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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.

We report on long-term multiwavelength monitoring of blazar Mrk 421 by the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in the period 2007-2015, characterized by several extreme flares. The ratio between the optical, X-ray and γ-ray fluxes is very variable. The γ-ray flux variations show a fair correlation with the optical ones starting from 2012. We analyse spectropolarimetric data and find wavelength-dependence of the polarization degree (P), which is compatible with the presence of the host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA). Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of the EVPA. We build broad-band spectral energy distributions with simultaneous near-infrared and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite. They show strong variability in both flux and X-ray spectral shape and suggest a shift of the synchrotron peak up to a factor of ~50 in frequency. The interpretation of the flux and spectral variability is compatible with jet models including at least two emitting regions that can change their orientation with respect to the line of sight.© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society., The Abastumani team acknowledges financial support by Shota Rustaveli NSF under contract FR/577/6-320/13. The research at BU was supported in part by US National Science Foundation grant AST-1615796 and NASA Fermi Guest Investigator grant NNX14AQ58G. This study makes use of 43 GHz VLBA data from the VLBA-BU Blazar Monitoring Program (VLBA-BUBLAZAR; http://www.bu.edu/blazars/VLBAproject.html), funded by NASA through the Fermi Guest Investigator Program. The PRISM camera at the Lowell Observatory was developed by K. Janes et al. at the BU and Lowell Observatory, with funding from the NSF, BU and Lowell Observatory. This research has made use of data from the MOJAVE data base, which is maintained by the MOJAVE team (Lister et al. 2009). The St. Petersburg University team acknowledges support from Russian RFBR grant 15-02-00949 and St. Petersburg University research grant 6.38.335.2015. This paper is partly based on observations carried out at the German Spanish Calar Alto Observatory, which is jointly operated by the MPIA and the IAA-CSIC. IA research is supported by a Ramon y Cajal grant of the Spanish Ministerio de Economia y Competitividad (MINECO). Acquisition of the MAPCAT data was supported in part by MINECO through grants AYA2010-14844, AYA2013-40825-P and AYA2016-80889-P, and by the Regional Government of Andalucia through grant P09-FQM-4784. This research was partially supported by the Scientific Research Fund of the Bulgarian Ministry of Education and Sciences under grants DO 02-137 (BIn-13/09) and DN 08/1. The Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology - Hellas and the Max-Planck-Institut fur Extrater-restrische Physik. GD and OV gratefully acknowledge the observing grant support from the Institute of Astronomy and Rozhen National Astronomical Observatory, Bulgaria Academy of Sciences, via the bilateral joint research project 'Observations of ICRF radio-sources visible in optical domain' (the head is Dr G. Damljanovic). This work is a part of the Projects No 176011 ('Dynamics and kinematics of celestial bodies and systems'), No 176004 ('Stellar physics') and No 176021 ('Visible and invisible matter in nearby galaxies: theory and observations') supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia. The Serbian station is the Astronomical Station Vidojevica (ASV) with the 60-cm ASV telescope (and from this year, the 1.4-m ASV one). This research was supported partly by funds of the project RD-08-81 of the Shumen University.

