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Full list of authors: Jorstad, S. G.; Marscher, A. P.; Raiteri, C. M.; Villata, M.; Weaver, Z. R.; Zhang, H.; Dong, L.; Gomez, J. L.; Perel, M., V; Savchenko, S. S.; Larionov, V. M.; Carosati, D.; Chen, W. P.; Kurtanidze, O. M.; Marchini, A.; Matsumoto, K.; Mortari, F.; Aceti, P.; Acosta-Pulido, J. A.; Andreeva, T.; Apolonio, G.; Arena, C.; Arkharov, A.; Bachev, R.; Bonnoli, G.; Borman, G. A.; Bozhilov, V; Carnerero, M., I; Damljanovic, G.; Ehgamberdiev, S. A.; Elsasser, D.; Frasca, A.; Gabellini, D.; Grishina, T. S.; Gupta, A. C.; Hagen-Thorn, V. A.; Hallum, M. K.; Hart, M.; Hasuda, K.; Hemrich, F.; Hsiao, H. Y.; Ibryamov, S.; Irsmambetova, T. R.; Ivanov, D., V; Joner, M. D.; Kimeridze, G. N.; Klimanov, S. A.; Knoett, J.; Kopatskaya, E. N.; Kurtanidze, S. O.; Kurtenkov, A.; Kuutma, T.; Larionova, E. G.; Leonini, S.; Lin, H. C.; Lorey, C.; Mannheim, K.; Marino, G.; Minev, M.; Mirzaqulov, D. O.; Morozova, D. A.; Nikiforova, A. A.; Nikolashvili, M. G.; Ovcharov, E.; Papini, R.; Pursimo, T.; Rahimov, I; Reinhart, D.; Sakamoto, T.; Salvaggio, F.; Semkov, E.; Shakhovskoy, D. N.; Sigua, L. A.; Steineke, R.; Stojanovic, M.; Strigachev, A.; Troitskaya, Y., V; Troitskiy, I. S.; Tsai, A.; Valcheva, A.; Vasilyev, A. A.; Vince, O.; Waller, L.; Zaharieva, E.; Chatterjee, R., Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales1,2,3. This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare4 in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet5, plasma instabilities6,7 or orbital motion in an accretion disc7,8. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref. 9). BL Lac, the prototype of a subclass of blazars10, is powered by a 1.7 × 108 MSun (ref. 11) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref. 12)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities6 near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image. © 2022 Springer Nature Limited., The research reported here is based on work supported in part by US National Science Foundation grants AST-2108622 and AST-2107806, and NASA Fermi GI grants 80NSSC20K1567, 80NSSC21K1917 and 80NSSC21K1951; by Shota Rustaveli National Science Foundation of Georgia under contract FR-19-6174; by the Bulgarian National Science Fund of the Ministry of Education and Science under grants DN 18-10/2017, DN 18-13/2017, KP-06-H28/3 (2018), KP-06-H38/4 (2019) and KP-06-KITAJ/2 (2020), and by National RI Roadmap Project D01-383/18.12.2020 of the Ministry of Education and Science of the Republic of Bulgaria; by JSPS KAKENHI grant #19K03930 of Japan; by the Ministry of Education, Science and Technological Development of the Republic of Serbia (contract 451-03-9/2021-14/200002) and observing grant support from the Institute of Astronomy and Rozhen NAO BAS through the bilateral joint research project ‘Gaia Celestial Reference Frame (CRF) and fast variable astronomical objects’; by the Agenzia Spaziale Italiana (ASI) through contracts I/037/08/0, I/058/10/0, 2014-025-R.0, 2014-025-R.1.2015 and 2018-24-HH.0 to the Italian Istituto Nazionale di Astrofisica (INAF). H.Z. is supported by the NASA Postdoctoral Program at Goddard Space Flight Center, administered by ORAU. M.V.P. is partially supported by the Russian Foundation for Basic Research grant 20-02-00490. G.B. acknowledges support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and from the Spanish ‘Ministerio de Ciencia e Innovacíon’ (MICINN) through grant PID2019-107847RB-C44. M.D.J. thanks the Brigham Young University Department of Physics and Astronomy for continued support of the extragalactic monitoring programme under way at the West Mountain Observatory. R.C. thanks ISRO for support under the AstroSat archival data utilization programme and BRNS for support through a project grant (sanction no. 57/14/10/2019-BRNS). The measurements at the