@article{RoelofsBlackburnLindahletal.2023, author = {Roelofs, Freek and Blackburn, Lindy and Lindahl, Greg and Doeleman, Sheperd S. and Johnson, Michael D. and Arras, Philipp and Chatterjee, Koushik and Emami, Razieh and Fromm, Christian and Fuentes, Antonio and Knollm{\"u}ller, Jakob and Kosogorov, Nikita and M{\"u}ller, Hendrik and Patel, Nimesh and Raymond, Alexander and Tiede, Paul and Traianou, Efthalia and Vega, Justin}, title = {The ngEHT analysis challenges}, series = {Galaxies}, volume = {11}, journal = {Galaxies}, number = {1}, issn = {2075-4434}, doi = {10.3390/galaxies11010012}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-304976}, year = {2023}, abstract = {The next-generation Event Horizon Telescope (ngEHT) will be a significant enhancement of the Event Horizon Telescope (EHT) array, with ∼10 new antennas and instrumental upgrades of existing antennas. The increased uv-coverage, sensitivity, and frequency coverage allow a wide range of new science opportunities to be explored. The ngEHT Analysis Challenges have been launched to inform the development of the ngEHT array design, science objectives, and analysis pathways. For each challenge, synthetic EHT and ngEHT datasets are generated from theoretical source models and released to the challenge participants, who analyze the datasets using image reconstruction and other methods. The submitted analysis results are evaluated with quantitative metrics. In this work, we report on the first two ngEHT Analysis Challenges. These have focused on static and dynamical models of M87* and Sgr A* and shown that high-quality movies of the extended jet structure of M87* and near-horizon hourly timescale variability of Sgr A* can be reconstructed by the reference ngEHT array in realistic observing conditions using current analysis algorithms. We identify areas where there is still room for improvement of these algorithms and analysis strategies. Other science cases and arrays will be explored in future challenges.}, language = {en} } @phdthesis{Meyer2008, author = {Meyer, Markus}, title = {Observations of a systematically selected sample of high frequency peaked BL Lac objects with the MAGIC telescope}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28115}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {At the beginning of regular observations with the MAGIC telescope in December 2004, all but one extragalactic sources detected at very high energy (VHE) gamma-rays belonged to the class of high frequency peaked BL Lac (HBL) objects. This motivated a systematic scan of candidate sources to increase the number of known sources and to study systematically their spectral properties. As candidate sources for VHE emission, X-ray bright HBLs were selected from a compilation of active galactic nuclei. The MAGIC observations took place from December 2004 to March 2006. The declination of the objects was restricted to values between -1.2° and +58.8° corresponding to a maximum zenith distance lower than 30° at culmination. Since gamma-rays are absorbed by photo-pair production in low energy background radiation fields, the redshift of the investigated objects was limitetd to z < 0.3. Under the assumption that HBLs generally emit the same energy flux at 1keV as at 200GeV, only the brightest X-ray sources were observed, leading to a cut in the X-ray flux of F(1keV) > 2µJy}. Of the fourteen sources observed, four have been detected: 1ES 1218+304 (for the first time at very high energies), 1ES 2344+514 (strong detection in a state of low activity), Mrk 421 and Mrk 501. A hint of a signal on a 3-sigma-level from the direction of 1ES 1011+496 has been observed. In the meantime the object has been confirmed as a source of VHE gamma-rays by a second MAGIC observation campaign triggered by an optical outburst. For ten sources, upper limits on their integral fluxes above 200GeV have been calculated on a 99\% confidence level. To cross calibrate the different data samples, collected during 14 months, bright muon ring images have been used, recorded as background events by the MAGIC telescope. Based on the development by Meyer (2003), the method has been improved and implemented into the automatic data analysis as a continuous monitor of the calibration and the point spread function of the optical system. While the ring images are generated by muons with small impact parameters, it could be shown that the image parameter distributions for muons with large impact parameters and gamma showers completely overlap, revealing these muons as the dominant background for gamma-ray observations below energies of 150GeV. The sample of HBLs (including all HBLs detected at VHE so far) has been investigated for correlations between broad-band spectral indices as determined from simultaneous optical, archival X-ray and radio luminosities, finding that the VHE emitting HBLs do not differ from the non-detected ones. In general the absorption corrected HBL gamma-ray luminosities at 200GeV are not higher than their X-ray luminosities at 1keV. Based on a complete X-ray BL Lac sample, the Hamburg/ROSAT X-ray BL Lac sample, the number of expected VHE sources has been estimated for the performed scan, finding a consistent number under the assumption of a 37\% completeness of the investigated sample and a 1keV-to-200GeV luminosity ratio of 1.4. An upper limit on the omnidirectional flux at 200GeV has been calculated by interpolating the sum over the observed fluxes and upper limits. Within the uncertainties, the result is in agreement with the expectations derived from the X-ray luminosity function of BL Lacs. For 1ES 1218+304 and 1ES 2344+514 the lightcurves have been derived, showing evidence for flux variability on a time scale of 17 days and 24h, respectively. In the case of 1ES 1218+304 variability has been reported for the first time at VHEs. For both sources the energy spectra have been reconstructed and discussed in the context of their broad band spectral energy distribution (SED), using a single zone synchrotron self Compton model. The SEDs are well fitted by the simulation even though the very high peak frequencies at gamma-rays push the model to its limits. The parameters derived from the simulation are in good agreement with the parameters found for similar HBLs.}, subject = {Aktiver galaktischer Kern}, 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} } @article{RomoliChakrabortyDorneretal.2018, author = {Romoli, Carlo and Chakraborty, Nachiketa and Dorner, Daniela and Taylor, Andrew and Blank, Michael}, title = {Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501}, series = {Galaxies}, volume = {6}, journal = {Galaxies}, number = {4}, organization = {FACT and H.E.S.S. Collaborations}, issn = {2075-4434}, doi = {10.3390/galaxies6040135}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-197580}, year = {2018}, abstract = {Flux distribution is an important tool to understand the variability processes in activegalactic nuclei. We now have available a great deal of observational evidences pointing towards thepresence of log-normal components in the high energy light curves, and different models have beenproposed to explain these data. Here, we collect some of the recent developments on this topic usingthe well-known blazar Mrk 501 as example of complex and interesting aspects coming from its fluxdistribution in different energy ranges and at different timescales. The observational data we refer toare those collected in a complementary manner by Fermi-LAT over multiple years, and by the FirstG-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the brightflare of June 2014}, language = {en} }