Browsing by Author "Kiehlmann, S."
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Item Association of IceCube neutrinos with radio sources observed at Owens Valley and Metsähovi Radio Observatories(EDP SCIENCES, 2021-06-10) Hovatta, T.; Lindfors, E.; Kiehlmann, S.; Max-Moerbeck, W.; Hodges, M.; Liodakis, I.; Lähteemäki, A.; Pearson, T. J.; Readhead, A.C.S.; Reeves, R. A.; Suutarinen, S.; Tammi, J.; Tornikoski, M.; Metsähovi Radio Observatory; Department of Electronics and Nanoengineering; Anne Lähteenmäki Group; University of Turku; Universidad de Chile; California Institute of Technology; Universidad de Concepción; Metsähovi Radio Observatory; Foundation for Research and Technology - HellasContext. Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei with relativistic jets, also known as blazars, have been considered to be one of the main candidates because of their ability to accelerate particles to high energies. Aims. We study the connection between radio emission and IceCube neutrino events using data from the Owens Valley Radio Observatory (OVRO) and Metsähovi Radio Observatory blazar monitoring programs. Methods. We identify sources in our radio monitoring sample that are positionally consistent with IceCube high-energy neutrino events. We estimate their mean flux density and variability amplitudes around the neutrino arrival time, and compare these with values from random samples to establish the significance of our results. Results. We find radio source associations within our samples with 15 high-energy neutrino events detected by IceCube. Nearly half of the associated sources are not detected in the -ray energies, but their radio variability properties and Doppler boosting factors are similar to the -ray detected objects in our sample, meaning that they could still be potential neutrino emitters. We find that the number of strongly flaring objects in our statistically complete OVRO samples is unlikely to be a random coincidence (at 2_ level). Conclusions. Based on our results, we conclude that although it is clear that not all neutrino events are associated with strong radio flaring blazars, observations of large-amplitude radio flares in a blazar at the same time as a neutrino event are unlikely to be a random coincidence.Item The Baikal-GVD neutrino telescope : search for high-energy cascades(Scuola Internazionale Superiore di Studi Avanzati (SISSA), 2022-03-18) Allakhverdyan, V. A.; Avrorin, A. D.; Avrorin, A. V.; Aynutdinov, V. M.; Bannasch, R.; Bardаčová, Z.; Belolaptikov, I. A.; Borina, I. V.; Brudanin, V. B.; Budnev, N. M.; Dik, V. Y.; Domogatsky, G. V.; Doroshenko, A. A.; Dvornický, R.; Dyachok, A. N.; Dzhilkibaev, Zh A.M.; Eckerová, E.; Elzhov, T. V.; Fajt, L.; Fialkovsky, S. V.; Gafarov, A. R.; Golubkov, K. V.; Gorshkov, N. S.; Gress, T. I.; Katulin, M. S.; Kebkal, K. G.; Kebkal, O. G.; Khramov, E. V.; Kolbin, M. M.; Konischev, K. V.; Kopański, K. A.; Korobchenko, A. V.; Koshechkin, A. P.; Kozhin, V. A.; Kruglov, M. V.; Kryukov, M. K.; Kulepov, V. F.; Malecki, Pa; Malyshkin, Y. M.; Milenin, M. B.; Mirgazov, R. R.; Naumov, D. V.; Nazari, V.; Noga, W.; Petukhov, D. P.; Pliskovsky, E. N.; Rozanov, M. I.; Rushay, V. D.; Hovatta, T.; Kiehlmann, S.; , Baikal-GVD Collaboration; Metsähovi Radio Observatory; Joint Institute for Nuclear Research; Institute for Nuclear Research of the Russian Academy of Sciences; EvoLogics GmbH; Comenius University Bratislava; Irkutsk State University; Czech Technical University in Prague; R.Y. Alekseev Nizhny Novgorod State Technical University; Institute of Nuclear Physics of the Polish Academy of Sciences; Lomonosov Moscow State University; Saint Petersburg State Marine Technical University; California Institute of TechnologyBaikal-GVD is a neutrino telescope currently under construction in Lake Baikal. GVD is formed by multi-megaton subarrays (clusters). The design of Baikal-GVD allows one to search for astrophysical neutrinos already at early phases of the array construction. We present here preliminary results of a search for high-energy neutrinos with GVD in 2019-2020.Item Early-time polarized optical light curve of GRB 131030A(2014) King, O.G.; Blinov, D.; Giannios, D.; Papadakis, I.; Angelakis, E.; Balokovic, M.; Fuhrmann, L.; Hovatta, T.; Khodade, P.; Kiehlmann, S.; Kylafis, N.; Kus, A.; Myserlis, I.; Modi, D.; Panopoulou, G.; Papamastorakis, I.; Pavlidou, V.; Pazderska, B.; Pazderski, E.; Pearson, T.J.; Rajarshi, C.; Ramaprakash, A.N.; Readhead, A.C.S.; Reig, P.; Tassis, K.; Zensus, J.A.Item First characterization of the emission behavior of Mrk 421 from radio to very high-energy gamma rays with simultaneous X-ray polarization measurements(EDP Sciences, 2024-04-01) Abe, S.; Abhir, J.; Acciari, V. A.; Agudo, I.; Aniello, T.; Ansoldi, S.; Antonelli, L. A.; Arbet-Engels, A.; Arcaro, C.; Artero, M.; Asano, K.; Babić, A.; Baquero, A.; de Almeida, U. Barres; Barrio, J. A.; Batković, I.; Baxter, J.; González, J. Becerra; Bednarek, W.; Bernardini, E.; Bernete, J.; Berti, A.; Besenrieder, J.; Bigongiari, C.; Biland, A.; Blanch, O.; Bonnoli, G.; Bošnjak; Burelli, I.; Busetto, G.; Campoy-Ordaz, A.; Carosi, A.; Carosi, R.; Carretero-Castrillo, M.; Castro-Tirado, A. J.; Ceribella, G.; Chai, Y.; Cifuentes, A.; Cikota, S.; Colombo, E.; Contreras, J. L.; Cortina, J.; Covino, S.; D'Ammando, F.; D'Amico, G.; D'Elia, V.; Da Vela, P.; Dazzi, F.; De Angelis, A.; De Lotto, B.; de Menezes, R.; Del Popolo, A.; Delgado, J.; Mendez, C. Delgado; Di Pierro, F.; Di Venere, L.; Prester, D. Dominis; Donini, A.; Dorner, D.; Doro, M.; Elsaesser, D.; Emery, G.; Escudero, J.; Fariña, L.; Fattorini, A.; Foffano, L.; Font, L.; Fröse, S.; Fukami, S.; Fukazawa, Y.; López, R. J.García; Garczarczyk, M.; Gasparyan, S.; Gaug, M.; Paiva, J. G.Giesbrecht; Giglietto, N.; Giordano, F.; Gliwny, P.; Godinović, N.; Gradetzke, T.; Grau, R.; Green, D.; Green, J. G.; Günther, P.; Hadasch, D.; Hahn, A.; Hassan, T.; Heckmann, L.; Herrera, J.; Hrupec, D.; Hütten, M.; Imazawa, R.; Inada, T.; Ishio, K.; Martínez, I. Jiménez; Jormanainen, J.; Kerszberg, D.; Kluge, G. W.; Kobayashi, Y.; Kouch, P. M.; Kubo, H.; Kushida, J.; Lezáun, M. Láinez; Lamastra, A.; Leone, F.; Lindfors, E.; Linhoff, L.; Lombardi, S.; Longo, F.; López-Coto, R.; López-Moya, M.; López-Oramas, A.; Loporchio, S.; Lorini, A.; de