Hans Haffner Observatory, Hettstadt, Germany, were supported by Baader Planetarium, Mammendorf, Germany. This study was based (in part) on observations conducted using the 1.8-m Perkins Telescope Observatory (PTO) in Arizona, USA, which is owned and operated by Boston University. These results made use of the Lowell Discovery Telescope (LDT) at Lowell Observatory. Lowell Observatory is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy, and operates the LDT in partnership with Boston University, the University of Maryland and the University of Toledo. This paper is partly based on observations made with the IAC-80 operated on the island of Tenerife by the Instituto de Astrofisica de Canarias in the Spanish Observatorio del Teide and on observations made with the LCOGT telescopes, one of whose nodes is located at the Observatorios de Canarias del IAC on the island of Tenerife in the Observatorio del Teide. This paper is partly based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The VLBA is an instrument of the NRAO, USA. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around the world. In mid 2021 the source showed its historical maximum, with R = 11.14. The light curves display many episodes of intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet. We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations in flux density and those in polarisation degree, with a time delay of about 13 h. This suggests a common physical origin. The overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time scales greater than several days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly induced by kink instabilities in the jet, can explain variability on shorter time scales. The latter scenario could also account for the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more closely aligned with our line of sight and the time scales are shortened by relativistic effects., Comment: 15 pages, 16 figures, submitted to MNRAS

Full list of authors: Acciari, V. A.; Ansoldi, S.; Antonelli, L. A.; Arbet Engels, A.; Artero, M.; Asano, K.; Babić, A.; Baquero, A.; Barres de Almeida, U.; Barrio, J. A.; Batković, I.; Becerra González, J.; Bednarek, W.; Bellizzi, L.; Bernardini, E.; Bernardos, M.; Berti, A.; Besenrieder, J.; Bhattacharyya, W.; Bigongiari, C.; Blanch, O.; Bošnjak, Ž.; 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 Espiñeira, E.; Dominis Prester, D.; Donini, A.; Doro, M.; Fallah Ramazani, V.; Fattorini, A.; Ferrara, G.; Fonseca, M. V.; Font, L.; Fruck, C.; Fukami, S.; García López, R. J.; Garczarczyk, M.; Gasparyan, S.; Gaug, M.; Giglietto, N.; Giordano, F.; Gliwny, P.; Godinović, N.; Green, J. G.; Green, D.; Hadasch, D.; Hahn, A.; Heckmann, L.; Herrera, J.; Hoang, J.; Hrupec, D.; Hütten, M.; Inada, T.; Inoue, S.; Ishio, K.; Iwamura, Y.; Jiménez, 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.; López-Coto, R.; López-Moya, M.; López-Oramas, A.; Loporchio, S.; Machado de Oliveira Fraga, B.; Maggio, C.; Majumdar, P.; Makariev, M.; Mallamaci, M.; Maneva, G.; Manganaro, M.; Maraschi, L.; Mariotti, M.; Martínez, M.; Mazin, D.; Menchiari, S.; Mender, S.; Mićanović, S.; Miceli, D.; Miener, T.; Minev, M.; Miranda, J. M.; Mirzoyan, R.; Molina, E.; Moralejo, A.; Morcuende, D.; Moreno, V.; 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.; Pavletić, L.; Peñil, P.; Perennes, C.; Persic, M.; Prada Moroni, P. G.; Prandini, E.; Priyadarshi, C.; Puljak, I.; Ribó, M.; Rico, J.; Righi, C.; Rugliancich, A.; Saha, L.; Sahakyan, N.; Saito, T.; Sakurai, S.; Satalecka, K.; Saturni, F. G.; Schmidt, K.; Schweizer, T.; Sitarek, J.; Šnidarić, I.; Sobczynska, D.; Spolon, A.; Stamerra, A.; Strom, D.; Strzys, M.; Suda, Y.; Surić, T.; Takahashi, M.; Tavecchio, F.; Temnikov, P.; Terzić, 