The object 4C 71.07 is a high-redshift blazar whose spectral energy distribution shows a prominent big blue bump and a strong Compton dominance. We present the results of a 2- yr multiwavelength campaign led by the Whole Earth Blazar Telescope (WEBT) to study both the quasar core and the beamed jet of this source. The WEBT data are complemented by ultraviolet and X-ray data from Swift, and by γ-ray data by Fermi. The big blue bump is modelled by using optical and near-infrared mean spectra obtained during the campaign, together with optical and ultraviolet quasar templates. We give prescriptions to correct the source photometry in the various bands for the thermal contribution, in order to derive the non-thermal jet flux. The role of the intergalactic medium absorption is analysed in both the ultraviolet and X-ray bands.We provide opacity values to deabsorb ultraviolet data, and derive a best-guess value for the hydrogen column density of Nbest H = 6.3 × 10 cmthrough the analysis of X-ray spectra.We estimate the disc and jet bolometric luminosities, accretion rate, and black hole mass. Light curves do not show persistent correlations among flux changes at different frequencies. We study the polarimetric behaviour and find no correlation between polarization degree and flux, even when correcting for the dilution effect of the big blue bump. Similarly, wide rotations of the electric vector polarization angle do not seem to be connected with the source activity.© 2019 The Author(s)., We acknowledge financial contribution from the agreement ASI-INAF n.2017-14-H.0 and from the contract PRIN-SKA-CTA-INAF 2016. PR and SV acknowledge contract ASI-INAF I/004/11/0. We acknowledge support by Bulgarian National Science Programme 'Young Scientists and Postdoctoral Students 2019', Bulgarian National Science Fund under grant DN18-10/2017 and National RI Roadmap Projects DO1-157/28.08.2018 and DO1-153/28.08.2018 of the Ministry of Education and Science of the Republic of Bulgaria. GD and OV gratefully acknowledge the observing grant support from the Institute of Astronomy and Rozhen National Astronomical Observatory, Bulgarian Academy of Sciences via bilateral joint research project 'Study of ICRF radio-sources and fast variable astronomical objects' (head -G.Damljanovic). This work is a part of the Projects No. 176011 ('Dynamics and Kinematics of Celestial Bodies and Systems'), No. 176004 ('Stellar Physics'), and No. 176021 ('Visible and Invisible Matter in Nearby Galaxies: Theory and Observations') supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia. This research was partially supported by the Bulgarian National Science Fund of theMinistry of Education and Science under grants DN 08-1/2016, DN 18-13/2017, and KP-06-H28/3 (2018). The Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology -Hellas, and the Max-Planck-Institut fur Extraterrestrische Physik. The St Petersburg University team acknowledges support from Russian Science Foundation grant no. 17-12-01029. The Abastumani team acknowledges financial support by the Shota Rustaveli National Science Foundation under contract FR/217950/16. This work was partly supported by the National Science Fund of the Ministry of Education and Science of Bulgaria under grant DN 08-20/2016, and by funds of the project RD-08-37/2019 of the University of Shumen. The Astronomical Observatory of the Autonomous Region of the Aosta Valley (OAVdA) is managed by the Fondazione Clement Fillietroz-ONLUS, which is supported by the Regional Government of the Aosta Valley, the Town Municipality of Nus and the Unite des Communes valdotaines Mont-Emilius'. The research at the OAVdA was partially funded by two 'Research and Education' grants from Fondazione CRT