Oliveira Fraga, B. Machado; Majumdar, P.; Makariev, M.; Maneva, G.; Mang, N.; Manganaro, M.; Mangano, S.; Mannheim, K.; Mariotti, M.; Martínez, M.; Martínez-Chicharro, M.; Mas-Aguilar, A.; Mazin, D.; Menchiari, S.; Mender, S.; Miceli, D.; Miener, T.; Miranda, J. M.; Mirzoyan, R.; González, M. Molero; Molina, E.; Mondal, H. A.; Moralejo, A.; Morcuende, D.; Nakamori, T.; Nanci, C.; Nava, L.; Neustroev, V.; Nickel, L.; Rosillo, M. Nievas; Nigro, C.; Nikolić, L.; Nilsson, K.; Nishijima, K.; Ekoume, T. Njoh; Noda, K.; Nozaki, S.; Ohtani, Y.; Okumura, A.; Otero-Santos, J.; Paiano, S.; Palatiello, M.; Paneque, D.; Paoletti, R.; Paredes, J. M.; Pavlović, D.; Peresano, M.; Persic, M.; Pihet, M.; Pirola, G.; Podobnik, F.; Moroni, P. G.Prada; Prandini, E.; Principe, G.; Priyadarshi, C.; Rhode, W.; Ribó, M.; Rico, J.; Righi, C.; Sahakyan, N.; Saito, T.; Satalecka, K.; Saturni, F. G.; Schleicher, B.; Schmidt, K.; Schmuckermaier, F.; Schubert, J. L.; Schweizer, T.; Sciaccaluga, A.; Sitarek, J.; Sliusar, V.; Sobczynska, D.; Stamerra, A.; Strišković, J.; Strom, D.; Strzys, M.; Suda, Y.; Suutarinen, S.; Tajima, H.; Takahashi, M.; Tavecchio, F.; Temnikov, P.; Terauchi, K.; Terzić, T.; Teshima, M.; Tosti, L.; Truzzi, S.; Tutone, A.; Ubach, S.; van Scherpenberg, J.; Acosta, M. Vazquez; Ventura, S.; Viale, I.; Vigorito, C. F.; Vitale, V.; Vovk, I.; Walter, R.; Will, M.; Wunderlich, C.; Yamamoto, T.; Liodakis, I.; Jorstad, S. G.; Di Gesu, L.; Donnarumma, I.; Kim, D. E.; Marscher, A. P.; Middei, R.; Perri, M.; Puccetti, S.; Verrecchia, F.; Leto, C.; De La Calle Pérez, I.; Jiménez-Bailón, E.; Blinov, D.; Bourbah, I. G.; Kiehlmann, S.; Kontopodis, E.; Mandarakas, N.; Skalidis, R.; Vervelaki, A.; Aceituno, F. J.; Agís-González, B.; Sota, A.; Sasada, M.; Kawabata, K. S.; Uemura, M.; Mizuno, T.; Akitaya, H.; Casadio, C.; Myserlis, I.; Sievers, A.; Lähteenmäki, A.; Syrjärinne, I.; Tornikoski, M.; Salomé, Q.; Gurwell, M.; Keating, G. K.; Rao, R.; Metsähovi Radio Observatory; School common, ELEC; Department of Electronics and Nanoengineering; Anne Lähteenmäki Group; University of Tokyo; Swiss Federal Institute of Technology Zurich; University of La Laguna; CSIC - Institute of Astrophysics of Andalusia; National Institute for Astrophysics (INAF); University of Udine; Max Planck Institute for Physics; University of Padova; Institute for High Energy Physics; University of Zagreb; Universidad Complutense de Madrid; Centro Brasileiro de Pesquisas Físicas; University of Łódź; Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas - CIEMAT; Autonomous University of Barcelona; University of Pisa; University of Barcelona; INAF - Istituto di Radioastronomia; University of Bergen; University of Turin; University of Catania; Polytechnic University of Bari; TU Dortmund University; University of Geneva; Hiroshima University; German Electron Synchrotron; University of Split; University of Würzburg; Josip Juraj Strossmayer University of Osijek; University of Turku; Tokai University; University of Siena; Homi Bhabha National Institute; Bulgarian Academy of Sciences; Yamagata University; University of Oulu; Chiba University; Nagoya University; Metsähovi Radio Observatory; Kyoto University; National Institute for Nuclear Physics; Boston University; Italian Space Agency; Urbanización Villafranca Del Castillo; Foundation for Research and Technology - Hellas; University of Crete; California Institute of Technology; Tokyo Institute of Technology; Chiba Institute of Technology; Max Planck Institute for Radio Astronomy; Aalto University; Harvard UniversityAims. We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE. Methods. The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC, Fermi-LAT, NuSTAR, XMM-Newton, Swift, and several optical and radio telescopes to complement IXPE data. Results. During the IXPE exposures, the measured 0.2-1 TeV flux was close to the quiescent state and ranged from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the very high-energy (VHE) and X-ray emission are positively correlated at a 4σ significance level. The IXPE measurements reveal an X-ray polarization degree that is a factor of 2-5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, the Swift-XRT monitoring reveals an X-ray flux increase with a clear spectral hardening. This suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation, NuSTAR data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counterclockwise), implying important changes in the particle acceleration efficiency on approximately hour timescales.Item The hunt for extraterrestrial high-energy neutrino counterparts(EDP SCIENCES, 2022-09-30) Liodakis, I.; Hovatta, T.; Pavlidou, Vassiliki; Readhead, A. C. S.; Blandford, R. D.; Kiehlmann, S.; Lindfors, E.; Max-Moerbeck, W.; Pearson, T. J.; Petropoulou, M.; Metsähovi Radio Observatory; University of Turku; Technical University of Crete; National and Kapodistrian University of Athens; Foundation for Research and Technology - Hellas; Stanford University; Universidad de ChileThe origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental to our understanding of the high-energy Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater than those of gravitational waves; the sources are unknown, hard to predict, and we lack clear signatures. Neutrino astronomy therefore represents the greatest challenge faced by the astronomy and physics communities thus far. The possible neutrino sources range from accretion disks and tidal disruption events, to relativistic jets and galaxy clusters with blazar TXS 0506+056 the most compelling