First characterization of the emission behavior of Mrk 421 from radio to very high-energy gamma rays with simultaneous X-ray polarization measurements^⋆

¹ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
² ETH Zürich, 8093 Zürich, Switzerland
³ Instituto de Astrofísica de Canarias and Dpto. de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
⁴ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁶ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁷ Max-Planck-Institut für Physik, 85748 Garching, Germany
⁸ Università di Padova and INFN, 35131 Padova, Italy
⁹ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra (Barcelona), Spain
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 University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹⁴ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
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, Norway
¹⁹ INFN MAGIC Group: INFN Sezione di Torino and Università degli Studi di Torino, 10125 Torino, Italy
²⁰ INFN MAGIC Group: INFN Sezione di Catania and Dipartimento di Fisica e Astronomia, University of Catania, 95123 Catania, 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 Technische Universität Dortmund, 44221 Dortmund, Germany
²⁴ University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
²⁵ Japanese MAGIC Group: Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 739-8526 Hiroshima, Japan
²⁶ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
²⁷ Armenian MAGIC Group: ICRANet-Armenia, 0019 Yerevan, Armenia
²⁸ Croatian MAGIC Group: University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture (FESB), 21000 Split, Croatia
²⁹ Universität Würzburg, 97074 Würzburg, Germany
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, University of Turku, 20014 Turku, Finland
³² Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka, 259-1292 Kanagawa, Japan
³³ Università di Siena and INFN Pisa, 53100 Siena, Italy
³⁴ Saha Institute of Nuclear Physics, A CI of Homi Bhabha National Institute, Kolkata, 700064 West Bengal, India
³⁵ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
³⁶ Japanese MAGIC Group: Department of Physics, Yamagata University, Yamagata 990-8560, Japan
³⁷ Finnish MAGIC Group: Space Physics and Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
³⁸ Japanese MAGIC Group: Chiba University, ICEHAP, 263-8522 Chiba, Japan
³⁹ 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
⁴⁰ Japanese MAGIC Group: Department of Physics, Kyoto University, 606-8502 Kyoto, Japan
⁴¹ INFN MAGIC Group: INFN Sezione di Perugia, 06123 Perugia, Italy
⁴² INFN MAGIC Group: INFN Roma Tor Vergata, 00133 Roma, Italy
⁴³ Japanese MAGIC Group: Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
⁴⁴ Also at International Center for Relativistic Astrophysics (ICRA), Rome, Italy
⁴⁵ Now at Institute for Astro- and Particle Physics, University of Innsbruck, 6020 Innsbruck, Austria
⁴⁶ Also at Port d’Informació Científica (PIC), 08193 Bellaterra, (Barcelona), Spain
⁴⁷ Also at Institute for Astro- and Particle Physics, University of Innsbruck, 6020 Innsbruck, Austria
⁴⁸ Also at Department of Physics, University of Oslo, Norway
⁴⁹ Also at Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁵⁰ Max-Planck-Institut für Physik, 85748 Garching, Germany
⁵¹ Also at INAF, 35122 Padova, Italy
⁵² Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁵³ INAF Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁵⁴ Finnish Centre for Astronomy with ESO, 20014 University of Turku, Finland
⁵⁵ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁵⁶ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁵⁷ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁵⁸ Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁵⁹ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁶⁰ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁶¹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
⁶² Quasar Science Resource S.L. for the European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁶³ Foundation for Research and Technology – IESL Hellas, Greece
⁶⁴ Institute of Astrophysics, Voutes, 7110 Heraklion, Greece
⁶⁵ Department of Physics, University of Crete, 70013 Heraklion, Greece
⁶⁶ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
⁶⁷ Owens Valley Radio Observatory, California Institute of Technology, MC 249-17, Pasadena, CA 91125, USA
⁶⁸ Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
⁶⁹ Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University Kagamiyama, 1-3-1 Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷⁰ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷¹ Core Research for Energetic Universe (Core-U), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷² Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
⁷³ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora, 7, Local 20, 18012 Granada, Spain
⁷⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷⁵ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁷⁶ Aalto University Department of Electronics and Nanoengineering, PO Box 15500 00076 Aalto, Finland
⁷⁷ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁷⁸ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁷⁹ Also affiliated at Physik Department, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany

Received: 15 September 2023
Accepted: 16 December 2023

Abstract

Aims. We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE.

Methods. The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC, Fermi-LAT, NuSTAR, XMM-Newton, Swift, and several optical and radio telescopes to complement IXPE data.

Results. During the IXPE exposures, the measured 0.2–1 TeV flux was close to the quiescent state and ranged from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the very high-energy (VHE) and X-ray emission are positively correlated at a 4σ significance level. The IXPE measurements reveal an X-ray polarization degree that is a factor of 2–5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, the Swift-XRT monitoring reveals an X-ray flux increase with a clear spectral hardening. This suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation, NuSTAR data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counterclockwise), implying important changes in the particle acceleration efficiency on approximately hour timescales.