Very high-energy gamma-ray detection and long-term multiwavelength view of the flaring blazar B2 1811+31

¹ Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka, 259-1292 Kanagawa, Japan
² Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
³ ETH Zürich, CH-8093 Zürich, Switzerland
⁴ Università di Siena and INFN Pisa, I-53100 Siena, Italy
⁵ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), E-08193 Bellaterra (Barcelona), Spain
⁶ Universitat de Barcelona, ICCUB, IEEC-UB, E-08028 Barcelona, Spain
⁷ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008, Granada, Spain
⁸ National Institute for Astrophysics (INAF), I-00136 Rome, Italy
⁹ Università di Udine and INFN Trieste, I-33100 Udine, Italy
¹⁰ Max-Planck-Institut für Physik, D-85748 Garching, Germany
¹¹ Università di Padova and INFN, I-35131 Padova, Italy
¹² Croatian MAGIC Group: University of Zagreb, Faculty of Electrical Engineering and Computing (FER), 10000 Zagreb, Croatia
¹³ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro (RJ), Brazil
¹⁴ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, E-28040 Madrid, Spain
¹⁵ Instituto de Astrofísica de Canarias and Dpto. de Astrofísica, Universidad de La Laguna, E-38200, La Laguna, Tenerife, Spain
¹⁶ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹⁷ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, E-28040 Madrid, Spain
¹⁸ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain
¹⁹ Università di Pisa and INFN Pisa, I-56126 Pisa, Italy
²⁰ INFN MAGIC Group: INFN Sezione di Bari and Dipartimento Interateneo di Fisica dell’Università e del Politecnico di Bari, I-70125 Bari, Italy
²¹ Armenian MAGIC Group: A. Alikhanyan National Science Laboratory, 0036 Yerevan, Armenia
²² Department for Physics and Technology, University of Bergen, Norway
²³ INFN MAGIC Group: INFN Sezione di Torino and Università degli Studi di Torino, I-10125 Torino, Italy
²⁴ Croatian MAGIC Group: University of Rijeka, Faculty of Physics, 51000 Rijeka, Croatia
²⁵ Universität Würzburg, D-97074 Würzburg, Germany
²⁶ Technische Universität Dortmund, D-44221 Dortmund, Germany
²⁷ Japanese MAGIC Group: Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 739-8526 Hiroshima, Japan
²⁸ Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany
²⁹ Armenian MAGIC Group: ICRANet-Armenia, 0019 Yerevan, Armenia
³⁰ Croatian MAGIC Group: Josip Juraj Strossmayer University of Osijek, Department of Physics, 31000 Osijek, Croatia
³¹ Finnish MAGIC Group: Finnish Centre for Astronomy with ESO, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
³² University of Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland
³³ Saha Institute of Nuclear Physics, A CI of Homi Bhabha National Institute, Kolkata 700064, West Bengal, India
³⁴ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, BG-1784 Sofia, Bulgaria
³⁵ Japanese MAGIC Group: Department of Physics, Yamagata University, Yamagata 990-8560, Japan
³⁶ Finnish MAGIC Group: Space Physics and Astronomy Research Unit, University of Oulu, FI-90014 Oulu, Finland
³⁷ Japanese MAGIC Group: Chiba University, ICEHAP, 263-8522 Chiba, Japan
³⁸ 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
³⁹ Japanese MAGIC Group: Department of Physics, Kyoto University, 606-8502 Kyoto, Japan
⁴⁰ INFN MAGIC Group: INFN Roma Tor Vergata, I-00133 Roma, Italy
⁴¹ Japanese MAGIC Group: Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
⁴² Also at International Center for Relativistic Astrophysics (ICRA), Rome, Italy
⁴³ Also at Port d’Informació Científica (PIC), E-08193 Bellaterra (Barcelona), Spain
⁴⁴ Now at Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
⁴⁵ Also at Department of Physics, University of Oslo, Norway
⁴⁶ Also at Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
⁴⁷ Max-Planck-Institut für Physik, D-85748 Garching, Germany
⁴⁸ Also at INAF Padova, I-35131 Padova, Italy
⁴⁹ INAF Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁵⁰ Institute of Astrophysics, Foundation for Research and Technology-Hellas, GR-71110 Heraklion, Greece
⁵¹ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
⁵² Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Straße 31, D-97074 Würzburg, Germany
⁵³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁵⁴ University of Siena, Department of Physical Sciences, Earth and Environment, Astronomical Observatory, Via Roma 56, 53100 Siena, Italy
⁵⁵ Hans-Haffner-Sternwarte (Hettstadt), Naturwissenschaftliches Labor für Schüler am FKG, Friedrich-Koenig-Gymnasium, D-97082 Würzburg, Germany
⁵⁶ Lehrstuhl für Astronomie, Universität Würzburg, D-97074 Würzburg, Germany
⁵⁷ Astroteilchenphysik, TU Dortmund, Otto-Hahn-Str. 4A, D-44227 Dortmund, Germany
⁵⁸ Dipartimento di Fisica ”M. Merlin” dell’Università e del Politecnico di Bari, Via Amendola 173, I-70126 Bari, Italy
⁵⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Bari, I-70126 Bari, Italy
⁶⁰ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
⁶¹ Department of Physics and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis 63130, USA
⁶² Remeis Observatory and Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁶³ Finnish Center for Astronomy with ESO (FINCA), Quantum, Vesilinnantie 5, FI-20014 University of Turku, Finland
⁶⁴ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁶⁵ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA91125, USA
⁶⁶ University of Trento, 38123, Trento, Italy