association thus far. Since that association, immense effort has been put into proving or disproving that jets are indeed neutrino emitters, but to no avail. By generating simulated neutrino counterpart samples, we explore the potential of detecting a significant correlation of neutrinos with jets from active galactic nuclei. We find that, given the existing challenges, even our best experiments could not have produced a > 3σ result. Larger programs over the next few years will be able to detect a significant correlation only if the brightest radio sources, rather than all jetted active galactic nuclei, are neutrino emitters. We discuss the necessary strategies required to steer future efforts into successful experiments.Item Jet kinematics in the transversely stratified jet of 3C 84 A two-decade overview(EDP SCIENCES, 2022-08-31) Paraschos, G. F.; Krichbaum, T. P.; Kim, J-Y; Hodgson, J. A.; Oh, J.; Ros, E.; Zensus, J. A.; Marscher, A. P.; Jorstad, S. G.; Gurwell, M. A.; Laehteenmaeki, A.; Tornikoski, M.; Kiehlmann, S.; Readhead, A. C. S.; Metsähovi Radio Observatory; Department of Electronics and Nanoengineering; Anne Lähteenmäki Group; Max Planck Institute of Neurobiology; Sejong University; University of Massachusetts Boston; Saint Petersburg State Electrotechnical University; Harvard-Smithsonian Center for Astrophysics; Jet Propulsion Laboratory; Foundation for Research and Technology - Hellas3C 84 (NGC 1275) is one of the brightest radio sources in the millimetre radio bands, which led to a plethora of very-long-baseline interferometry (VLBI) observations at numerous frequencies over the years. They reveal a two-sided jet structure, with an expanding but not well-collimated parsec-scale jet, pointing southward. High-resolution millimetre-VLBI observations allow the study and imaging of the jet base on a sub-parsec scale. This could facilitate the investigation of the nature of the jet origin, also in view of the previously detected two-railed jet structure and east-west oriented core region seen with RadioAstron at 22 GHz. We produced VLBI images of this core and inner jet region, observed over the past twenty years at 15, 43, and 86 GHz. We determined the kinematics of the inner jet and ejected features at 43 and 86 GHz and compared their ejection times with radio and -ray variability. For the moving jet features, we find an average velocity of βavg app = 0:055 - 0:22c (μavg = 0:04 - 0:18 mas yr-1). From the time-averaged VLBI images at the three frequencies, we measured the transverse jet width along the bulk flow. On the ≤1:5 parsec scale, we find a clear trend of the jet width being frequency dependent, with the jet being narrower at higher frequencies. This stratification is discussed in the context of a spine-sheath scenario, and we compare it to other possible interpretations. From quasi-simultaneous observations at 43 and 86 GHz, we obtain spectral index maps, revealing a time-variable orientation of the spectral index gradient due to structural variability of the inner jet.Item MOJAVE. XVII. Jet Kinematics and Parent Population Properties of Relativistically Beamed Radio-loud Blazars(IOP PUBLISHING LTD, 2019-03-20) Lister, M. L.; Homan, D. C.; Hovatta, T.; Kellermann, K.; Kiehlmann, S.; Kovalev, Y. Y.; Max-Moerbeck, W.; Pushkarev, A. B.; Readhead, A. C. S.; Ros, E.; Savolainen, T.; Metsähovi Radio Observatory; Department of Electronics and Nanoengineering; Anne Lähteenmäki Group; Purdue University; Denison University; National Radio Astronomy Observatory; Lebedev Physical Institute; Universidad de Chile; California Institute of Technology; Max-Planck-Institut für RadioastronomieWe present results from a parsec-scale jet kinematics study of 409 bright radio-loud active galactic nuclei (AGNs) based on 15 GHz Very Long Baseline Array (VLBA) data obtained between 1994 August 31 and 2016 December 26 as part of the 2 cm VLBA survey and Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) programs. We tracked 1744 individual bright features in 382 jets over at least 5 epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the >3σ level. Although most features within a jet typically have speeds within ∼40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc deprojected) of the core feature. Our results, combined with other speeds from the literature, indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best-fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power-law luminosity function incorporating pure luminosity evolution and a power-law Lorentz factor distribution ranging from 1.25 to 50 with slope -1.4 ± 0.2. The parent jets of the brightest radio quasars have a space density of 261 ± 19 Gpc -3 and unbeamed 15 GHz luminosities above ∼10 24.5 W Hz -1 , consistent with FR II class radio galaxies.Item A multi-band study and exploration of the radio wave-gamma-ray connection in 3C 84(EDP Sciences, 2023-01-03) Paraschos, G. F.; Mpisketzis, V.; Kim, J. Y.; Witzel, G.; Krichbaum, T. P.; Zensus, J. A.; Gurwell, M. A.; Lähteenmäki, A.; Tornikoski, M.; Kiehlmann, S.; Readhead, A. C.S.; Department of Electronics and Nanoengineering; Metsähovi Radio Observatory; Anne Lähteenmäki Group; Max Planck Institute for Radio Astronomy; National and Kapodistrian University of Athens; Harvard University; Foundation for Research and Technology - Hellas; California Institute of TechnologyTotal intensity variability light curves offer a unique insight into the ongoing debate about the launching mechanism of jets. For this work, we utilised the availability of radio and γ-ray light curves over a few decades of the radio source 3C 84 (NGC 1275). We calculated the multi-band time-lags between the flares identified in the light curves via discrete cross-correlation and Gaussian process regression. We find that the jet particle and magnetic field energy densities are in equipartition (kr-=-1.08-±-0.18). The jet apex is located z91.5-GHz-=-22-645-Rs (2- -20 ×10-3-pc) upstream of the 3 mm radio core; at that position, the magnetic field amplitude is Bcore91.5-GHz-=-3-10 G. Our results are in good agreement with earlier studies that utilised very-long-baseline interferometry. Furthermore, we investigated the temporal relation between the ejection of radio and γ-ray flares. Our results are in favour of the γ-ray emission being associated with the radio emission. We are able to tentatively connect the ejection of features identified at 43 and 86 GHz to prominent γ-ray flares. Finally, we computed the multiplicity parameter λ and the Michel magnetisation Ï M, and find that they are consistent with a jet launched by the Blandford & Znajek (1977, MNRAS, 179, 433) mechanism.Item Multi-epoch monitoring of TXS 0506+056 with MAGIC and MWL partners(Scuola Internazionale Superiore di Studi Avanzati (S I S S A), 2022-03-18) Satalecka, K.; Aniello, T.; Bernardini, E.; Bhattacharyya, W.; Cerruti, M.; D'Ammando, F.; Prandini, E.; Righi, C.; Sahakyan, N.; Viale, I.; Edwards, P. G.; Ojha, R.; Stevens, J.; Hovatta, Talvikki; Kiehlmann, S.; Readhead, A. C.S.; Eppel, F.; Gokus, A.; Heßdörfer, J.; Kadler, M.; Paraschos, G. F.; Sinapius, J.; Rösch, F.; , ATCA; , OVRO; , TELAMON Collaboration; , MAGIC Collaboration; Metsähovi Radio Observatory; German Electron Synchrotron; University of Padova; University of Barcelona; Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna; Osservatorio Astronomico di Brera; Commonwealth Scientific and Industrial Research Organisation (CSIRO); National Aeronautics and Space Administration; University of Crete; California Institute of Technology; University of Würzburg; Max Planck Institute for Radio AstronomyThe measurement of an astrophysical flux of high-energy neutrinos by IceCube is an important step towards finding the long-sought sources of cosmic rays. Nevertheless, the long exposure neutrino sky map shows no significant indication of point sources so far. The real-time followup of neutrino events turned out to be the most successful approach in neutrino point-source searches. It brought, among others, the most compelling evidence for a neutrino point source: the flaring gamma-ray blazar TXS 0506+056 in coincidence with a single high-energy neutrino from IceCube (IceCube-170922A). The fast multiwavelength(MWL) follow-up of this alert was key for establishing this coincidence and constraining the subsequent theoretical modeling for this event. In the long run, accurate and contemporaneous MWL spectral measurements are essential ingredients in investigating the physical processes leading to particle acceleration and emission of radiation. A deeper understanding of those processes allows us to put constraints on the potential neutrino emission. Here we present the light curves and simultaneous spectral energy distributions from November 2017 till February 2021 of MAGIC and MWL monitoring of TXS 0506+056. The more than two-year-long rise and high state of the radio light curve of TXS 0506+056, which started near the time of the IceCube neutrino detection, seems to have ended, as indicated by a steep decrease in the first half of 2021. We also present the theoretical interpretation of our observations.Item Multiwavelength behaviour of the blazar 3C 279: decade-long study from γ-ray to radio(OXFORD UNIV PRESS INC, 2020-03-01) Larionov, V. M.; Jorstad, S. G.; Marscher, A. P.; Villata, M.; Raiteri, C. M.; Smith, P. S.; Agudo, Ivan; Savchenko, S. S.; Morozova, D. A.; Acosta-Pulido, J. A.; Aller, M. F.; Aller, H. D.; Andreeva, T. S.; Arkharov, A. A.; Bachev, R.; Bonnoli, G.; Borman, G. A.; Bozhilov, V.; Calcidese, P.; Carnerero, M.; Carosati, D.; Casadio, C.; Chen, W-P; Damljanovic, G.; Dementyev, A.; Di Paola, A.; Frasca, A.; Fuentes, A.; Gomez, J. L.; Gonzalez-Morales, P.; Giunta, A.; Grishina, T. S.; Gurwell, M. A.; Hagen-Thorn, V. A.; Hovatta, Talvikki; Ibryamov, S.; Joshi, M.; Kiehlmann, S.; Kim, J-Y; Kimeridze, Givi N.; Kopatskaya, E. N.; Kovalev, Yu A.; Kovalev, Y.; Kurtanidze, O. M.; Kurtanidze, Sofia O.; Lahteenmaki, Anne; Lazaro, C.; Larionova, L.; Larionova, E. G.; Leto, G.; Marchini, A.; Matsumoto, K.; Mihov, B.; Minev, M.; Mingaliev, M. G.; Mirzaqulov, D.; Dimitrova, R. V. Munoz; Myserlis, I.; Nikiforova, A. A.; Nikolashvili, M. G.; Nizhelsky, N. A.; Ovcharov, E.; Pressburger, L. D.; Rakhimov, I. A.; Righini, S.; Rizzi, N.; Sadakane, K.; Sadun, A. C.; Samal, M. R.; Sanchez, R. Z.; Semkov, E.; Sergeev, S. G.; Sigua, L. A.; Slavcheva-Mihova, L.; Sola, P.; Sotnikova, Yu; Strigachev, A.; Thum, C.; Traianou, Efthalia; Troitskaya, Yu; Troitsky, I. S.; Tsybulev, P. G.; Vasilyev, A. A.; Vince, O.; Weaver, Z. R.; Williamson, K. E.; Zhekanis, G.; Metsähovi Radio Observatory; Department of Electronics and Nanoengineering; Anne Lähteenmäki Group; St. Petersburg State University; Boston University; Istituto Nazionale di Astrofisica (INAF); University of Arizona; Instituto de Astrofísica de Andalucía; University of La Laguna; University of Michigan; Russian Academy of Sciences; RAS - Pulkovo Astronomical Observatory; Bulgarian Academy of Sciences; University of Siena; Crimean Astrophysical Observatory; Technical University of Sofia; Osservatorio Astronomico della Regione Autonoma Valle d'Aosta; INAF - Osservatorio Astrofisico di Torino; EPT Observatories; Max-Planck-Institut für Radioastronomie; National Central University; Astronomical Observatory Belgrade; Osservatorio Astronomico di Roma; INAF - Osservatorio Astrofisico di Catania; Italian Space Agency; Harvard University; Konstantin Preslavsky University of Shumen; Abastumani Observatory; Lebedev Physical Institute; Osaka Kyoiku University; Kazan Federal University; Academy of Sciences of the Republic of Uzbekistan; INAF - Istituto di Radioastronomia; Osservatorio Astronomico Sirio; University of Colorado Denver; Instituto de Astrofísica de Canarias; Instituto de Radioastronomía Milimétrica; Special Astrophysical Observatory