Magnetic field properties inside the jet of Mrk 421

Multiwavelength polarimetry, including the Imaging X-ray Polarimetry Explorer

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: dawoon.kim@inaf.it
² Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
³ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁵ Finnish Centre for Astronomy with ESO, 20014 University of Turku, Finland
⁶ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁷ Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia
⁸ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
¹⁰ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹¹ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹³ Dipartimento di Fisica, Universitá degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
¹⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
¹⁵ University of Maryland, Baltimore County, Baltimore, MD 21250, USA
¹⁶ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁷ Louisiana State University, Baton Rouge, LA 70803, USA
¹⁸ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁹ Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
²⁰ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²¹ Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University Kagamiyama, 1-3-1 Higashi-Hiroshima, Hiroshima 739-8526, Japan
²² Core Research for Energetic Universe (Core-U), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²³ Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
²⁴ Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
²⁵ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
²⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²⁷ Section of Astrophysics, Astronomy & Mechanics, Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos 15784, Greece
²⁸ Korea Astronomy and Space Science Institute, Daedeokdae-ro 776, Yuseong-gu, Daejeon 34055, Republic of Korea
²⁹ University of Science and Technology, Gajeong-ro 217, Yuseong-gu, Daejeon 34113, Republic of Korea
³⁰ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
³¹ Department of Physics and Astronomy, 20014 University of Turku, Finland
³² Department of Physics, University of Crete, 70013 Heraklion, Greece
³³ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
³⁴ Owens Valley Radio Observatory, California Institute of Technology, MC 249-17, Pasadena, CA 91125, USA
³⁵ Special Astrophysical Observatory, Russian Academy of Sciences, 369167 Nizhnii Arkhyz, Russia
³⁶ Pulkovo Observatory, St.Petersburg 196140, Russia
³⁷ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
³⁸ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
³⁹ Dipartimento di Fisica, Universitá di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
⁴⁰ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁴¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁴² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴³ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
⁴⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
⁴⁵ Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA
⁴⁶ Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, California 94305, USA
⁴⁷ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁴⁸ Astronomical Institute of the Czech Academy of Sciences, Boní II 1401/1, 14100 Praha 4, Czech Republic
⁴⁹ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁵⁰ California Institute of Technology, Pasadena, CA 91125, USA
⁵¹ Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
⁵² Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
⁵³ University of British Columbia, Vancouver, BC V6T 1Z4, Canada
⁵⁴ Department of Physics, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
⁵⁵ Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
⁵⁶ Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
⁵⁷ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁵⁸ Graduate School of Science, Division of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
⁵⁹ Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong
⁶⁰ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
⁶¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶² Dipartimento di Fisica e Astronomia, Universitá degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁶³ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁶⁴ Anton Pannekoek Institute for Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶⁵ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China

Received: 9 July 2023
Accepted: 6 October 2023

Abstract

Aims. We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes.

Methods. We conducted a polarimetry campaign from radio to X-ray wavelengths of the high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry Explorer (IXPE) measurements from 2022 December 6–8. During the IXPE observation, we also monitored Mrk 421 using Swift-XRT and obtained a single observation with XMM-Newton to improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months.

Results. We detected X-ray polarization of Mrk 421 with a degree of Π_X = 14 ± 1% and an electric-vector position angle ψ_X = 107 ± 3° in the 2–8 keV band. From the time variability analysis, we find a significant episodic variation in ψ_X. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May, ψ_X varied in the range 0° to 180°, while Π_X remained relatively constant within ∼10–15%. Furthermore, a swing in ψ_X in 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that Π_X was generally ∼2–3 times greater than Π at longer wavelengths, while ψ fluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation of ψ occurred in the opposite direction with respect to the rotation of ψ_X and over longer timescales at similar epochs.

Conclusions. The polarization behavior observed across multiple wavelengths is consistent with previous IXPE findings for HSP blazars. This result favors the energy-stratified shock model developed to explain variable emission in relativistic jets. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation of ψ_X. The accompanying spectral variation during the ψ_X rotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction of ψ between the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode of ψ_X rotation, although it is unclear whether there is any connection between the two events.