Insights into the broadband emission of the TeV blazar Mrk 501 during the first 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 Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
⁵ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁷ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁸ Max-Planck-Institut für Physik, 85748 Garching bei München, Germany
⁹ Università di Padova and INFN, 35131 Padova, Italy
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 Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, (Barcelona), Spain
¹⁶ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁷ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
¹⁸ Department for Physics and Technology, University of Bergen, 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 Also at Port d’Informació Científica (PIC), 08193 Bellaterra, (Barcelona), Spain
⁴⁶ Also at Institute for Astro- and Particle Physics, University of Innsbruck, 6020 Innsbruck, Austria
⁴⁷ Also at Department of Physics, University of Oslo, Oslo, Norway
⁴⁸ Also at Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁴⁹ Max-Planck-Institut für Physik, 80805 München, Germany
⁵⁰ Also at INAF Padova, Italy
⁵¹ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁵² Finnish Centre for Astronomy with ESO, 20014 University of Turku, Finland
⁵³ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁵⁴ Department of Physics & McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
⁵⁵ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁵⁶ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc 00133 Roma, Italy
⁵⁷ INAF Osservatorio Astronomico di Roma, Via Frascati 33 00078 Monte Porzio Catone, (RM), Italy
⁵⁸ Department of Physics and Astronomy, 20014 University of Turku, Finland
⁵⁹ Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Ruhr University Bochum, 44780 Bochum, Germany
⁶⁰ Institute of Astrophysics, Foundation for Research and Technology – Hellas, 100 Nikolaou Plastira str. Vassilika Vouton 70013 Heraklion, Crete, Greece
⁶¹ Department of Physics, University of Crete, Vasilika Bouton 70013 Heraklion, Greece
⁶² Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
⁶³ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora, 7, Local 20 18012 Granada, Spain
⁶⁴ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n 18008 Granada, Spain
⁶⁵ Geological and Mining Institute of Spain (IGME-CSIC), Calle Ríos Rosas 23 28003 Madrid, Spain
⁶⁶ University of Siena, Department of Physical Sciences, Earth and Environment, Astronomical Observatory, Via Roma 56 53100 Siena, Italy
⁶⁷ Aalto University Metsähovi Radio Observatory, Metsähovintie 114 02540 Kylmälä, Finland
⁶⁸ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁶⁹ Departamento de Astronomía Universidad de Conceptión, Concepción, Chile
⁷⁰ Aalto University Department of Electronics and Nanoengineering, PO BOX 15500 00076 Aalto, Finland
⁷¹ Unidad Asociada al CSIC, Departamento de Ingeniería de Sistemas y Automática, Escuela de Ingenierías, Universidad de Málaga, Málaga, Spain
⁷² Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
⁷³ Hiroshima Astrophysical Science Center, Hiroshima University 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷⁴ Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University Kagamiyama, 1-3-1 Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷⁵ Core Research for Energetic Universe (Core-U), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷⁶ Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
⁷⁷ Italian Space Agency, ASI, Via del Politecnico snc 00133 Roma, Italy
⁷⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁷⁹ INAF – Istituto di Radioastronomia, Via Gobetti 101 40129 Bologna, Italy

Received: 22 November 2023
Accepted: 14 January 2024

Abstract

Aims. We present the first multiwavelength study of Mrk 501 that contains simultaneous very-high-energy (VHE) γ-ray observations and X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE).

Methods. We used radio-to-VHE data from a multiwavelength campaign carried out between March 1, 2022, and July 19, 2022 (MJD 59639 to MJD 59779). The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and several other instruments that cover the optical and radio bands to complement the IXPE pointings. We characterized the dynamics of the broadband emission around the X-ray polarization measurements through its multiband fractional variability and correlations, and compared changes observed in the polarization degree to changes seen in the broadband emission using a multi-zone leptonic scenario.

Results. During the IXPE pointings, the VHE state is close to the average behavior, with a 0.2–1 TeV flux of 20%–50% of the emission of the Crab Nebula. Additionally, it shows low variability and a hint of correlation between VHE γ-rays and X-rays. Despite the average VHE activity, an extreme X-ray behavior is measured for the first two IXPE pointings, taken in March 2022 (MJD 59646 to 59648 and MJD 59665 to 59667), with a synchrotron peak frequency > 1 keV. For the third IXPE pointing, in July 2022 (MJD 59769 to 59772), the synchrotron peak shifts toward lower energies and the optical/X-ray polarization degrees drop. All three IXPE epochs show an atypically low Compton dominance in the γ-rays. The X-ray polarization is systematically higher than at lower energies, suggesting an energy stratification of the jet. While during the IXPE epochs the polarization angles in the X-ray, optical, and radio bands align well, we find a clear discrepancy in the optical and radio polarization angles in the middle of the campaign. Such results further support the hypothesis of an energy-stratified jet. We modeled broadband spectra taken simultaneous to the IXPE pointings, assuming a compact zone that dominates in the X-rays and the VHE band, and an extended zone stretching farther downstream in the jet that dominates the emission at lower energies. NuSTAR data allow us to precisely constrain the synchrotron peak and therefore the underlying electron distribution. The change between the different states observed in the three IXPE pointings can be explained by a change in the magnetization and/or the emission region size, which directly connects the shift in the synchrotron peak to lower energies with the drop in the polarization degree.