of the Russian Academy of Sciences; California Institute of TechnologyWe report the results of decade-long (2008-2018) γ -ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, Fermi and Swift data, as well as polarimetric and spectroscopic data. The X-ray and γ -ray light curves correlate well, with no delay ≳ 3 h, implying general cospatiality of the emission regions. The γ -ray-optical flux-flux relation changes with activity state, ranging from a linear to amore complex dependence. The behaviour of the Stokes parameters at optical and radio wavelengths, including 43 GHz Very Long Baseline Array images, supports either a predominantly helical magnetic field or motion of the radiating plasma along a spiral path. Apparent speeds of emission knots range from 10 to 37c, with the highest values requiring bulk Lorentz factors close to those needed to explain γ -ray variability on very short time-scales. The MgII emission line flux in the 'blue' and 'red' wings correlates with the optical synchrotron continuum flux density, possibly providing a variable source of seed photons for inverse Compton scattering. In the radio bands, we find progressive delays of the most prominent light-curve maxima with decreasing frequency, as expected from the frequency dependence of the τ= 1 surface of synchrotron self-absorption. The global maximum in the 86 GHz light curve becomes less prominent at lower frequencies, while a local maximum, appearing in 2014, strengthens toward decreasing frequencies, becoming pronounced at ∼5 GHz. These tendencies suggest different Doppler boosting of stratified radio-emitting zones in the jet.Item Multiwavelength Observations of the Blazar BL Lacertae: A New Fast TeV Gamma-Ray Flare(2018-04-01) Abeysekara, A. U.; Benbow, W.; Bird, R.; Brantseg, T.; Brose, R.; Buchovecky, M.; Buckley, J. H.; Bugaev, V.; Connolly, M. P.; Cui, W.; Daniel, M. K.; Falcone, A.; Feng, Q.; Finley, J. P.; Fortson, L.; Furniss, A.; Gillanders, G. H.; Gunawardhana, I.; Hütten, M.; Hanna, D.; Hervet, O.; Holder, J.; Hughes, G.; Humensky, T. B.; Johnson, C. A.; Kaaret, P.; Kar, P.; Kertzman, M.; Krennrich, F.; Lang, M. J.; Lin, T. T.Y.; McArthur, S.; Moriarty, P.; Mukherjee, R.; O'Brien, S.; Ong, R. A.; Otte, A. N.; Park, N.; Petrashyk, A.; Pohl, Martin; Pueschel, E.; Quinn, J.; Ragan, K.; Reynolds, P. T.; Richards, G. T.; Roache, E.; Rulten, C.; Sadeh, I.; Santander, M.; Sembroski, G. H.; Shahinyan, K.; Wakely, S. P.; Weinstein, A.; Wells, R. M.; Wilcox, P.; Williams, D. A.; Zitzer, B.; Jorstad, S. G.; Marscher, A. P.; Lister, M. L.; Kovalev, Y. Y.; Pushkarev, A. B.; Savolainen, T.; Agudo, I.; Molina, S. N.; Gómez, J. L.; Larionov, V. M.; Borman, G. A.; Mokrushina, A. A.; Tornikoski, M.; Lahteenmaki, A.; Chamani, Wara; Enestam, S.; Kiehlmann, S.; Hovatta, T.; Smith, P. S.; Pontrelli, P.; Metsähovi Radio Observatory; Department of Electronics and Nanoengineering; Anne Lähteenmäki Group; University of Utah; Harvard University; University of California, Los Angeles; Iowa State University; University of Potsdam; Washington University St. Louis; National University of Ireland, Galway; Purdue University; Pennsylvania State University; McGill University; University of Minnesota Twin Cities; California State University, East Bay; German Electron Synchrotron; University of California, Santa Cruz; University of Delaware; Columbia University; University of Iowa; DePauw University; University College Dublin; Georgia Institute of Technology; University of Chicago; Cork Institute of Technology; Boston University; RAS - P.N. Lebedev Physics Institute; CSIC; St. Petersburg State University; Crimean Astrophysical Observatory; University of Arizona; California Institute of Technology; University of TurkuCombined with measurements made by very-long-baseline interferometry, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL Lacertae observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). with a rise time of ∼2.3 hr and a decay time of ∼36 min. The peak flux above 200 GeV is (4.2 ± 0.6) ×10-6 photon m-2 s-1 measured with a 4-minute-binned light curve, corresponding to ∼180% of the flux that is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in Very Long Baseline Array observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models that invoke relativistic plasma passing stationary shocks.Item Multiwavelength observations of the lensed quasar PKS 1830-211 during the 2019 γ-ray flare(Oxford University Press, 2024-01-01) Vercellone, S.; Donnarumma, I.; Pittori, C.; Capitanio, F.; De Rosa, A.; Di Gesu, L.; Kiehlmann, S.; Iacolina, M. N.; Pellizzoni, P. A.; Egron, E.; Pacciani, L.; Piano, G.; Puccetti, S.; Righini, S.; Valente, G.; Verrecchia, F.; Vittorini, V.; Tavani, M.; Brocato, E.; Chen, A. W.; Hovatta, T.; Melis, A.; Max-Moerbeck, W.; Perrodin, D.; Pilia, M.; Pili, M.; Readhead, A. C.S.; Reeves, R.; Ridolfi, A.; Vitali, F.; Bulgarelli, A.; Cattaneo, P. W.; Lucarelli, F.; Morselli, A.; Trois, A.; Metsähovi Radio Observatory; Osservatorio Astronomico di Brera; Osservatorio Astronomico di Roma; National Institute for Astrophysics (INAF); Institute of Astrophysics; Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna; University of the Witwatersrand, Johannesburg; Universidad de Chile; California Institute of Technology; Universidad de Concepción; National Institute for Nuclear PhysicsPKS 1830 -211 is a γ-ray emitting, high-redshift ( z = 2.507 ±0.002), lensed flat-spectrum radio quasar. During the period 2019 mid-February to mid-April, this source underwent a series of strong γ-ray flares that were detected by both AGILE- GRID (Gamma-Ray Imaging Detector) and Fermi Large Area Telescope ( Fermi -LAT), reaching a maximum γ-ray flux of F E > 100 MeV ≈2 . 