@article{GonzalezDornerBretzetal.2019,
  author    = {Gonz{\´a}lez, Mar{\´i}a Magdalena and Dorner, Daniela and Bretz, Thomas and Garc{\´i}a-Gonz{\´a}lez, Jos{\´e} Andr{\´e}s},
  title     = {Unbiased long-term monitoring at TeV energies},
  series = {Galaxies},
  volume    = {7},
  journal   = {Galaxies},
  number    = {2},
  issn      = {2075-4434},
  doi       = {10.3390/galaxies7020051},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-197389},
  year      = {2019},
  abstract  = {For the understanding of the variable, transient and non-thermal universe, unbiased long-term monitoring is crucial. To constrain the emission mechanisms at the highest energies, it is important to characterize the very high energy emission and its correlation with observations at other wavelengths. At very high energies, only a limited number of instruments is available. This article reviews the current status of monitoring of the extra-galactic sky at TeV energies.},
  language  = {en}
}
@phdthesis{Bretz2006,
  author    = {Bretz, Thomas},
  title     = {Observations of the Active Galactic Nucleus 1ES1218+304 with the MAGIC-telescope},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-19240},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2006},
  abstract  = {The astronomical exploration at energies between 30\,GeV and \$\lesssim\$\,350\,GeV was the main motivation for building the \MAGIC-telescope. With its 17\,m \diameter\ mirror it is the worldwide largest imaging air-Cherenkov telescope. It is located at the Roque de los Muchachos at the Canary island of San Miguel de La Palma at 28.8\$^\circ\$\,N, 17.8\$^\circ\$\,W, 2200\,m a.s.l. The telescope detects Cherenkov light produced by relativistic electrons and positrons in air showers initiated by cosmic gamma-rays. The imaging technique is used to powerfully reject the background due to hadronically induced air showers from cosmic rays. Their inverse power-law energy-distribution leads to an increase of the event rate with decreasing energy threshold. For \MAGIC this implies a trigger rate in the order of 250\,Hz, and a correspondingly large data stream to be recorded and analyzed. A robust analysis software package, including the general framework \MARS, was developed and commissioned to allow automation, necessary for data taken under variable observing conditions. Since many of the astronomical sources of high-energy radiation, in particular the enigmatic gamma-ray bursts, are of a transient nature, the telescope was designed to allow repositioning in several tens of seconds, keeping a tracking accuracy of \$\lesssim\,\$0.01\$^\circ\$. Employing a starguider, a tracking accuracy of \$\lesssim\,\$1.3\,minutes of arc was obtained. The main class of sources at very high gamma-ray energies, known from previous imaging air-Cherenkov telescopes, are Active Galactic Nuclei with relativistic jets, the so-called high-peaked Blazars. Their spectrum is entirely dominated by non-thermal emission, spanning more than 15 orders of magnitude in energy, from radio to gamma-ray energies. Predictions based on radiation models invoking a synchrotron self-Compton or hadronic origin of the gamma-rays suggest, that a fairly large number of them should be detectable by \MAGIC. Promising candidates have been chosen from existing compilations, requiring high (synchrotron) X-ray flux, assumed to be related to a high (possibly inverse-Compton) flux at GeV energies, and a low distance, in oder to avoid strong attenuation due to pair-production in interactions with low-energy photons from the extragalactic background radiation along the line of sight. Based on this selection the first \AGN, emitting gamma-rays at 100\,GeV, 1ES\,1218+304 at a redshift of \$z=0.182\$, was discovered, one of the two farthest known \AGN emitting in the TeV energy region. In this context, the automated analysis chain was successfully demonstrated. The source was observed in January 2005 during six moonless nights for 8.2\,h. At the same time the collaborating \KVA-telescope, located near the \MAGIC site, observed in the optical band. The lightcurve calculated showed no day-to-day variability and is compatible with a constant flux of \$F(\$\,\$>\$\,\$100\,\mbox{GeV})=(8.7\pm1.4) \cdot 10^{-7}\,\mbox{m}^{-2}\,\mbox{s}^{-1}\$ within the statistical errors. A differential spectrum between 87\,GeV and 630\,GeV was calculated and is compatible with a power law of \$F_E(E) = (8.1\pm 2.1) \cdot 10^{-7}(E/\mbox{250\,GeV})^{-3.0\pm0.4}\,\mbox{TeV}^{-1}\,\mbox{m}^{-2}\,\mbox{s}^{-1}\$ within the statistical errors. The spectrum emitted by the source was obtained by taking into account the attenuation due to pair-production with photons of the extragalactic background at low photon energies. A homogeneous, one-zone synchrotron self-Compton model has been fitted to the collected multi-wavelength data. Using the simultaneous optical data, a best fit model could be obtained from which some physical properties of the emitting plasma could be inferred. The result was compared with the so-called {\em Blazar sequence}.},