^⋆ Corresponding authors; D. Cerasole, S. Loporchio and L. Pavletić, contact.magic@mpp.mpg.de

Received: 9 November 2024
Accepted: 20 March 2025

Abstract

Context. Among the blazars whose emission has been detected up to very high-energy (VHE; 100 GeV<E<100 TeV) γ rays, intermediate synchrotron-peaked BL Lacs (IBLs) are quite rare. The IBL B2 1811+31 (z = 0.117) exhibited intense flaring activity in 2020. Detailed characterization of the source emission from radio to γ-ray energies was achieved with quasi-simultaneous observations, which led to the first-time detection of VHE γ-ray emission from the source with the MAGIC telescopes.

Aims. In this work, we present a comprehensive multiwavelength (MWL) view of B2 1811+31, with a specific focus on the 2020 VHE flare, employing data from MAGIC, Fermi-LAT, Swift-XRT, Swift-UVOT, and several optical and radio ground-based telescopes.

Methods. Long-term MWL data were employed to contextualize the high-state episode within the source emissions over 18 years. We investigated the variability, cross-correlations, and classification of the source emissions during low and high states. We propose an interpretative leptonic model for the observed radiative high state.

Results. During the 2020 flaring state, the synchrotron peak frequency shifted to higher values and reached the limit of the IBL classification. Variability in timescales of a few hours in the high-energy (HE; 100 MeV<E<100 GeV) γ-ray band poses an upper limit of 6×10¹⁴ δ_D cm on the size of the emission region responsible for the γ-ray flare, with δ_D being the relativistic Doppler factor of the region. During the 2020 high state, the average spectrum became harder in the HE γ-ray band compared to the low states. A similar behavior has been observed in X-rays. Conversely, during different activity periods, we find harder-when-brighter trends in X-rays and a hint of softer-when-brighter trends at HE γ rays. A long-term HE γ-ray and optical correlation indicates that the same emission regions dominate the radiative output in both ranges, whereas the evolution at 15 GHz shows no correlation with the fluxes at higher frequencies. We test one-zone and two-zone synchrotron-self-Compton models for describing the broadband spectral energy distribution during the 2020 flaring state and investigate the self-consistency of the proposed scenario.