3 ×10 -5 photons cm -2 s -1 . Here, we report on a coordinated campaign from both on-ground [Medicina, Owens Valley Radio Observatory (OVRO), Rapid Eye Mount (REM), and Sardinia Radio Telescope (SRT)] and orbiting facilities (AGILE, Fermi , INTEGRAL , NuSTAR , Swift , and Chandra ), with the aim of investigating the multiwavelength properties of PKS 1830-211 through nearly simultaneous observations presented here for the first time. We find a possible break in the radio spectra in different epochs abo v e 15 GHz, and a clear maximum of the 15 GHz data approximately 110 d after the γ-ray main activity periods. The spectral energy distribution shows a very pronounced Compton dominance ( > 200) which challenges the canonical one-component emission model. Therefore, we propose that the cooled electrons of the first component are re- accelerated to a second component by, for example, kink or tearing instability during the γ-ray flaring periods. We also note that PKS 1830-211 could be a promising candidate for future observations with both Compton satellites [e.g. enhanced ASTROGAM (e-ASTROGAM)] and Cherenkov arrays [Cherenkov Telescope Array Observatory (CTAO)] which will help, thanks to their impro v ed sensitivity, in extending the data availability in energy bands currently unco v ered.Item Multiwavelength study of the gravitationally lensed blazar QSO B0218+357 between 2016 and 2020(OXFORD UNIV PRESS INC, 2022-02-01) Acciari, V. A.; Ansoldi, S.; Antonelli, L. A.; Engels, A. Arbet; Artero, M.; Asano, K.; Baack, D.; Babic, A.; Baquero, A.; Barres de Almeida, U.; Barrio, J. A.; Batkovic; Becerra Gonzalez, J.; Bednarek, W.; Bellizzi, L.; Bernardini, E.; Bernardos, M.; Berti, A.; Besenrieder, J.; Bhattacharyya, W.; Bigongiari, C.; Biland, A.; Blanch, O.; Bonnoli, G.; Bosnjak, Z.; Busetto, G.; Carosi, R.; Ceribella, G.; Cerruti, M.; Chai, Y.; Chilingarian, A.; Cikota, S.; Colak, S. M.; Colombo, E.; Contreras, J. L.; Cortina, J.; Covino, S.; D'Amico, G.; D'Elia, V.; Da Vela, P.; Dazzi, F.; De Angelis, A.; De Lotto, B.; Delfino, M.; Delgado, J.; Delgado Mendez, C.; Depaoli, D.; Di Pierro, F.; Di Venere, L.; Do Souto Espineira, E.; Prester, D. Dominis; Donini, A.; Dorner, D.; Doro, M.; Elsaesser, D.; Ramazani, V. Fallah; Fattorini, A.; Ferrara, G.; Fonseca, M.; Font, L.; Fruck, C.; Fukami, S.; Garcia Lopez, R. J.; Garczarczyk, M.; Gasparyan, S.; Gaug, M.; Giglietto, N.; Giordano, F.; Gliwny, P.; Godinovic, N.; Green, J. G.; Green, D.; Hadasch, D.; Hahn, A.; Heckmann, L.; Herrera, J.; Hoang, J.; Hrupec, D.; Hutten, M.; Inada, T.; Inoue, S.; Ishio, K.; Iwamura, Y.; Jimenez, I.; Jormanainen, J.; Jouvin, L.; Kajiwara, Y.; Karjalainen, M.; Kerszberg, D.; Kobayashi, Y.; Kubo, H.; Kushida, J.; Lamastra, A.; Lelas, D.; Leone, F.; Lindfors, E.; Lombardi, S.; Longo, F.; Lopez-Coto, R.; Lopez-Moya, M.; Lopez-Oramas, A.; Loporchio, S.; Machado de Oliveira Fraga, B.; Maggio, C.; Majumdar, P.; Makariev, M.; Mallamaci, M.; Maneva, G.; Manganaro, M.; Mannheim, K.; Maraschi, L.; Mariotti, M.; Martinez, M.; Mazin, D.; Menchiari, S.; Mender, S.; Micanovic, S.; Miceli, D.; Miener, T.; Minev, M.; Miranda, J. M.; Mirzoyan, R.; Molina, E.; Moralejo, A.; Morcuende, D.; Moreno, Diana; Moretti, E.; Neustroev, V.; Nigro, C.; Nilsson, K.; Nishijima, K.; Noda, K.; Nozaki, S.; Ohtani, Y.; Oka, T.; Otero-Santos, J.; Paiano, S.; Palatiello, M.; Paneque, D.; Paoletti, R.; Paredes, J. M.; Pavletic, L.; Penil, P.; Perennes, C.; Persic, M.; Moroni, P. G. Prada; Prandini, E.; Priyadarshi, C.; Puljak, Ivica; Rhode, W.; Ribo, M.; Rico, J.; Righi, C.; Rugliancich, A.; Saha, L.; Sahakyan, N.; Saito, T.; Sakurai, S.; Satalecka, K.; Saturni, F. G.; Schleicher, B.; Schmidt, K.; Schweizer, T.; Sitarek, J.; Snidaric; Sobczynska, D.; Spolon, A.; Stamerra, A.; Strom, D.; Strzys, M.; Suda, Y.; Suric, T.; Takahashi, M.; Tavecchio, F.; Temnikov, P.; Terzic, T.; Teshima, M.; Tosti, L.; Truzzi, S.; Tutone, A.; Ubach, S.; van Scherpenberg, J.; Vanzo, G.; Vazquez Acosta, M.; Ventura, S.; Verguilov, V.; Vigorito, C. F.; Vitale, V.; Vovk, I.; Will, M.; Wunderlich, C.; Zaric, D.; de Palma, F.; D'Ammando, F.; Barnacka, A.; Sahu, D. K.; Hodges, M.; Hovatta, T.; Kiehlmann, S.; Max-Moerbeck, W.; Readhead, A. C. S.; Reeves, R.; Pearson, T. J.; Lahteenmaki, A.; Bjorklund; Tornikoski, M.; Tammi, J.; Suutarinen, S.; Hada, K.; Niinuma, K.; Metsähovi Radio Observatory; Department of Electronics and Nanoengineering; School common, ELEC; Anne Lähteenmäki Group; Department of Electronics and Nanoengineering; School common, ELECWe report multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 in 2016-2020. Optical, X-ray, and GeV flares were detected. The contemporaneous MAGIC observations do not show significant very high energy (VHE; ≳100 GeV) gamma-ray emission. The lack of enhancement in radio emission measured by The Owens Valley Radio Observatory indicates the multizone nature of the emission from this object. We constrain the VHE duty cycle of the source to be <16 2014-like flares per year (95 per cent confidence). For the first time for this source, a broad-band low-state spectral energy distribution is constructed with a deep exposure up to the VHE range. A flux upper limit on the low-state VHE gamma-ray emission of an order of magnitude below that of the 2014 flare is determined. The X-ray data are used to fit the column density of (8.10 ± 0.93stat) × 1021 cm-2 of the dust in the lensing galaxy. VLBI observations show a clear radio core and jet components in both lensed images, yet no significant movement of the components is seen. The radio measurements are used to model the source-lens-observer geometry and determine the magnifications and time delays for both components. The quiescent emission is modelled with the high-energy bump explained as a combination of synchrotron-self-Compton and external Compton emission from a region located outside of the broad-line region. The bulk of the low-energy emission is explained as originating from a tens-of-parsecs scale jet.Item New Tests of Milli-lensing in the Blazar PKS 1413 + 135(IOP Publishing Ltd., 