field lines, enabling particle acceleration and contributing to the mass loading of the jets. Magnetised structures, containing bunches of relativistic electrons, propagate away from the galactic nucleus along the jets. Assuming that these electrons emit synchrotron radiation and that they inverse-Compton (IC) up-scatter abundant target photons, which can either be the synchrotron photons themselves or photons from external emitters, the emitted spectrum can be theoretically determined. Additionally taking into account that these emission regions move relativistically themselves and that the emission is Doppler-boosted and beamed in forward direction, the typical two-hump spectral energy distribution (SED) of blazars is recovered. There are however findings that challenge this well-established model. Short-time variability, reaching down to minute scales at very high energy gamma rays, is today known to be a widespread phenomenon of blazars, calling for very compact emission regions. In most models of such optically thick emission regions, the gamma-ray flux is usually pair-absorbed exponentially, without considering the cascade evolving from the pair-produced electrons. From the observed flux, it is often concluded that emission emanates from larger distances where the region is optically thin, especially from outside of the BLR. Only in few blazars gamma-ray attenuation associated with pair absorption in the BLR was clearly reported. With the advent of sophisticated high-energy or very high energy gamma-ray detectors, like the Fermi Large Area Telescope or the Major Atmospheric Gamma-ray Imaging Cherenkov telescopes, besides the extraordinarily fast variability spectral features have been found that cannot be explained by conventional models reproducing the two-hump SED. Two such narrow spectral features are discussed in this work. For the nearby blazar Markarian 501, hints to a sharp peak around 3 TeV have been reported from a multi-wavelength campaign carried out in July 2014, while for 3C 279 a spectral dip was found in 2018 data, that can hardly be described with conventional fitting functions. In this work it is examined whether these spectral peculiarities of blazar jet emission can be explained, if the full radiation reprocessing through an IC pair cascade is accounted for. Such a cascade is the multiple concatenation of IC scattering events and pair production events. In the cascades generally considered in this work, relativistic electrons and high-energy photons are injected into a fixed soft target photon field. A mathematical description for linear IC pair cascades with escape terms is delivered on the basis of preliminary works. The steady-state kinetic equations for the electrons and for the photons are determined, whereby it is paid attention to an explicit formulation and to motivating the correct integration borders of all integrals from kinematic constraints. In determining the potentially observable gamma-ray flux, both the attenuated injected flux and the flux evolving as an effect of IC up-scattering, pair absorption and escape are incorporated, giving the emerging spectra very distinct imprints. Much effort is dedicated to the numerical solution of the electrons' kinetic equation via iterative schemes. It is explained why pointwise iteration from higher to lower Lorentz factors is more efficient than iterating the whole set of sampling points. The algorithm is parallelised at two positions. First, several workers can perform pointwise iterations simultaneously. Second, the most demanding integral is cut into a number of part integrals which can be determined by multiple workers. Through these measures, the Python code can be readily applied to simulate steady-state IC pair cascades with escape. In the case of Markarian 501 the developed framework is as follows. The AGN hosts an advection-dominated accretion flow with a normalised accretion rate of several \(10^{-4}\) and an electron temperature near \(10^{10}\) K. On the one hand, the accretion flow illuminates the few ambient gas clouds with approximate radius \(10^{11}\) m, which reprocess a fraction 0.01 of the luminosity into hydrogen and helium emission lines. On the other hand, the gamma rays from the accretion flow create electrons and positrons in a sporadically active vacuum gap in the BH magnetosphere. In the active gap, a power of roughly 0.001 of the Blandford-Znajek power is extracted from the rotating BH through a gap potential drop of several \(10^{18}\) V, generating ultra-relativistic electrons, which subsequently are multiplied by a factor of about \(10^6\) through interaction with the accretion flow photons. This electron beam propagates away from the central engine and encounters the photon field of one passing ionised cloud. The resulting IC pair cascade is simulated and the evolving gamma-ray spectrum is determined. Just above the absorption