Multi-year characterisation of the broad-band emission from the intermittent extreme BL Lac 1ES 2344+514

¹ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
² Instituto de Astrofísica de Canarias and Dpto. de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
³ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁵ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁶ Max-Planck-Institut für Physik, 80805 München, Germany
⁷ Università di Padova and INFN, 35131 Padova, Italy
⁸ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Barcelona, Spain
⁹ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁰ Croatian MAGIC Group: University of Zagreb, Faculty of Electrical Engineering and Computing (FER), 10000 Zagreb, Croatia
¹¹ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹² Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro, RJ, Brazil
¹³ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁴ ETH Zürich, 8093 Zürich, Switzerland
¹⁵ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁶ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
¹⁷ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
¹⁸ Department for Physics and Technology, University of Bergen, Bergen, Norway
¹⁹ INFN MAGIC Group: INFN Sezione di Catania and Dipartimento di Fisica e Astronomia, University of Catania, 95123 Catania, Italy
²⁰ INFN MAGIC Group: INFN Sezione di Torino and Università degli Studi di Torino, 10125 Torino, Italy
²¹ INFN MAGIC Group: INFN Sezione di Bari and Dipartimento Interateneo di Fisica dell’Università e del Politecnico di Bari, 70125 Bari, Italy
²² Croatian MAGIC Group: University of Rijeka, Faculty of Physics, 51000 Rijeka, Croatia
²³ University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
²⁴ Japanese MAGIC Group: Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 739-8526 Hiroshima, Japan
²⁵ Armenian MAGIC Group: ICRANet-Armenia, 0019 Yerevan, Armenia
²⁶ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
²⁷ Croatian MAGIC Group: Josip Juraj Strossmayer University of Osijek, Department of Physics, 31000 Osijek, Croatia
²⁸ Universität Würzburg, 97074 Würzburg, Germany
²⁹ Finnish MAGIC Group: Finnish Centre for Astronomy with ESO, University of Turku, 20014 Turku, Finland
³⁰ Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka, 259-1292 Kanagawa, Japan
³¹ Università di Siena and INFN Pisa, 53100 Siena, Italy
³² Saha Institute of Nuclear Physics, A CI of Homi Bhabha National Institute, Kolkata, 700064 West Bengal, India
³³ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
³⁴ Japanese MAGIC Group: Department of Physics, Yamagata University, Yamagata 990-8560, Japan
³⁵ Finnish MAGIC Group: Space Physics and Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
³⁶ Japanese MAGIC Group: Chiba University, ICEHAP, 263-8522 Chiba, Japan
³⁷ Japanese MAGIC Group: Institute for Space-Earth Environmental Research and Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, 464-6801 Nagoya, Japan
³⁸ Japanese MAGIC Group: Department of Physics, Kyoto University, 606-8502 Kyoto, Japan
³⁹ INFN MAGIC Group: INFN Sezione di Perugia, 06123 Perugia, Italy
⁴⁰ INFN MAGIC Group: INFN Roma Tor Vergata, 00133 Roma, Italy
⁴¹ Japanese MAGIC Group: Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
⁴² International Center for Relativistic Astrophysics (ICRA), Rome, Italy
⁴³ Institute for Astro- and Particle Physics, University of Innsbruck, 6020 Innsbruck, Austria
⁴⁴ Port d’Informació Científica (PIC), 08193 Bellaterra, Barcelona, Spain
⁴⁵ Institute for Astro- and Particle Physics, University of Innsbruck, 6020 Innsbruck, Austria
⁴⁶ Department of Physics, University of Oslo, Oslo, Norway
⁴⁷ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁴⁸ Max-Planck-Institut für Physik, 80805 München, Germany
⁴⁹ INAF Padova, Padova, Italy
⁵⁰ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁵¹ Space Science Data Center (SSDC) – ASI, Via del Politecnico, s.n.c., 00133 Roma, Italy
⁵² INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monteporzio, Italy
⁵³ Italian Space Agency, ASI, Via del Politecnico snc, 00133 Roma, Italy
⁵⁴ School of Astrophysics, Presidency University, 86/1 College Street, Kolkata 700073, India
⁵⁵ INAF-Osservatorio Astrofisico di Torino, 10025 Pino Torinese, TO, Italy
⁵⁶ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, Sofia, Bulgaria
⁵⁷ Department of Physics and Astronomy, Faculty of Natural Sciences, University of Shumen, 115, Universitetska Str., 9712 Shumen, Bulgaria
⁵⁸ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
⁵⁹ Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia
⁶⁰ Pulkovo Observatory, St.-Petersburg 196140, Russia
⁶¹ Special Astrophysical Observatory, Russian Academy of Sciences, 369167 Nizhnii Arkhyz, Russia
⁶² Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli 32001, Taiwan
⁶³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave, Boston, MA 02215, USA
⁶⁴ Astronomical Institute of St. Petersburg State University, 28 Universitetskij Pr., 198504 St. Petersburg, Russia
⁶⁵ EPT Observatories, Tijarafe, 38780 La Palma, Spain
⁶⁶ INAF, TNG Fundaci ón Galileo Galilei, 38712 La Palma, Spain
⁶⁷ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
⁶⁸ Zentrum für Astronomie der Universitüt Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁶⁹ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
⁷⁰ School of Cosmic Physics, Dublin Institute For Advanced Studies, Dublin, Ireland
⁷¹ Section of Astrophysics, Astronomy and Mechanics, Department of Physics, National and Kapodistrian University of Athens, 15784 Zografos, Athens, Greece
⁷² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
⁷³ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁷⁴ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
⁷⁵ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
⁷⁶ Department of Physics, Univ. of Crete, 70013 Heraklion, Greece
⁷⁷ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁷⁸ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁷⁹ Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Ruhr University Bochum, 44780 Bochum, Germany