2022-03-01) Peirson, A. L.; Liodakis, I.; Readhead, A. C.S.; Lister, M. L.; Perlman, E. S.; Aller, M. F.; Blandford, R. D.; Grainge, K. J.B.; Green, D. A.; Gurwell, M. A.; Hodges, M. W.; Hovatta, T.; Kiehlmann, S.; Lähteenmäki, A.; Max-Moerbeck, W.; McAloone, T.; O'Neill, S.; Pavlidou, V.; Pearson, T. J.; Ravi, V.; Reeves, R. A.; Scott, P. F.; Taylor, G. B.; Titterington, D. J.; Tornikoski, M.; Vedantham, H. K.; Wilkinson, P. N.; Williams, D. T.; Zensus, J. A.; Metsähovi Radio Observatory; Department of Electronics and Nanoengineering; Anne Lähteenmäki Group; Stanford University; University of Turku; California Institute of Technology; Purdue University; Florida Institute of Technology; University of Michigan, Ann Arbor; University of Manchester; University of Cambridge; Harvard University; Universidad de Chile; University of Crete; Universidad de Concepción; University of New Mexico; Netherlands Institute for Radio Astronomy; Max Planck Institute for Radio Astronomy; Foundation for Research and Technology - HellasSymmetric achromatic variability (SAV) is a rare form of radio variability in blazars that has been attributed to gravitational milli-lensing by a ∼102-105 M ⊙ mass condensate. Four SAVs have been identified between 1980 and 2020 in the long-term radio monitoring data of the blazar PKS 1413 + 135. We show that all four can be fitted with the same, unchanging, gravitational lens model. If SAV is due to gravitational milli-lensing, PKS 1413 + 135 provides a unique system for studying active galactic nuclei with unprecedented microarcsecond resolution, as well as for studying the nature of the milli-lens itself. We discuss two possible candidates for the putative milli-lens: a giant molecular cloud hosted in the intervening edge-on spiral galaxy, and an undetected dwarf galaxy with a massive black hole. We find a significant dependence of SAV crossing time on frequency, which could indicate a fast shock moving in a slower underlying flow. We also find tentative evidence for a 989 day periodicity in the SAVs, which, if real, makes possible the prediction of future SAVs: the next three windows for possible SAVs begin in 2022 August, 2025 May, and 2028 February.Item Optical EVPA rotations in blazars(OXFORD UNIV PRESS INC, 2017-12) Kiehlmann, S.; Blinov, D.; Pearson, T. J.; Liodakis, I.; Metsähovi Radio Observatory; St. Petersburg State University; California Institute of Technology; Stanford University; Department of Radio Science and EngineeringWe identify rotations of the polarization angle in a sample of blazars observed for three seasons with the RoboPol instrument. A simplistic stochastic variability model is tested against this sample of rotation events. The model is capable of producing samples of rotations with parameters similar to the observed ones, but fails to reproduce the polarization fraction at the same time. Even though we can neither accept nor conclusively reject the model, we point out various aspects of the observations that are fully consistent with a random walk process.Item Optical polarization map of the Polaris Flare with RoboPol(2015) Panopoulou, G.; Tassis, K.; Blinov, D.; Pavlidou, V.; King, O.G.; Paleologou, E.; Ramaprakash, A.; Angelakis, E.; Balokovic, M.; Das, H.K.; Feiler, R.; Hovatta, T.; Khodade, P.; Kiehlmann, S.; Kus, A.; Kylafis, N.; Liodakis, I.; Mahabal, A.; Modi, D.; Myserlis, I.; Papadakis, I.; Papamastorakis, I.; Pazderska, B.; Pazderski, E.; Pearson, T.J.; Rajarshi, C.; Readhead, A.C.S.; Reig, P.; Zensus, J.A.; Metsähovi Radio ObservatoryItem Optical polarization of high-energy BL Lacertae objects(2016-12-01) Hovatta, T.; Lindfors, E.; Blinov, D.; Pavlidou, V.; Nilsson, K.; Kiehlmann, S.; Angelakis, E.; Fallah Ramazani, V.; Liodakis, I.; Myserlis, I.; Panopoulou, G. V.; Pursimo, T.; Department of Radio Science and Engineering; Metsähovi Radio Observatory; University of Turku; St. Petersburg State University; University of Crete; Max Planck Institute for Radio Astronomy; Nordic Optical TelescopeContext. We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. Aims. We wish to understand if there are differences between the BL Lac objects that have been detected with the current-generation TeV instruments and those objects that have not yet been detected. Methods. We used a maximum-likelihood method to investigate the optical polarization fraction and its variability in these sources. In order to study the polarization position angle variability, we calculated the time derivative of the electric vector position angle (EVPA) change. We also studied the spread in the Stokes Q/I-U/I plane and rotations in the polarization plane. Results. The mean polarization fraction of the TeV-detected BL Lacs is 5%, while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q/I-U/I plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seem to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations. Conclusions. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and non-TeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location.Item Polarization angle swings in blazars: The case of 3C 279(2016-04-28) Kiehlmann, S.; Savolainen, T.; Jorstad, S. G.; Sokolovsky, K. V.; Schinzel, F. K.; Marscher, A. P.; Larionov, V. M.; Agudo, I.; Akitaya, H.; Benítez, E.; Berdyugin, A.; Blinov, D. A.; Bochkarev, N. G.; Borman, G. A.; Burenkov, A. N.; Casadio, C.; Doroshenko, V. T.; Efimova, N. V.; Fukazawa, Y.; Gómez, J. L.; Grishina, T. S.; Hagen-Thorn, V. A.; Heidt, J.; Hiriart, D.; Itoh, R.; Joshi, M.; Kawabata, K. S.; Kimeridze, G. N.; Kopatskaya, E. N.; Korobtsev, I. V.; Krajci, T.; Kurtanidze, O. M.; Kurtanidze, S. O.; Larionova, E. G.; Larionova, L. V.; Lindfors, E.; López, J. M.; McHardy, I. M.; Molina, S. N.; Moritani, Y.; Morozova, D. A.; Nazarov, S. V.; Nikolashvili, M. G.; Nilsson, K.; Pulatova, N. G.; Reinthal, R.; Sadun, A.; Sasada, M.; Savchenko, S. S.; Sergeev, S. G.; Sigua, L. A.; Smith, P. S.; Sorcia, M.; Spiridonova, O. I.; Takaki, K.; Takalo, L. O.; Taylor, B.; Troitsky, I. S.; Uemura, M.; Ugolkova, L. S.; Ui, T.; Yoshida, M.; Zensus, J. A.; Zhdanova, V. E.; Metsähovi