troughs due to the hydrogen lines, the spectrum exhibits a narrow bump around 3 TeV. When the cascaded emission is added to the emission generated at larger distances, the observed multi-wavelength SED including the sharp peak at 3 TeV is reproduced, underlining that radiation processes beyond conventional models are motivated by distinct spectral features. The dip in the spectrum of 3C 279 is addressed by a similar cascade model. Three types of injection are considered, varying in the ratio of the photon density to the electron density and varying in the spectral shape. The IC pair cascade is assumed to happen either in the dense BLR photon field with a luminosity of several \(10^{37}\) W and a radial size of few \(10^{14}\) m or in the diluted photon field outside of the BLR. The latter scenario is however rejected as the spectral slope around several 100 MeV and the dip at few 10 GeV cannot be reconciled within this model. The radiation cascaded in the BLR can explain the observational data, irrespective of the assumed injected rate. It is therefore concluded that for this period of gamma-ray emission, the radiation production happens at the edge of the BLR of 3C 279. Both investigations show that IC pair cascades can account for fine structure seen in blazar SEDs. It is insufficient to restrict the radiation transport to pure exponential absorption of an injection term. Pair production and IC up-scattering by all generations of photons and electrons in the optically thick regime critically shape the emerging spectra. As the advent of future improved detectors will provide more high-precision spectra, further observations of narrow spectral features can be expected. It seems therefore recommendable to incorporate cascading into conventional radiation production models or to extend the model developed in this work by synchrotron radiation.},
  subject      = {Active galactic nucleus},
  language  = {en}
}
@phdthesis{Dorner2008,
  author    = {Dorner, Daniela},
  title     = {Observations of PG 1553+113 with the MAGIC telescope},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28196},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {Blazars are among the most luminous sources in the universe. Their extreme short-time variability indicates emission processes powered by a supermassive black hole. With the current generation of Imaging Air Cherenkov Telescopes, these sources are explored at very high energies. Lowering the threshold below 100 GeV and improving the sensitivity of the telescopes, more and more blazars are discovered in this energy regime. For the MAGIC telescope, a low energy analysis has been developed allowing to reach energies of 50 GeV for the first time. The method is presented in this thesis at the example of PG 1553+113 measuring a spectrum between 50 GeV and 900 GeV. In the energy regime observed by MAGIC, strong attenuation of the gamma-rays is expected from pair production due to interactions of gamma-rays with low-energy photons from the extragalactic background light. For PG 1553+113, this provides the possibility to constrain the redshift of the source, which is still unknown. Well studied from radio to x-ray energies, PG 1553+113 was discovered in 2005 in the very high energy regime. In total, it was observed with the MAGIC telescope for 80~hours between April 2005 and April 2007. From more than three years of data taking, the MAGIC telescope provides huge amounts of data and a large number of files from various sources. To handle this data volume and to provide monitoring of the data quality, an automatic procedure is essential. Therefore, a concept for automatic data processing and management has been developed. Thanks to its flexibility, the concept is easily applicable to future projects. The implementation of an automatic analysis is running stable since three years in the data center in W{\"u}rzburg and provides consistent results of all MAGIC data, i.e. equal processing ensures comparability. In addition, this database controlled system allows for easy tests of new analysis methods and re-processing of all data with a new software version at the push of a button. At any stage, not only the availability of the data and its processing status is known, but also a large set of quality parameters and results can be queried from the database, facilitating quality checks, data selection and continuous monitoring of the telescope performance. By using the automatic analysis, the whole data sample can be analyzed in a reasonable amount of time, and the analyzers can concentrate on interpreting the results instead. For PG 1553+113, the tools and results of the automatic analysis were used. Compared to the previously published results, the software includes improvements as absolute pointing correction, absolute light calibration and improved quality and background-suppression cuts. In addition, newly developed