VHE gamma-ray detection of FSRQ QSO B1420+326 and modeling of its enhanced broadband state in 2020

¹ Instituto de Astrofísica de Canarias and Dpto. de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
² Università di Udine and INFN Trieste, 33100 Udine, Italy
³ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁴ ETH Zürich, 8093 Zürich, Switzerland
⁵ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Barcelona, Spain
⁶ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa 277-8582, Chiba, Japan
⁷ Technische Universität Dortmund, 44221 Dortmund, Germany
⁸ Croatian MAGIC Group: University of Zagreb, Faculty of Electrical Engineering and Computing (FER), 10000 Zagreb, Croatia
⁹ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁰ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro, RJ, Brazil
¹¹ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹² Università di Siena and INFN Pisa, 53100 Siena, Italy
¹³ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹⁴ Università di Padova and INFN, 35131 Padova, Italy
¹⁵ INFN MAGIC Group: INFN Sezione di Torino and Università degli Studi di Torino, 10125 Torino, Italy
¹⁶ Max-Planck-Institut für Physik, 80805 München, Germany
¹⁷ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
¹⁸ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
¹⁹ Armenian MAGIC Group: A. Alikhanyan National Science Laboratory, Yerevan, Armenia
²⁰ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
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, Department of Physics, 51000 Rijeka, Croatia
²³ Universität Würzburg, 97074 Würzburg, Germany
²⁴ Finnish MAGIC Group: Finnish Centre for Astronomy with ESO, University of Turku, 20014 Turku, Finland
²⁵ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁶ Armenian MAGIC Group: ICRANet-Armenia at NAS RA, Yerevan, Armenia
²⁷ Croatian MAGIC Group: University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture (FESB), 21000 Split, Croatia
²⁸ Croatian MAGIC Group: Josip Juraj Strossmayer University of Osijek, Department of Physics, 31000 Osijek, Croatia
²⁹ Japanese MAGIC Group: Department of Physics, Kyoto University, 606-8502 Kyoto, Japan
³⁰ Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka 259-1292, Kanagawa, Japan
³¹ Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
³² Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
³³ Finnish MAGIC Group: Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
³⁴ Croatian MAGIC Group: Ruđer Bošković Institute, 10000 Zagreb, Croatia
³⁵ INFN MAGIC Group: INFN Sezione di Perugia, 06123 Perugia, Italy
³⁶ INFN MAGIC Group: INFN Roma Tor Vergata, 00133 Roma, Italy
³⁷ Now at University of Innsbruck, Innsbruck, Austria
³⁸ Also at Port d’Informació Científica (PIC), 08193 Bellaterra, Barcelona, Spain
³⁹ Also at Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁴⁰ Max-Planck-Institut für Physik, 80805 München, Germany
⁴¹ Also at INAF Trieste and Dept. of Physics and Astronomy, University of Bologna, Bologna, Italy
⁴² Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa 277-8582, Chiba, Japan
⁴³ ASI Space Science Data Center, Via del Politecnico, snc., 00133 Rome, Italy
⁴⁴ INFN – Roma Tor Vergata, Via della Ricerca Scientifica, 1., 00133 Rome, Italy
⁴⁵ INAF – IRA Bologna, Via P. Gobetti 101, 40129 Bologna, Italy
⁴⁶ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
⁴⁷ Naval Research Laboratory, Washington, DC 20375, USA
⁴⁸ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere, 100, 00133 Rome, Italy
⁴⁹ Physical Research Laboratory, Ahmedabad, India
⁵⁰ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose Blvd., 1784 Sofia, Bulgaria
⁵¹ Department of Astronomy, Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria
⁵² Università di Siena, 53100 Siena, Italy
⁵³ Instituto Nacional de Astrofisica, Optica & Electronics Tonantzintla, Puebla, Mexico
⁵⁴ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁵⁵ Aalto University Department of Electronics and Nanoengineering, PO Box 15500, 00076 Aalto, Finland
⁵⁶ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Vesilinnantie 5, 20014 Turku, Finland
⁵⁷ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁵⁸ Astronomical Institute, St. Petersburg University, Universitetskij Pr. 28, Petrodvorets 198504, St. Petersburg, Russia
⁵⁹ Pulkovo Observatory, St. Petersburg, Russia
⁶⁰ Crimean Astrophysical Observatory, Crimea
⁶¹ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
⁶² Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
⁶³ Department of Physics, Univ. of Crete, 70013 Heraklion, Greece
⁶⁴ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁶⁵ CePIA, Departamento de Astronomía, Universidad de Concepción, Concepción, Chile

Received: 15 October 2020
Accepted: 18 December 2020

Abstract

Context. QSO B1420+326 is a blazar classified as a flat-spectrum radio quasar (FSRQ). At the beginning of the year 2020, it was found to be in an enhanced flux state and an extensive multiwavelength campaign allowed us to trace the evolution of the flare.

Aims. We search for very high-energy (VHE) gamma-ray emission from QSO B1420+326 during this flaring state. We aim to characterize and model the broadband emission of the source over different phases of the flare.

Methods. The source was observed with a number of instruments in radio, near-infrared, optical (including polarimetry and spectroscopy), ultraviolet, X-ray, and gamma-ray bands. We use dedicated optical spectroscopy results to estimate the accretion disk and the dust torus luminosity. We performed spectral energy distribution modeling in the framework of combined synchrotron-self-Compton and external Compton scenario in which the electron energy distribution is partially determined from acceleration and cooling processes.

Results. During the enhanced state, the flux of both SED components of QSO B1420+326 drastically increased and the peaks were shifted to higher energies. Follow-up observations with the MAGIC telescopes led to the detection of VHE gamma-ray emission from this source, making it one of only a handful of FSRQs known in this energy range. Modeling allows us to constrain the evolution of the magnetic field and electron energy distribution in the emission region. The gamma-ray flare was accompanied by a rotation of the optical polarization vector during a low -polarization state. Also, a new superluminal radio knot contemporaneously appeared in the radio image of the jet. The optical spectroscopy shows a prominent FeII bump with flux evolving together with the continuum emission and a MgII line with varying equivalent width.