Insights from the first flaring activity of a high synchrotron peaked blazar with X-ray polarization and VHE gamma rays

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, 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: University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture (FESB), 21000 Split, Croatia
³¹ Croatian MAGIC Group: Josip Juraj Strossmayer University of Osijek, Department of Physics, 31000 Osijek, Croatia
³² Finnish MAGIC Group: Finnish Centre for Astronomy with ESO, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
³³ University of Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland
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, BG-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, FI-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 Roma Tor Vergata, I-00133 Roma, Italy
⁴² Japanese MAGIC Group: Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
⁴³ also at International Center for Relativistic Astrophysics (ICRA), Rome, Italy
⁴⁴ also at Port d’Informació Científica (PIC), E-08193 Bellaterra (Barcelona), Spain
⁴⁵ also at Department of Physics, University of Oslo, 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, Padova, Italy
⁴⁹ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁵⁰ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁵¹ Institute of Astrophysics, Foundation for Research and Technology – Hellas, GR-70013 Heraklion, Crete, Greece
⁵² INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁵³ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁵⁴ Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁵⁵ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵⁶ Science & Technology Institute, Universities Space Research Association, 320 Sparkman Drive, Huntsville, AL 35805, USA
⁵⁷ 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
⁵⁸ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁵⁹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
⁶⁰ Hans-Haffner-Sternwarte, Hettstadt; Naturwissenschaftliches Labor für Schüler am FKG; Friedrich-Koenig-Gymnasium, D-97082 Würzburg, Germany
⁶¹ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park IC2, 146 Brownlow Hill, Liverpool, UK
⁶² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁶³ Perkins Telescope Observatory, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁶⁴ Institute of Integrated Research, Institute of Science Tokyo, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
⁶⁵ Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University Kagamiyama, 1-3-1 Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁶⁶ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁶⁷ Core Research for Energetic Universe (Core-U), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁶⁸ Astronomy Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
⁶⁹ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora, 7, Local 20, E–18012 Granada, Spain
⁷⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁷¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁷² Orchideenweg 8, 53123 Bonn, Germany
⁷³ INAF Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy
⁷⁴ now at Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France

^⋆ Corresponding authors; A. Arbet Engels, L. Heckmann, D. Paneque, E-mail: contact.magic@mpp.mpg.de

Received: 28 October 2024
Accepted: 17 January 2025

Abstract

Context. Blazars exhibit strong variability across the entire electromagnetic spectrum, including periods of high-flux states commonly known as flares. The physical mechanisms in blazar jets responsible for flares remain poorly understood to date.

Aims. Our aim is to better understand the emission mechanisms during blazar flares using X-ray polarimetry and broadband observations from the archetypical TeV blazar Mrk 421, which can be studied with higher accuracy than other blazars that are dimmer and/or located farther away.

Methods. We studied a flaring activity from December 2023 that was characterized from radio to very high-energy (VHE; E > 0.1 TeV) gamma rays with MAGIC, Fermi-LAT, Swift, XMM-Newton, and several optical and radio telescopes. These observations included, for the first time for a gamma-ray flare of a blazar, simultaneous X-ray polarization measurements with IXPE, in addition to optical and radio polarimetry data. We quantify the variability and correlations among the multi-band flux and polarization measurements, and describe the varying broadband emission within a theoretical scenario constrained by the polarization data.

Results. We find substantial variability in both X-rays and VHE gamma rays throughout the campaign, with the highest VHE flux above 0.2 TeV occurring during the IXPE observing window, and exceeding twice the flux of the Crab Nebula. However, the VHE and X-ray spectra are on average softer, and the correlation between these two bands is weaker than those reported in the previous flares of Mrk 421. IXPE reveals an X-ray polarization degree significantly higher than that at radio and optical frequencies, similar to previous results for Mrk 421 and other high synchrotron peaked blazars. Differently to past observations, the X-ray polarization angle varies by ∼100° on timescales of days, and the polarization degree changes by more than a factor of 4. The highest X-ray polarization degree, analyzed in 12 h time intervals, reaches 26 ± 2%, around which an X-ray counter-clockwise hysteresis loop is measured with XMM-Newton. It suggests that the X-ray emission comes from particles close to the high-energy cutoff, hence possibly probing an extreme case of the Turbulent Extreme Multi-Zone model for which the chromatic trend in the polarization may be more pronounced than theoretically predicted. We model the broadband emission with a simplified stratified jet model throughout the flare. The polarization measurements imply an electron distribution in the X-ray emitting region with a very high minimum Lorentz factor ($\gamma {\prime }_{\min }\gtrsim {10}^4$), which is expected in electron-ion plasma, as well as a variation of the emitting region size of up to a factor of 3 during the flaring activity. We find no correlation between the fluxes and the evolution of the model parameters, which indicates a stochastic nature of the underlying physical mechanism that likely explains the lack of a tight X-ray/VHE correlation during this flaring activity. Such behavior would be expected in a highly turbulent electron-ion plasma crossing a shock front.