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.; Zarić, D.; Baack, D.; Balbo, M.; Biederbeck, N.; Biland, A.; Bretz, T.; Buss, J.; Dorner, D.; Eisenberger, L.; Elsaesser, D.; Hildebrand, D.; Iotov, R.; Mannheim, K.; Neise, D.; Noethe, M.; Paravac, A.; Rhode, W.; Schleicher, B.; Sliusar, V.; Walter, R.; D'Ammando, F.; Horan, D.; Lien, A. Y.; Baloković, M.; Madejski, G. M.; Perri, M.; Verrecchia, F.; Leto, C.; Lähteenmäki, A.; Tornikoski, M.; Ramakrishnan, V.; Järvelä, E.; Vera, R. J. C.; Chamani, W.; Villata, M.; Raiteri, C. M.; Gupta, A. C.; Pandey, A.; Fuentes, A.; Agudo, I.; Casadio, C.; Semkov, E.; Ibryamov, S.; Marchini, A.; Bachev, R.; Strigachev, A.; Ovcharov, E.; Bozhilov, V.; Valcheva, A.; Zaharieva, E.; Damljanovic, G.; Vince, O.; Larionov, V. M.; Borman, G. A.; Grishina, T. S.; Hagen-Thorn, V. A.; Kopatskaya, E. N.; Larionova, E. G.; Larionova, L. V.; Morozova, D. A.; Nikiforova, A. A.; Savchenko, S. S.; Troitskiy, I. S.; Troitskaya, Y. V.; Vasilyev, A. A.; Merkulova, O. A.; Chen, W. P; Samal, M.; Lin, H. C.; Moody, J. W.; Sadun, A. C.; Jorstad, S. G.; Marscher, A. P.; Weaver, Z. R.; Feige, M.; Kania, J.; Kopp, M.; Kunkel, L.; Reinhart, D.; Scherbantin, A.; Schneider, L.; Lorey, C.; Acosta-Pulido, J. A.; Carnerero, M. I.; Carosati, D.; Kurtanidze, S. O.; Kurtanidze, O. M.; Nikolashvili, M. G.; Chigladze, R. A.; Ivanidze, R. Z.; Kimeridze, G. N.; Sigua, L. A.; Joner, M. D.; Spencer, M.; Giroletti, M.; Marchili, N.; Righini, S.; Rizzi, N.; Bonnoli, G.; MAGIC Collaboration; Fact Collaboration.-- This is an Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., Aims. We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations. Methods. The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina, and Metsahovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. Results. The very-high-energy (VHE; E 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow dierent patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3. A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario.We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from 0 min = 2104 to 0 max = 6105. © 2021 Georg Thieme Verlag. All rights reserved., The MAGIC Collaboration would like to thank the Instituto de Astrofísica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF, MPG and HGF; the Italian INFN and INAF; the Swiss National Fund SNF; the ERDF under the Spanish Ministerio de Ciencia e Innovación (MICINN) (FPA2017-87859-P, FPA2017-85668-P, FPA2017-82729-C6-5-R, FPA2017-90566-REDC, PID2019-104114RB-C31, PID2019-104114RB-C32, PID2019-105510GB-C31,PID2019-107847RB-C41, PID2019-107847RB-C42, PID2019-107847RB-C44, PID2019-107988GB-C22); the Indian Department of Atomic Energy; the Japanese ICRR, the University of Tokyo, JSPS, and MEXT; the Bulgarian Ministry of Education and Science, National RI Roadmap Project DO1-268/16.12.2019 and the Academy of Finland grant nr. 320045 is gratefully acknowledged. This work was also supported by the Spanish Centro de Excelencia “Severo Ochoa” SEV-2016-0588, SEV-2017-0709 and CEX2019-000920-S, and “María de Maeztu” CEX2019-000918-M, the Unidad de Excelencia “María de Maeztu” MDM-2015-0509-18-2 and the “la Caixa” Foundation (fellowship LCF/BQ/PI18/11630012) and by the CERCA program of the Generalitat de Catalunya; by the Croatian Science Foundation (HrZZ) Project IP-2016-06-9782 and the University of Rijeka Project 13.12.1.3.02; by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3; the Polish National Research Centre grant UMO-2016/22/M/ST9/00382; and by the Brazilian MCTIC, CNPq and FAPERJ. The important contributions from ETH Zurich grants ETH-10.08-2 and ETH-27.12-1 as well as the funding by the Swiss SNF and the German BMBF (Verbundforschung Astro- und Astroteilchenphysik) and HAP (Helmoltz Alliance for Astroparticle Physics) are gratefully acknowledged. Part of this work is supported by Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Center SFB 876 “Providing Information by Resource-Constrained