Received: 31 August 2023
Accepted: 28 September 2023

Abstract

Aims. The BL Lac 1ES 2344+514 is known for temporary extreme properties characterised by a shift of the synchrotron spectral energy distribution (SED) peak energy ν_synch, p above 1 keV. While those extreme states have only been observed during high flux levels thus far, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to very high energy (VHE) performed so far, focussing on a systematic characterisation of the intermittent extreme states.

Methods. We organised a monitoring campaign covering a 3-year period from 2019 to 2021. More than ten instruments participated in the observations in order to cover the emission from radio to VHE. In particular, sensitive X-ray measurements by XMM-Newton, NuSTAR, and AstroSat took place simultaneously with multi-hour MAGIC observations, providing an unprecedented constraint of the two SED components for this blazar.

Results. While our results confirm that 1ES 2344+514 typically exhibits ν_synch, p > 1 keV during elevated flux periods, we also find periods where the extreme state coincides with low flux activity. A strong spectral variability thus happens in the quiescent state, and is likely caused by an increase in the electron acceleration efficiency without a change in the electron injection luminosity. On the other hand, we also report a strong X-ray flare (among the brightest for 1ES 2344+514) without a significant shift of ν_synch, p. During this particular flare, the X-ray spectrum is among the softest of the campaign. It unveils complexity in the spectral evolution, where the common harder-when-brighter trend observed in BL Lacs is violated. By combining Swift-XRT and Swift-UVOT measurements during a low and hard X-ray state, we find an excess of the UV flux with respect to an extrapolation of the X-ray spectrum to lower energies. This UV excess implies that at least two regions significantly contribute to the infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC, XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly associated with the 10 GHz radio core may explain such an excess. Finally, we investigate a VHE flare, showing an absence of simultaneous variability in the 0.3−2 keV band. Using time-dependent leptonic modelling, we show that this behaviour, in contradiction to single-zone scenarios, can instead be explained by a two-component model.