We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013-2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well as data from the Swift (optical-UV and X-rays) and Fermi (γ -rays) satellites to study flux and spectral variability and the correlation between flux changes at different wavelengths. Unprecedented γ -ray flaring activity was observed during 2016 November-2017 February, with four major outbursts. A peak flux of (2158 ± 63) × 10−8 ph cm−2 s−1, corresponding to a luminosity of (2.2 ± 0.1) × 1050 erg s−1, was reached on 2016 December 28. These four γ -ray outbursts have corresponding events in the near-infrared, optical, and UV bands, with the peaks observed at the same time. A general agreement between X-ray and γ -ray activity is found. The γ -ray flux variations show a general, strong correlation with the optical ones with no time lag between the two bands and a comparable variability amplitude. This γ -ray/optical relationship is in agreement with the geometrical model that has successfully explained the low-energy flux and spectral behaviour, suggesting that the long-term flux variations are mainly due to changes in the Doppler factor produced by variations of the viewing angle of the emitting regions. The difference in behaviour between radio and higher energy emission would be ascribed to different viewing angles of the jet regions producing their emission. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society, The data collected by the WEBT collaboration are stored in the WEBT archive at the Osservatorio Astrofisico di Torino INAF (http://www.oato.inaf.it/blazars/webt/); for questions regarding their availability, contact the WEBT President Massimo Villata(massimo.villata@inaf.it).We acknowledge financial contribution from the agreementASI-INAFn. 2017-14-H.0 and from the contract PRIN-SKA-CTA-INAF 2016. This research was partially supported by the Bulgarian National Science Fund of the Ministry of Education and Science under grants DN 08-1/2016, DN 18-13/2017, and KP-06-H28/3 (2018). The Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology -Hellas, and the Max-Planck-Institut fur Extraterrestrische Physik. The Abastumani team acknowledges financial support by the Shota Rustaveli National Science Foundation under contract FR/217554/16. The St. Petersburg University team acknowledges support from Russian Science Foundation grant 17-12-01029. GD and OV gratefully acknowledge the observing grant support from the Institute of Astronomy and Rozhen National Astronomical Observatory, Bulgarian Academy of Sciences, via bilateral joint research project 'Study of ICRF radio-sources and fast variable astronomical objects' (head -G.Damljanovic). This work is a part of the Projects No. 176011 ('Dynamics and Kinematics of Celestial Bodies and Systems'), No. 176004 ('Stellar Physics'), and No. 176021 ('Visible and Invisible Matter in Nearby Galaxies: Theory and Observations') supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia. JE is indebted to DGAPA (Universidad Nacional Autonoma de M ' exico) for financial support, PAPIIT project IN114917. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. Data from the Steward Observatory blazar monitoring project were used. This program is supported by NASA/Fermi Guest Investigator grants NNX12AO93G and NNX15AU81G. We acknowledge support by Bulgarian National Science Programme 'Young Scientists and Postdoctoral Students 2019', Bulgarian National Science Fund under grant DN18-10/2017 and National RI Roadmap Projects DO1-157/28.08.2018 and DO1-153/28.08.2018 of the Ministry of Education and Science of the Republic of Bulgaria. This publication makes use of data obtained at Mets ahovi Radio Observatory, operated by Aalto University in Finland. The Astronomical Observatory of the Autonomous Region of theAostaValley (OAVdA) is managed by the Fondazione Clment Fillietroz-ONLUS, which is supported by the Regional Government of the Aosta Valley, the TownMunicipality of Nus and the 'Unit des Communes valdtaines Mont-milius'. The research at the OAVdA was partially funded by two 'Research and Education' grants from Fondazione CRT. RR acknowledges support from CONICYT project Basal AFB-170002. MM and TM acknowledge support through the Russian Government Program of Competitive Growth of Kazan Federal University. 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 a l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucleaire 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'Etudes Spatiales in France. This work performed in part under DOE Contract DE-AC02-76SF00515. The OVRO 40-m monitoring program is supported in part by NASA grants NNX08AW31G, NNX11A043G, and NNX14AQ89G, and NSF grants AST-0808050 and AST-1109911. We thank the Swift team for making these observations possible, the duty scientists, and science planners. This research has made use 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. FD thanks S. Covino for his help with the REM data reduction. This research has made use of data obtained from the high-energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA's Goddard Space Flight Center.

We present the first results from a reverberation-mapping campaign undertaken during the first half of 2012, with additional data on one AGN (NGC 3227) from a 2014 campaign. Our main goals are (1) to determine the black hole masses from continuum-Hbeta reverberation signatures, and (2) to look for velocity-dependent time delays that might be indicators of the gross kinematics of the broad-line region. We successfully measure Hbeta time delays and black hole masses for five AGNs, four of which have previous reverberation mass measurements. The values measured here are in agreement with earlier estimates, though there is some intrinsic scatter beyond the formal measurement errors. We observe velocity dependent Hbeta lags in each case, and find that the patterns have changed in the intervening five years for three AGNs that were also observed in 2007., Accepted for publication on ApJ; 32 pages, 16 figures, 10 tables

We report on the optical observations and analysis of the high-energy peaked BL Lac object (HBL), Mrk 501, at redshift z = 0.033. We can confirm microvariable behavior over the course of minutes on several occasions per night. As an alternative to the commonly understood dynamical model of random variations in intensity of the AGN, we develop a relativistic beaming model with a minimum of free parameters, which allows us to infer changes in the line of sight angles for the motion of the different relativistic components. We hope our methods can be used in future studies of beamed emission in other active microvariable sources, similar to the one we explored.