Analysis”, project C3. We are thankful for the very valuable contributions from E. Lorenz, D. Renker and G. Viertel during the early phase of the project. We thank the Instituto de Astrofísica de Canarias for allowing us to operate the telescope at the Observatorio del Roque de los Muchachos in La Palma, the Max-Planck-Institut für Physik for providing us with the mount of the former HEGRA CT3 telescope, and the MAGIC collaboration for their support. The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l’Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d’Études Spatiales in France. This work performed in part under DOE Contract DE-AC02-76SF00515. This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC; Italy) and the California Institute of Technology (USA). This research has also made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. A.A.E and D.P acknowledge support from the Deutsche Forschungs gemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2094 – 390783311. M. B. acknowledges support from the YCAA Prize Postdoctoral Fellowship and from the Black Hole Initiative at Harvard University, which is funded in part by the Gordon and Betty Moore Foundation (grant GBMF8273) and in part by the John Templeton Foundation. This publication makes use of data obtained at the Metsähovi Radio Observatory, operated by Aalto University in Finland. This research has made use of data from the OVRO 40-m monitoring program (Richards et al. 2011) which is supported in part by NASA grants NNX08AW31G, NNX11A043G, and NNX14AQ89G and NSF grants AST-0808050 and AST-1109911. I.A. acknowledges financial support from the Spanish “Ministerio de Ciencia e Innovación” (MCINN) through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía-CSIC (SEV-2017-0709). Acquisition and reduction of the MAPCAT data was supported in part by MICINN through grants AYA2016-80889-P and PID2019-107847RB-C44. The MAPCAT observations were carried out at the German-Spanish Calar Alto Observatory, which is jointly operated by Junta de Andalucía and Consejo Superior de Investigaciones Científicas. C.C. acknowledges support from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program under the grant agreement No 771282. This research was partially supported by the Bulgarian National Science Fund of the Ministry of Education and Science under grants KP-06-H28/3 (2018), KP-06-H38/4 (2019) and KP-06-KITAJ/2 (2020). We acknowledge support by Bulgarian National Science Fund under grant DN18-10/2017 and National RI Roadmap Projects DO1-277/16.12.2019 and DO1-268/16.12.2019 of the Ministry of Education and Science of the Republic of Bulgaria. This research was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (contract No 451-03-68/2020-14/200002). G.D. acknowledges observing grant support from the Institute of Astronomy and Rozhen NAO BAS through the bilateral joint research project “Gaia Celestial Reference Frame (CRF) and fast variable astronomical objects” (2020–2022, head – G. Damljanovic). The BU group was supported in part by NASA Fermi guest investigator program grants 80NSSC19K1505 and 80NSSC20K1566. This study was based in part on observations conducted using the 1.8 m Perkins Telescope Observatory (PTO) in Arizona, which is owned and operated by Boston University. This article is partly based on observations made with the LCOGT Telescopes, one of whose nodes is located at the Observatorios de Canarias del IAC on the island of Tenerife in the Observatorio del Teide. This article is also based partly on data obtained with the STELLA robotic telescopes in Tenerife, an AIP facility jointly operated by AIP and IAC. The Abastumani team acknowledges financial support by the Shota Rustaveli National Science Foundation under contract FR-19-6174. Based on observations with the Medicina telescope operated by INAF – Istituto di Radioastronomia.