analysis methods taking into account timing information were used. Based on the automatically produced results, the presented analysis was enhanced using a special low energy analysis. Part of the data were affected by absorption due to the Saharan Air Layer, i.e. sanddust in the atmosphere. Therefore, a new method has been developed, correcting for the effect of this meteorological phenomenon. Applying the method, the affected data could be corrected for apparent flux variations and effects of absorption on the spectrum, allowing to use the result for further studies. This is especially interesting, as these data were taken during a multi-wavelength campaign. For the whole data sample of 54 hours after quality checks, a signal from the position of PG 1553+113 was found with a significance of 15 standard deviations. Fitting a power law to the combined spectrum between 75 GeV and 900 GeV, yields a spectral slope of 4.1 +/- 0.2. Due to the low energy analysis, the spectrum could be extended to below 50 GeV. Fitting down to 48 GeV, the flux remains the same, but the slope changes to 3.7 +/- 0.1. The determined daily light curve shows that the integral flux above 150 GeV is consistent with a constant flux. Also for the spectral shape no significant variability was found in three years of observations. In July 2006, a multi-wavelength campaign was performed. Simultaneous data from the x-ray satellite Suzaku, the optical telescope KVA and the two Cherenkov experiments MAGIC and H.E.S.S. are available. Suzaku measured for the first time a spectrum up to 30 keV. The source was found to be at an intermediate flux level compared to previous x-ray measurements, and no short time variability was found in the continuous data sample of 41.1 ksec. Also in the gamma regime, no variability was found during the campaign. Assuming a maximum slope of 1.5 for the intrinsic spectrum, an upper limit of z < 0.74 was determined by deabsorbing the measured spectrum for the attenuation of photons by the extragalactic background light. For further studies, a redshift of z = 0.3 was assumed. Collecting various data from radio, infrared, optical, ultraviolet, x-ray and gama-ray energies, a spectral energy distribution was determined, including the simultaneous data of the multi-wavelength campaign. Fitting the simultaneous data with different synchrotron-self-compton models shows that the observed spectral shape can be explained with synchrotron-self-compton processes. The best result was obtained with a model assuming a log-parabolic electron distribution.},
  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{Langejahn2022,
  author    = {Langejahn, Marcus},
  title     = {Hard X-ray Properties of Relativistically Beamed Jets from Radio- and Gamma-Ray-Bright Blazars},
  doi       = {10.25972/OPUS-28200},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-282009},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {In this work I characterize the hard X-ray properties of blazars, active galactic nuclei with highly beamed emission, which are notoriously hard to detect in this energy range. I employ pre-defined samples of beamed AGN: the radio-selected MOJAVE and TANAMI samples, as well as the most recent gamma-ray-selected Fermi/LAT 4LAC catalog. The hard X-ray data is extracted from the 105-month all-sky survey maps of the Swift/BAT (Burst Alert Telescope) in the energy band of 20 keV to 100 keV. A great majority of both the MOJAVE and TANAMI samples are significantly detected, with signal-to noise ratios of the sources often just below the X-ray catalog signal thresholds. All blazar sub-types (FSRQs, BL Lacs) and radio galaxies show characteristic ranges of X-ray flux, luminosity, and photon index. Their properties are correlated with the corresponding SED's shape / peak frequency. The LogN-LogS distributions of the samples show a scarcity of blazars in the middle and lower X-ray flux range, indicating differing evolutionary paths between radio and X-ray emission, which is also suggested by the corresponding luminosity functions. Compared to the radio samples, the 4LAC sources are on average significantly less bright in the BAT band since this range often coincides with the spectral gap region between the two big SED emission bumps. Also, the spectral shapes differ notably, especially for the sub-type of BL Lacs. Using the parameter space of X-ray and gamma-ray photon indices, 35 blazar candidate sources can be assigned to either the FSRQ or BL Lac type with high certainty. The reason why many blazars are weak in this energy band can be traced back to a number of factors: the selection bias of the initial sample, differential evolution of the X-rays and the wavelengths in which the sample is defined, and the limited sensitivity of the observing instruments.},
  subject      = {Aktiver galaktischer Kern},
  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}
}