Blazar emission is dominated by non-thermal radiation from a relativistic jet pointing toward us, therefore undergoing Doppler beaming. This is responsible for flux enhancement and contraction of the variability time scales, so that most blazars appear as luminous sources characterized by noticeable and fast flux changes at all frequencies. The mechanisms producing their unpredictable variability are debated and include injection, acceleration and cooling of particles, with possible intervention of shock waves or turbulence. Changes in the viewing angle of the emitting knots or jet regions have also been suggested to explain flaring events or specific properties such as intraday variability, quasi-periodicities, or the delay of radio flux variations relative to optical changes. However, such a geometric interpretation has not been universally accepted because alternative explanations based on changes of physical conditions can also work in many cases. Here we report the results of optical-to-radio monitoring of the blazar CTA 102 by the Whole Earth Blazar Telescope Collaboration and show that the observed long-term flux and spectral variability is best explained by an inhomogeneous, curved jet that undergoes orientation changes. We propose that magnetohydrodynamic instabilities or rotation of a twisted jet cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the recent extreme optical outburst (six magnitudes) occurred when the corresponding jet emitting region acquired a minimum viewing angle., Comment: 31 pages, 11 figures. This is the accepted version (before Nature editing) of the paper published by Nature, online version at http://dx.doi.org/10.1038/nature24623, Nature 2017/12/04/online

In 2015 July 29 - September 1 the satellite XMM-Newton pointed at the BL Lac object PG 1553+133 six times, collecting data for 218 hours. During one of these epochs, simultaneous observations by the Swift satellite were requested to compare the results of the X-ray and optical-UV instruments. Optical, near-infrared and radio monitoring was carried out by the Whole Earth Blazar Telescope (WEBT) collaboration for the whole observing season. We here present the results of the analysis of all these data, together with an investigation of the source photometric and polarimetric behaviour over the last three years. The 2015 EPIC spectra show slight curvature and the corresponding light curves display fast X-ray variability with a time scale of the order of 1 hour. In contrast to previous results, during the brightest X-ray states detected in 2015 the simple log-parabolic model that best-fits the XMM-Newton data also reproduces reasonably well the whole synchrotron bump, suggesting a peak in the near-UV band. We found evidence of a wide rotation of the polarization angle in 2014, when the polarization degree was variable, but the flux remained almost constant. This is difficult to interpret with deterministic jet emission models, while it can be easily reproduced by assuming some turbulence of the magnetic field., Comment: 15 pages, 12 figures, accepted by Monthly Notices of the Royal Astronomical Society