The optical polarization plane of some blazars occasionally exhibits smooth hundred degree long rotations. Multiple theoretical models have been proposed to explain the nature of such events. A deterministic origin of these rotations, however, remains uncertain. We aim to find repeating patterns of flares in gamma-ray light curves of blazars, which accompany optical polarization plane rotations. Such patterns have been predicted to occur by one of the models explaining this phenomenon. For the blazar 3C 279, where multiple polarization plane rotations have been reported in the literature, we obtain the Fermi-LAT gamma-ray light curve and analyze its intervals adjacent to polarization plane rotations. We find a complex characteristic pattern of flares in the gamma-ray light curve that is repeated during periods adjacent to three large amplitude EVPA rotation events in 3C 279. We discover a "hidden EVPA rotation", which can only be seen in the relative Stokes parameters plane and that occurred simultaneously with the fourth repetition of the pattern. This finding strongly favors the hypothesis of emission features propagating in the jet as the reason of optical polarization plane rotations. Furthermore, it is compatible with the hypothesis of a sheath in the jet comprised of more slowly propagating emission features., Accepted to MNRAS; Press release http://users.physics.uoc.gr/~blinov/3c279.html

Full list of authors: Raiteri, C. M.; Villata, M.; Larionov, V. M.; Jorstad, S. G.; Marscher, A. P.; Weaver, Z. R.; Acosta-Pulido, J. A.; Agudo, I.; Andreeva, T.; Arkharov, A.; Bachev, R.; Benítez, E.; Berton, M.; Björklund, I.; Borman, G. A.; Bozhilov, V.; Carnerero, M. I.; Carosati, D.; Casadio, C.; Chen, W. P. Damljanovic, G.; D'Ammando, F.; Escudero, J.; Fuentes, A.; Giroletti, M.; Grishina, T. S.; Gupta, A. C.; Hagen-Thorn, V. A.; Hart, M.; Hiriart, D.; Hou, W. -J.; Ivanov, D.; Kim, J. -Y.; Kimeridze, G. N.; Konstantopoulou, C.; Kopatskaya, E. N.; Kurtanidze, O. M.; Kurtanidze, S. O.; Lähteenmäki, A.; Larionova, E. G.; Larionova, L. V.; Marchili, N.; Markovic, G.; Minev, M.; Morozova, D. A.; Myserlis, I.; Nakamura, M.; Nikiforova, A. A.; Nikolashvili, M. G.; Otero-Santos, J.; Ovcharov, E.; Pursimo, T.; Rahimov, I.; Righini, S.; Sakamoto, T.; Savchenko, S. S.; Semkov, E. H.; Shakhovskoy, D.; Sigua, L. A.; Stojanovic, M.; Strigachev, A.; Thum, C.; Tornikoski, M.; Traianou, E.; Troitskaya, Y. V.; Troitskiy, I. S.; Tsai, A.; Valcheva, A.; Vasilyev, A. A.; Vince, O.; Zaharieva, E., Among active galactic nuclei, blazars show extreme variability properties. We here investigate the case of the BL Lac object S4 0954+65 with data acquired in 2019-2020 by the Transiting Exoplanet Survey Satellite (TESS) and by the Whole Earth Blazar Telescope (WEBT) Collaboration. The 2-min cadence optical light curves provided by TESS during three observing sectors of nearly 1 month each allow us to study the fast variability in great detail. We identify several characteristic short-term time-scales, ranging from a few hours to a few days. However, these are not persistent, as they differ in the various TESS sectors. The long-term photometric and polarimetric optical and radio monitoring undertaken by the WEBT brings significant additional information, revealing that (i) in the optical, long-term flux changes are almost achromatic, while the short-term ones are strongly chromatic; (ii) the radio flux variations at 37 GHz follow those in the optical with a delay of about 3 weeks; (iii) the range of variation of the polarization degree and angle is much larger in the optical than in the radio band, but the mean polarization angles are similar; (iv) the optical long-term variability is characterized by a quasi-periodicity of about 1 month. We explain the source behaviour in terms of a rotating inhomogeneous helical jet, whose pitch angle can change in time. © 2021 The Author(s)., This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. Partly