Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017^⋆

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 Max-Planck-Institut für Physik, 80805 München, Germany
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 Università di Padova and INFN, 35131 Padova, Italy
¹² University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹³ Università di Siena and INFN Pisa, 53100 Siena, Italy
¹⁴ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹⁵ INFN MAGIC Group: INFN Sezione di Torino and Università degli Studi di Torino, 10125 Torino, Italy
¹⁶ ETH Zürich, 8093 Zürich, Switzerland
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: RIKEN, Wako, Saitama, 351-0198, Japan
³⁰ Japanese MAGIC Group: Department of Physics, Kyoto University, 606-8502 Kyoto, Japan
³¹ Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka, 259-1292 Kanagawa, Japan
³² Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata, 700064, India
³³ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
³⁴ Finnish MAGIC Group: Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
³⁵ Croatian MAGIC Group: Ruđer Bošković Institute, 10000 Zagreb, Croatia
³⁶ INFN MAGIC Group: INFN Sezione di Perugia, 06123 Perugia, Italy
³⁷ INFN MAGIC Group: INFN Roma Tor Vergata, 00133 Roma, Italy
³⁸ Now at University of Innsbruck, Innsbruck, Austria
³⁹ Also at Port d’Informació Científica (PIC), 08193 Bellaterra, Barcelona, Spain
⁴⁰ Now at Ruhr-Universität Bochum, Fakultät für Physik und Astronomie, Astronomisches Institut (AIRUB), 44801 Bochum, Germany
⁴¹ Also at Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁴² Also at INAF Trieste and Dept. of Physics and Astronomy, University of Bologna, Bologna, Italy
⁴³ University of Geneva, Department of Astronomy, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
⁴⁴ Also at RWTH Aachen University, Aachen, Germany
⁴⁵ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁴⁶ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD, 20771, USA
⁴⁷ Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD, 21250, USA
⁴⁸ Yale Center for Astronomy & Astrophysics, 52 Hillhouse Avenue, New Haven, CT, 06511, USA
⁴⁹ Department of Physics, Yale University, PO Box 2018120, New Haven, CT, 06520, USA
⁵⁰ W.W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator, Menlo Park, USA
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 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 Astronomy Department, Universidad de Concepción, Casilla 160-C Concepción, Chile
⁵⁷ European Space Agency, European Space Astronomy Centre, C/ Bajo el Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁵⁸ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital, 263 001, India
⁵⁹ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶⁰ Foundation for Research and Technology – Hellas, IESL & Institute of Astrophysics, Voutes, 7110 Heraklion, Greece
⁶¹ Department of Physics, University of Crete, 71003 Heraklion, Greece
⁶² Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, Sofia, Bulgaria
⁶³ Department of Physics and Astronomy, Faculty of Natural Sciences, University of Shumen, 115, Universitetska Str., 9712 Shumen, Bulgaria
⁶⁴ University of Siena, Department of Physical Sciences, Earth and Environment, Astronomical Observatory, Via Roma 56, 53100 Siena, Italy
⁶⁵ Department of Astronomy, Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria
⁶⁶ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
⁶⁷ Astronomical Institute, St. Petersburg State University, St. Petersburg, 198504, Russia
⁶⁸ Pulkovo Observatory, St.-Petersburg, 196140, Russia
⁶⁹ Crimean Astrophysical Observatory RAS, P/O Nauchny, 298409, Crimea
⁷⁰ Special Astrophysical Observatory, Russian Academy of Sciences, 369167 Nizhnii Arkhyz, Russia
⁷¹ Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli, 32001, Taiwan
⁷² INAF–Osservatorio Astrofisico di Torino, 10025 Pino Torinese (TO), Italy
⁷³ Department of Physics and Astronomy, Brigham Young University, Provo, UT, 84602, USA
⁷⁴ Department of Physics, University of Colorado Denver, Denver, Colorado, CO, 80217-3364, USA
⁷⁵ Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave, Boston, MA, 02215, USA
⁷⁶ Sobolev Astronomical Institute, St. Petersburg State University, St. Petersburg, Russia
⁷⁷ Hans-Haffner-Sternwarte, Naturwissenschaftliches Labor für Schüler; Friedrich-Koenig-Gymnasium, 97082 Würzburg, Germany
⁷⁸ EPT Observatories, Tijarafe, 38780 La Palma, Spain
⁷⁹ INAF, TNG Fundaci ón Galileo Galilei, 38712 La Palma, Spain
⁸⁰ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
⁸¹ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁸² Samtskhe-Javakheti State University, Rustaveli Str. 113, 0080 Akhaltsikhe, Georgia
⁸³ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
⁸⁴ INAF Istituto di Radioastronomia, Stazione di Medicina, Via Fiorentina 3513, 40059 Villafontana (BO), Italy
⁸⁵ Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
⁸⁶ Osservatorio Astronomico Sirio, 70013 Castellana Grotte, Italy
⁸⁷ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain

Received: 6 April 2021
Accepted: 7 June 2021

Abstract

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 Metsähovi. 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 different 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 3σ. 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 γ′_min=2×10⁴ to γ′_max=6×10⁵.