Astronomy and astrophysics 603, A31 (2017). doi:10.1051/0004-6361/201629540, We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1, which includes, among other instruments, MAGIC, VERITAS, Whipple 10 m, and Fermi-LAT to cover the γ-ray range from 0.1 GeV to 20 TeV; RXTE and Swift to cover wavelengths from UV tohard X-rays; and GASP-WEBT, which provides coverage of radio and optical wavelengths. Optical polarization measurements were provided for a fraction of the campaign by the Steward and St. Petersburg observatories. We evaluate the variability of the source and interband correlations, the γ-ray flaring activity occurring in May 2009, and interpret the results within two synchrotron self-Compton (SSC) scenarios. Aims. The multiband variability observed during the full campaign is addressed in terms of the fractional variability, and the possible correlations are studied by calculating the discrete correlation function for each pair of energy bands where the significance was evaluated with dedicated Monte Carlo simulations. The space of SSC model parameters is probed following a dedicated grid-scan strategy, allowing for a wide range of models to be tested and offering a study of the degeneracy of model-to-data agreement in the individual model parameters, hence providing a less biased interpretation than the “single-curve SSC model adjustment” typically reported in the literature. Methods. We find an increase in the fractional variability with energy, while no significant interband correlations of flux changes are found on the basis of the acquired data set. The SSC model grid-scan shows that the flaring activity around May 22 cannot be modeled adequately with a one-zone SSC scenario (using an electron energy distribution with two breaks), while it can be suitably described within a two (independent) zone SSC scenario. Here, one zone is responsible for the quiescent emission from the averaged 4.5-month observing period, while the other one, which is spatially separated from the first, dominates the flaring emission occurring at X-rays and very-high-energy (>100 GeV, VHE) γ rays. The flaring activity from May 1, which coincides with a rotation of the electric vector polarization angle (EVPA), cannot be satisfactorily reproduced by either a one-zone or a two-independent-zone SSC model, yet this is partially affected by the lack of strictly simultaneous observations and the presence of large flux changes on sub-hour timescales (detected at VHE γ rays). Results. The higher variability in the VHE emission and lack of correlation with the X-ray emission indicate that, at least during the 4.5-month observing campaign in 2009, the highest energy (and most variable) electrons that are responsible for the VHE γ rays do not make a dominant contribution to the ~1 keV emission. Alternatively, there could be a very variable component contributing to the VHE γ-ray emission in addition to that coming from the SSC scenario. The studies with our dedicated SSC grid-scan show that there is some degeneracy in both the one-zone and the two-zone SSC scenarios probed, with several combinations of model parameters yielding a similar model-to-data agreement, and some parameters better constrained than others. The observed γ-ray flaring activity, with the EVPA rotation coincident with the first γ-ray flare, resembles those reported previously for low frequency peaked blazars, hence suggesting that there are many similarities in the flaring mechanisms of blazars with different jet properties., Published by EDP Sciences, Les Ulis

Here we report on the results of the WEBT photo-polarimetric campaign targeting the blazar S5~0716+71, organized in March 2014 to monitor the source simultaneously in BVRI and near IR filters. The campaign resulted in an unprecedented dataset spanning $\sim 110$\,h of nearly continuous, multi-band observations, including two sets of densely sampled polarimetric data mainly in R filter. During the campaign, the source displayed pronounced variability with peak-to-peak variations of about $30\%$ and "bluer-when-brighter" spectral evolution, consisting of a day-timescale modulation with superimposed hourlong microflares characterized by $\sim 0.1$\,mag flux changes. We performed an in-depth search for quasi-periodicities in the source light curve; hints for the presence of oscillations on timescales of $\sim 3$\,h and $\sim 5$\,h do not represent highly significant departures from a pure red-noise power spectrum. We observed that, at a certain configuration of the optical polarization angle relative to the positional angle of the innermost radio jet in the source, changes in the polarization degree led the total flux variability by about 2\,h; meanwhile, when the relative configuration of the polarization and jet angles altered, no such lag could be noted. The microflaring events, when analyzed as separate pulse emission components, were found to be characterized by a very high polarization degree ($> 30\%$) and polarization angles which differed substantially from the polarization angle of the underlying background component, or from the radio jet positional angle. We discuss the results in the general context of blazar emission and energy dissipation models., 16 pages, 17 Figures; ApJ accepted

The Seyfert 1 galaxy Zw 229-015 is among the brightest active galaxies being monitored by the Kepler mission. In order to determine the black hole mass in Zw 229-015 from H-beta reverberation mapping, we have carried out nightly observations with the Kast Spectrograph at the Lick 3m telescope during the dark runs from June through December 2010, obtaining 54 spectroscopic observations in total. We have also obtained nightly V-band imaging with the Katzman Automatic Imaging Telescope at Lick Observatory and with the 0.9m telescope at the Brigham Young University West Mountain Observatory over the same period. We detect strong variability in the source, which exhibited more than a factor of 2 change in broad H-beta flux. From cross-correlation measurements, we find that the H-beta light curve has a rest-frame lag of 3.86(+0.69,-0.90) days with respect to the V-band continuum variations. We also measure reverberation lags for H-alpha and H-gamma and find an upper limit to the H-delta lag. Combining the H-beta lag measurement with a broad H-beta width of sigma = 1590+/-47 km/s measured from the root-mean-square variability spectrum, we obtain a virial estimate of M_BH = 1.00(-0.24,+0.19)*10^7 solar masses for the black hole in Zw 229-015. As a Kepler target, Zw 229-015 will eventually have one of the highest-quality optical light curves ever measured for any active galaxy, and the black hole mass determined from reverberation mapping will serve as a benchmark for testing relationships between black hole mass and continuum variability characteristics in active galactic nuclei., 13 pages, 8 figures. Accepted for publication in ApJ