based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. This article is partly based on observations made in the Observatorios de Canarias del IAC with the Liverpool telescope operated on the island of La Palma by the Liverpool John Moores University in the Observatorio del Roque de Los Muchachos. This article is partly based on observations made with the LCOGT Telescopes, one of whose nodes is located at the Observatorios de Canarias del IAC on the island of Tenerife in the Observatorio del Teide. This article is partly based on observations made with the IAC-80 operated on the island of Tenerife by the Instituto de Astrofisica de Canarias in the Spanish Observatorio del Teide. Many thanks are due to the IAC support astronomers and telescope operators for supporting the observations at the IAC-80 telescope. This publication makes use of data obtained at Metsähovi Radio Observatory, operated by Aalto University in Finland. This research has made use of NASA’s Astrophysics Data System and of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The research at Boston University was supported by NASA grants 80NSSC20K1566 (Fermi Guest Investigator Program) and 80NSSC21K0243 (TESS Guest Investigator Program). This study was based (in part) on observations conducted using the 1.8 m Perkins Telescope Observatory (PTO) in Arizona (USA), which is owned and operated by Boston University. GD, MS, GM, and OV acknowledge the observing grant support from the Institute of Astronomy and Rozhen NAO BAS through the bilateral joint research project ‘Gaia Celestial Reference Frame (CRF) and fast variable astronomical objects’ (2020–2022, leader is G.Damljanovic), and support by the Ministry of Education, Science and Technological Development of the Republic of Serbia (contract No 451-03-68/2020-14/200002) This research was partially supported by the Bulgarian National Science Fund of the Ministry of Education and Science under grants DN 18-13/2017, KP-06-H28/3 (2018), KP-06-H38/4 (2019), and KP-06-KITAJ/2 (2020). SOK acknowledges financial support by Shota Rustaveli National Science Foundation of Georgia under contract PHDF-18-354 EB acknowledges support from DGAPA-PAPIIT GRANT IN113320. This work is partly based upon observations carried out at the Observatorio Astronómico Nacional on the Sierra San Pedro Mártir (OAN- SPM), Baja California, Mexico. We acknowledge support by Bulgarian National Science Fund under grant DN18-10/2017 and National RI Roadmap Project DO1-383/18.12.2020 of the Ministry of Education and Science of the Republic of Bulgaria. IA acknowledges financial support from the Spanish ‘Ministerio de Ciencia e Innovación’ (MCINN) through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía-CSIC (SEV-2017-0709). Acquisition and reduction of the POLAMI data was supported in part by MICINN through grants AYA2016-80889-P and PID2019-107847RB-C44. The POLAMI observations were carried out at the IRAM 30m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

We present the results of flux density, spectral index, and polarization intra-night monitoring studies of a sample of eight optically bright blazars, carried out by employing several small to moderate aperture (0.4\,m to 1.5\,m diameter) telescopes fitted with CCDs and polarimeters located in Europe, India, and Japan. The duty cycle of flux variability for the targets is found to be $\sim 45$ percent, similar to that reported in earlier studies. The computed two-point spectral indices are found to be between 0.65 to 1.87 for our sample, comprised of low- and intermediate frequency peaked blazars, with one exception; they are also found to be statistically variable for about half the instances where `confirmed' variability is detected in flux density. In the analysis of the spectral evolution of the targets on hourly timescale, a counter-clockwise loop (soft-lagging) is noted in the flux-spectral index plane on two occasions, and