Characterization of Markarian 421 during its most violent year: Multiwavelength variability and correlations

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 Technische Universität Dortmund, D-44221 Dortmund, Germany
¹³ Croatian MAGIC Group: University of Zagreb, Faculty of Electrical Engineering and Computing (FER), 10000 Zagreb, Croatia
¹⁴ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro, (RJ), Brazil
¹⁵ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, E-28040 Madrid, Spain
¹⁶ Instituto de Astrofísica de Canarias and Dpto. de Astrofísica, Universidad de La Laguna, E-38200 La Laguna, Tenerife, Spain
¹⁷ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹⁸ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, E-28040 Madrid, Spain
¹⁹ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain
²⁰ Università di Pisa and INFN Pisa, I-56126 Pisa, Italy
²¹ INFN MAGIC Group: INFN Sezione di Bari and Dipartimento Interateneo di Fisica dell’Università e del Politecnico di Bari, I-70125 Bari, Italy
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 Japanese MAGIC Group: Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 739-8526 Hiroshima, Japan
²⁷ Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany
²⁸ Armenian MAGIC Group: ICRANet-Armenia, 0019 Yerevan, Armenia
²⁹ Croatian MAGIC Group: University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture (FESB), 21000 Split, Croatia
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, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
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 University of Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland
⁴¹ Japanese MAGIC Group: Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan
⁴² also at International Center for Relativistic Astrophysics (ICRA), Rome, Italy
⁴³ also at Port d’Informació Científica (PIC), E-08193 Bellaterra, (Barcelona), Spain
⁴⁴ also at Department of Physics, University of Oslo, Oslo, Norway
⁴⁵ also at Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
⁴⁶ also at INAF, 35122 Padova, Italy
⁴⁷ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁴⁸ INAF Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy
⁴⁹ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁵⁰ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁵¹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, (RM), Italy
⁵² ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁵³ Department of Astronomy, University of Michigan, 323 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA
⁵⁴ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁵⁵ Institute of Astrophysics, Foundation for Research and Technology – Hellas, 100 Nikolaou Plastira Str. Vassilika Vouton, 70013 Heraklion, Crete, Greece
⁵⁶ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
⁵⁷ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁵⁸ Aalto University Department of Electronics and Nanoengineering, PO Box 15500 FI-00076 Aalto, Finland
⁵⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁶⁰ Space Science Institute, 4765 Walnut St., Suite B, Boulder, CO 80301, USA
⁶¹ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland

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

Received: 23 July 2024
Accepted: 27 December 2024

Abstract

Aims. Mrk 421 was in its most active state around early 2010, which led to the highest TeV gamma-ray flux ever recorded from any active galactic nuclei (AGN). We aim to characterize the multiwavelength behavior during this exceptional year for Mrk 421, and evaluate whether it is consistent with the picture derived with data from other less exceptional years.

Methods. We investigated the period from November 5, 2009, (MJD 55140) until July 3, 2010, (MJD 55380) with extensive coverage from very-high-energy (VHE; E > 100 GeV) gamma rays to radio with MAGIC, VERITAS, Fermi-LAT, RXTE, Swift, GASP-WEBT, VLBA, and a variety of additional optical and radio telescopes. We characterized the variability by deriving fractional variabilities as well as power spectral densities (PSDs). In addition, we investigated images of the jet taken with VLBA and the correlation behavior among different energy bands.

Results. Mrk 421 was in widely different states of activity throughout the campaign, ranging from a low-emission state to its highest VHE flux ever recorded. We find the strongest variability in X-rays and VHE gamma rays, and PSDs compatible with power-law functions with indices around 1.5. We observe strong correlations between X-rays and VHE gamma rays at zero time lag with varying characteristics depending on the exact energy band. We also report a marginally significant (∼3σ) positive correlation between high-energy (HE; E > 100 MeV) gamma rays and the ultraviolet band. We detected marginally significant (∼3σ) correlations between the HE and VHE gamma rays, and between HE gamma rays and the X-ray, that disappear when the large flare in February 2010 is excluded from the correlation study, hence indicating the exceptionality of this flaring event in comparison with the rest of the campaign. The 2010 violent activity of Mrk 421 also yielded the first ejection of features in the VLBA images of the jet of Mrk 421. Yet the large uncertainties in the ejection times of these unprecedented radio features prevent us from firmly associating them to the specific flares recorded during the 2010 campaign. We also show that the collected multi-instrument data are consistent with a scenario where the emission is dominated by two regions, a compact and extended zone, which could be considered as a simplified implementation of an energy-stratified jet as suggested by recent IXPE observations.