The KPNO International Spectroscopic Survey (KISS) is an objective-prism survey for extragalactic emission-line objects. It combines many of the features of previous slitless spectroscopic surveys with the advantages of modern CCD detectors, and is the first purely digital objective-prism survey for emission-line galaxies. Here we present the second list of emission-line galaxy candidates selected from our red spectral data, which cover the wavelength range 6400 to 7200 A. In most cases, the detected emission line is H-alpha. The current survey list covers a 1.6-degree-wide strip located at Dec(1950) = 43d 30' and spans the RA range 11h 55m to 16h 15m. The survey strip runs through the center of the Bootes Void, and has enough depth to adequately sample the far side of the void. An area of 65.8 sq. deg. is covered. A total of 1029 candidate emission-line objects have been selected for inclusion in the survey list (15.6 per sq. deg.). We tabulate accurate coordinates and photometry for each source, as well as estimates of the redshift and emission-line flux and equivalent width based on measurements of the digital objective-prism spectra. The properties of the KISS emission-line galaxies are examined using the available observational data. Although the current survey covers only a modest fraction of the total volume of the Bootes Void, we catalog at least twelve objects that appear to be located within the void. Only one of these objects has been recognized previously as a void galaxy., Comment: Accepted for publication in the Astronomical Journal (April 2004); 20 pages, 12 figures

The KPNO International Spectroscopic Survey (KISS) is a new objective-prism survey for extragalactic emission-line objects. It combines many of the features of previous slitless spectroscopic surveys that were carried out with Schmidt telescopes using photographic plates with the advantages of modern CCD detectors. It is the first purely digital objective-prism survey, and extends previous photographic surveys to substantially fainter flux limits. In this, the first paper in the series, we give an overview of the survey technique, describe our data processing procedures, and present examples of the types of objects found by KISS. Our first H-alpha-selected survey list detects objects at the rate of 18.1 per square degree, which is 181 times higher than the surface density of the Markarian survey.Since the sample is line-selected, there is an imposed redshift limit of z ~ 0.095 due to the filter employed for the objective-prism observations. We evaluate the quality of the observed parameters derived from the survey data, which include accurate astrometry, photometry, redshifts, and line fluxes. Finally, we describe some of the many applications the KISS database will have for addressing specific questions in extragalactic astronomy. Subsequent papers in this series will present our survey lists of emission-line galaxy candidates., 19 pages, 8 figures, A.J. in press

The KPNO International Spectroscopic Survey (KISS) is a new objective-prism survey for extragalactic emission-line objects. It combines many of the features of previous slitless spectroscopic surveys with the advantages of modern CCD detectors, and is the first purely digital objective-prism survey for emission-line galaxies. Here we present the first list of emission-line galaxy candidates selected from our red spectral data, which cover the spectral range 6400 to 7200 Angstroms. In most cases, the detected emission line is H-alpha. The current survey list covers a one-degree-wide strip located at Dec (1950) = 29.5 degrees and spanning the RA range 12h 15m to 17h 0m. An area of 62.2 sq. degrees is covered. A total of 1128 candidate emission-line objects have been selected for inclusion in the survey list (18.1 per sq. degree). We tabulate accurate coordinates and photometry for each source, as well as estimates of the redshift and emission-line flux and equivalent width based on measurements of the digital objective-prism spectra. The properties of the KISS emission-line galaxies are examined using the available observational data., Comment: 18 pages, 9 figures, A.J. in press. Full resolution versions of Figures 1 & 2 are available at http://www.astro.wesleyan.edu/~slaz/research/kiss/kisspubs.html