in one case a clear spectral flattening with the decreasing flux is observed. In our data set, we also observe a variety of flux-polarization degree variability patterns, including instances with a relatively straightforward anti-correlation, correlation, or counter-clockwise looping. These changes are typically reflected in the flux-polarization angle plane: the anti-correlation between the flux and polarization degree is accompanied by an anti-correlation between the polarization angle and flux, while the counter-clockwise flux-PD looping behaviour is accompanied by a clockwise looping in the flux-polarization angle representation. We discuss our findings in the framework of the internal shock scenario for blazar sources., MNRAS accepted

The quasar 3C454.3 underwent a uniquely-structured multi-frequency outburst in June 2016. The blazar was observed in the optical $R$ band by several ground-based telescopes in photometric and polarimetric modes, at $\gamma$-ray frequencies by the \emph{Fermi}\ Large Area Telescope, and at 43 GHz with the Very Long Baseline Array. The maximum flux density was observed on 2016 June 24 at both optical and $\gamma$-ray frequencies, reaching $S^\mathrm{max}_\mathrm{opt}=18.91\pm0.08$ mJy and $S_\gamma^\mathrm{max} =22.20\pm0.18\times10^{-6}$ ph cm$^{-2}$ s$^{-1}$, respectively. The June 2016 outburst possessed a precipitous decay at both $\gamma$-ray and optical frequencies, with the source decreasing in flux density by a factor of 4 over a 24-hour period in $R$ band. Intraday variability was observed throughout the outburst, with flux density changes between 1 and 5 mJy over the course of a night. The precipitous decay featured statistically significant quasi-periodic micro-variability oscillations with an amplitude of $\sim 2$-$3\%$ about the mean trend and a characteristic period of 36 minutes. The optical degree of polarization jumped from $\sim3\%$ to nearly 20\% during the outburst, while the position angle varied by $\sim120\degr$. A knot was ejected from the 43 GHz core on 2016 Feb 25, moving at an apparent speed $v_\mathrm{app}=20.3c\pm0.8c$. From the observed minimum timescale of variability $\tau_\mathrm{opt}^\mathrm{min}\approx2$ hr and derived Doppler factor $\delta=22.6$, we find a size of the emission region $r\lesssim2.6\times10^{15}$ cm. If the quasi-periodic micro-variability oscillations are caused by periodic variations of the Doppler factor of emission from a turbulent vortex, we derive a rotational speed of the vortex $\sim0.2c$., Comment: 19 pages, 13 figures, 3 tables, accepted to the Astrophysical Journal 2019 March 9

Blazar CTA 102 underwent exceptional optical and high-energy outbursts in 2012 and 2016–2017. We analyze its behaviour during these events, focusing on polarimetry as a tool that allows us to trace changes in the physical conditions and geometric configuration of the emission source close to the central black hole. We also use Fermi γ -ray data in conjunction with optical photometry in an effort to localize the origin of the outbursts.

We analyze the behavior of the BL Lac object S4 0954+658 during an unprecedented bright optical flare in early 2015. The optical flare was accompanied by a powerful γ -ray flare and the detection of very-high-energy γ -ray emission. We analyze total and polarized intensity images obtained with the VLBA at 43 GHz and discover a new bright polarized superluminal knot, which was ejected from the VLBI-core during the peak of the flare.

We present the results of photo-polarimetric monitoring observations of the blazar Markarian 421 carried out with different telescopes (the 0.4 m telescopes of St. Petersburg State University and the Pulkovo Observatory, the 0.7 m telescope of the Crimean Astrophysical Observatory) during 2008–2015. We analyse the optical data as well as gamma-ray ligh t curves obtained with the Fermi Large Area Telescope. The multiwavelength flux variations are discussed.