Time-dependent modelling of short-term variability in the TeV-blazar VER J0521+211 during the major flare in 2020

¹ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
² ETH Zürich, CH-8093 Zürich, Switzerland
³ Università di Siena and INFN Pisa, I-53100 Siena, Italy
⁴ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), E-08193 Bellaterra, (Barcelona), Spain
⁵ Universitat de Barcelona, ICCUB, IEEC-UB, E-08028 Barcelona, Spain
⁶ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ National Institute for Astrophysics (INAF), I-00136 Rome, Italy
⁸ Università di Udine and INFN Trieste, I-33100 Udine, Italy
⁹ Max-Planck-Institut für Physik, D-85748 Garching, Germany
¹⁰ Università di Padova and INFN, I-35131 Padova, Italy
¹¹ Technische Universität Dortmund, D-44221 Dortmund, Germany
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 Department for Physics and Technology, University of Bergen, Bergen, Norway
²² INFN MAGIC Group: INFN Sezione di Torino and Università degli Studi di Torino, I-10125 Torino, Italy
²³ Croatian MAGIC Group: University of Rijeka, Faculty of Physics, 51000 Rijeka, Croatia
²⁴ Universität Würzburg, D-97074 Würzburg, Germany
²⁵ Japanese MAGIC Group: Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 739-8526 Hiroshima, Japan
²⁶ Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany
²⁷ Armenian MAGIC Group: ICRANet-Armenia, 0019 Yerevan, Armenia
²⁸ Croatian MAGIC Group: Josip Juraj Strossmayer University of Osijek, Department of Physics, 31000 Osijek, Croatia
²⁹ Finnish MAGIC Group: Finnish Centre for Astronomy with ESO, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
³⁰ Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka, 259-1292 Kanagawa, Japan
³¹ University of Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland
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, BG-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, FI-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 Roma Tor Vergata, I-00133 Roma, Italy
⁴⁰ Japanese MAGIC Group: Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
⁴¹ Also at International Center for Relativistic Astrophysics (ICRA), Rome, Italy
⁴² Also at Port d’Informació Científica (PIC), E-08193 Bellaterra, (Barcelona), Spain
⁴³ Also at Department of Physics, University of Oslo, Oslo, Norway
⁴⁴ Also at Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
⁴⁵ Max-Planck-Institut für Physik, D-85748 Garching, Germany
⁴⁶ Also at INAF Padova, Italy
⁴⁷ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁴⁸ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
⁴⁹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, FI-20014 Turku, Finland
⁵⁰ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁵¹ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁵² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁵³ Institute of Astrophysics, Foundation for Research and Technology-Hellas, GR-70013 Heraklion, Greece
⁵⁴ Department of Physics, Univ. of Crete, GR-70013 Heraklion, Greece
⁵⁵ Aalto University Department of Electronics and Nanoengineering, PO BOX 15500, FI-00076 AALTO, Finland
⁵⁶ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁵⁷ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁵⁸ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Universitätsstraße 150, 44801 Bochum, Germany
⁵⁹ Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
⁶⁰ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁶¹ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
⁶² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁶³ Excellence Cluster ORIGINS, Boltzmannstr. 2, D-85748 Garching bei München, Germany
⁶⁴ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland

^⋆ Corresponding authors; contact.magic@mpp.mpg.de

Received: 4 July 2024
Accepted: 16 December 2024

Abstract

The BL Lacertae object VER J0521+211 underwent a notable flaring episode in February 2020. A short-term monitoring campaign, led by the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) collaboration, covering a wide energy range from radio to very high-energy (VHE, 100 GeV < E < 100 TeV) gamma rays was organised to study its evolution. These observations resulted in a consistent detection of the source over six consecutive nights in the VHE gamma-ray domain. Combining these nightly observations with an extensive set of multi-wavelength data made modelling of the blazar’s spectral energy distribution (SED) possible during the flare. This modelling was performed with a focus on two plausible emission mechanisms: (i) a leptonic two-zone synchrotron-self-Compton scenario, and (ii) a lepto-hadronic one-zone scenario. Both models effectively replicated the observed SED from radio to the VHE gamma-ray band. Furthermore, by introducing a set of evolving parameters, both models were successful in reproducing the evolution of the fluxes measured in different bands throughout the observing campaign. Notably, the lepto-hadronic model predicts enhanced photon and neutrino fluxes at ultra-high energies (E > 100 TeV). While the photon component, generated via decay of neutral pions, is not directly observable as it is subject to intense pair production (and therefore extinction) through interactions with the cosmic microwave background photons, neutrino detectors (e.g. IceCube) can probe the predicted neutrino component. Finally, the analysis of the gamma-ray spectra, observed by MAGIC and the Fermi-LAT telescopes, yielded a conservative 95% confidence upper limit of z ≤ 0.244 for the redshift of this blazar.