  subject      = {Aktiver galaktischer Kern},
  language  = {en}
}
@article{TemmeAdamAhnenetal.2017,
  author    = {Temme, Fabian and Adam, Jan and Ahnen, Max L. and Baack, Dominik and Balbo, Matteo and Bergmann, Matthias and Biland, Adrian and Blank, Michael and Bretz, Thomas and Br{\"u}gge, Kai A. and Buss, Jens and Dmytriiev, Anton and Dorner, Daniela and Einecke, Sabrina and Hempfling, Christina and Hildebrand, Dorothee and Hughes, Gareth and Linhoff, Lena and Mannheim, Karl and M{\"u}ller, Sebastian and Neise, Dominik and Neronov, Andrii and N{\"o}the, Max and Paravac, Aleksander and Pauss, Felicitas and Rhode, Wolfgang and Shukla, Amit and Thaele, Julia and Walter, Roland},
  title     = {Long-Term monitoring of bright blazars in the multi-GeV to TeV range with FACT},
  series = {Galaxies},
  volume    = {5},
  journal   = {Galaxies},
  number    = {1},
  publisher = {MDPI},
  issn      = {2075-4434},
  doi       = {10.3390/galaxies5010018},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-198088},
  pages     = {18},
  year      = {2017},
  abstract  = {Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable, with changing activity levels on timescales between minutes, months, and even years. Several questions are part of the current research, such as the question of the emission regions or the engine of the AGN and the particle acceleration. A dedicated longterm monitoring program is necessary to investigate the properties of blazars in detail. A densely sampled and unbiased light curve allows for observation of both high and low states of the sources, and the combination with multi-wavelength observation could contribute to the answer of several questions mentioned above. FACT (First G-APD Cherenkov Telescope) is the first operational telescope using silicon photomultiplier (SiPM, also known as Geigermode—Avalanche Photo Diode, G-APD) as photon detectors. SiPM have a very homogenous and stable longterm performance, and allow operation even during full moon without any filter, leading to a maximal duty cycle for an Imaging Air Cherenkov Telescope (IACT). Hence, FACT is an ideal device for such a longterm monitoring of bright blazars. A small set of sources (e.g., Markarian 421, Markarian 501, 1ES 1959+650, and 1ES 2344+51.4) is currently being monitored. In this contribution, the FACT telescope and the concept of longterm monitoring of bright blazars will be introduced. The results of the monitoring program will be shown, and the advantages of densely sampled and unbiased light curves will be discussed.},
  language  = {en}
}
@article{SchleicherArbetEngelsBaacketal.2019,
  author    = {Schleicher, Bernd and Arbet-Engels, Axel and Baack, Dominik and Balbo, Matteo and Biland, Adrian and Blank, Michael and Bretz, Thomas and Bruegge, Kai and Bulinski, Michael and Buss, Jens and Doerr, Manuel and Dorner, Daniela and Elsaesser, Dominik and Grischagin, Sergej and Hildebrand, Dorothee and Linhoff, Lena and Mannheim, Karl and Mueller, Sebastian Achim and Neise, Dominik and Neronov, Andrii and Noethe, Maximilian and Paravac, Aleksander and Rhode, Wolfgang and Schulz, Florian and Sedlaczek, Kevin and Shukla, Amit and Sliusar, Vitalii and Willert, Elan and Walter, Roland},
  title     = {Fractional Variability—A Tool to Study Blazar Variability},
  series = {Galaxies},
  volume    = {7},
  journal   = {Galaxies},
  number    = {2},
  issn      = {2075-4434},
  doi       = {10.3390/galaxies7020062},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-197348},
  year      = {2019},
  abstract  = {Active Galactic Nuclei emit radiation over the whole electromagnetic spectrum up to TeV energies. Blazars are one subtype with their jets pointing towards the observer. One of their typical features is extreme variability on timescales, from minutes to years. The fractional variability is an often used parameter for investigating the degree of variability of a light curve. Different detection methods and sensitivities of the instruments result in differently binned data and light curves with gaps. As they can influence the physics interpretation of the broadband variability, the effects of these differences on the fractional variability need to be studied. In this paper, we study the systematic effects of completeness in time coverage and the sampling rate. Using public data from instruments monitoring blazars in various energy ranges, we study the variability of the bright TeV blazars Mrk 421 and Mrk 501 over the electromagnetic spectrum, taking into account the systematic effects, and compare our findings with previous results. Especially in the TeV range, the fractional variability is higher than in previous studies, which can be explained by the much longer (seven years compared to few weeks) and more complete data sample.},
  language  = {en}
}