Radio Observatory; Department of Radio Science and Engineering; Anne Lähteenmäki Group; St. Petersburg State University; National Observatory of Athens; University of New Mexico; Boston University; RAS - Pulkovo Astronomical Observatory; CSIC; Hiroshima University; Universidad Nacional Autónoma de México; University of Turku; University of Crete; Lomonosov Moscow State University; Crimean Astrophysical Observatory; Special Astrophysical Observatory of the Russian Academy of Sciences; Heidelberg University ; Abastumani Observatory; RAS - Institute of Solar-Terrestrial Physics, Siberian Branch; Center for Backyard Astrophysics; Kazan Federal University; University of Southampton; University of Tokyo; University of Colorado Boulder; University of Arizona; Lowell Observatory; Max Planck Institute for Radio AstronomyContext. Over the past few years, on several occasions, large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy γ-ray flares and they have attracted considerable attention, since they could allow us to probe the magnetic field structure in the γ-ray emitting region of the jet. The flat-spectrum radio quasar 3C 279 is one of the most prominent examples showing this behaviour. Aims. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. Methods. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C 279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of 2008-2012. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data. Results. 3C 279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth ∼360° rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. Conclusions. We conclude that, during the two different optical flux states, two different processes govern polarization variation, which is possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity.Item The presence of interstellar scintillation in the 15 GHz interday variability of 1158 OVRO-monitored blazars(OXFORD UNIV PRESS INC, 2019-11) Koay, J. Y.; Jauncey, D. L.; Hovatta, Talvikki; Kiehlmann, S.; Bignall, H. E.; Max-Moerbeck, W.; Pearson, T. J.; Readhead, A. C. S.; Reeves, R.; Reynolds, C.; Vedantham, H.; Metsähovi Radio Observatory; Academia Sinica Institute of Astronomy and Astrophysics; Commonwealth Scientific and Industrial Research Organisation (CSIRO); Universidad de Chile; California Institute of Technology; Universidad de ConcepciónWe have conducted the first systematic search for interday variability in a large sample of extragalactic radio sources at 15 GHz. From the sample of 1158 radio-selected blazars monitored over an similar to 10 yr span by the Owens Valley Radio Observatory 40-m telescope, we identified 20 sources exhibiting significant flux density variations on 4-d time-scales. The sky distribution of the variable sources is strongly dependent on the line-of-sight Galactic H alpha intensities from the Wisconsin H alpha Mapper Survey, demonstrating the contribution of interstellar scintillation (ISS) to their interday variability. 21 per cent of sources observed through sightlines with H alpha intensities larger than 10 rayleighs exhibit significant ISS persistent over the similar to 10 yr period. The fraction of scintillators is potentially larger when considering less significant variables missed by our selection criteria, due to ISS intermittency. This study demonstrates that ISS is still important at 15 GHz, particularly through strongly scattered sightlines of the Galaxy. Of the 20 most significant variables, 11 are observed through the Orion-Eridanus superbubble, photoionized by hot stars of the Orion OB1 association. The high-energy neutrino source TXS 0506+056 is observed through this region, so ISS must be considered in any interpretation of its short-term radio variability. J0616-1041 appears to exhibit large similar to 20 per cent interday flux density variations, comparable in magnitude to that of the very rare class of extreme, intrahour scintillators that includes PKS0405-385, J1819+3845, and PKS1257-326; this needs to be confirmed by higher cadence follow-up observations.Item RoboPol: A four-channel optical imaging polarimeter(OXFORD UNIV PRESS INC, 2019-05) Ramaprakash, A. N.; Rajarshi, C. V.; Das, H. K.; Khodade, P.; Modi, D.; Panopoulou, G.; Maharana, S.; Blinov, D.; Angelakis, E.; Casadio, C.; Fuhrmann, L.; Hovatta, T.; Kiehlmann, S.; King, O. G.; Kylafis, N.; Kougentakis, A.; Kus, A.; Mahabal, A.; Marecki, A.; Myserlis, I.; Paterakis, G.; Paleologou, E.; Liodakis, I.; Papadakis, I.; Papamastorakis, I.; Pavlidou, V.; Pazderski, E.; Pearson, T. J.; Readhead, A. C.S.; Reig, P.; Słowikowska, A.; Tassis, K.; Zensus, J. A.; Metsähovi Radio Observatory; Inter-University Centre for Astronomy and Astrophysics India; California Institute of Technology; Foundation for Research and Technology - Hellas; Max Planck Institute for Radio Astronomy; Nicolaus Copernicus University in Toruń; Stanford UniversityWe present the design and performance of RoboPol, a four-channel optical polarimeter operating at the Skinakas Observatory in Crete, Greece. RoboPol is capable of measuring both relative linear Stokes parameters q and u (and the total intensity I) in one sky exposure. Though primarily used to measure the polarization of point sources in the R band, the instrument features additional filters (B, V, and I), enabling multiwavelength imaging polarimetry over a large field of view (13.6 × 13.6). We demonstrate the accuracy and stability of the instrument throughout its 5 yr of operation. Best performance is achieved within the central region of the field of view and in the R band. For such measurements the systematic uncertainty is below 0.1 per cent in fractional linear polarization, p (0.05 per cent maximum likelihood). Throughout all observing seasons the instrumental polarization varies within 0.1 per cent in p and within ∼1◦ in polarization angle.