IXPE observation of the low-synchrotron peaked blazar S4 0954+65 during an optical-X-ray flare

¹ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
² Finnish Centre for Astronomy with ESO (FINCA), Quantum, Vesilinnantie 5, University of Turku, FI-20014 Turku, Finland
³ Institute of Astrophysics, FORTH, N. Plastira 100, GR-70013 Heraklion, Greece
⁴ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁵ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁶ University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁷ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁹ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹⁰ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
¹¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
¹² Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
¹⁴ Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia
¹⁵ Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
¹⁶ Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
¹⁷ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁸ Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
¹⁹ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
²⁰ Department of Physics, University of Crete, 70013 Heraklion, Greece
²¹ Department of Mathematics and Applied Mathematics – Applied Mathematics, University of Crete, 70013 Heraklion, Greece
²² Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Crimea
²³ Pulkovo Observatory, St. Petersburg 196140, Russia
²⁴ Geological and Mining Institute of Spain (IGME-CSIC), Calle Ríos Rosas 23, E-28003 Madrid, Spain
²⁵ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora, 7, Local 20, E-18012 Granada, Spain
²⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
²⁷ Orchideenweg 8, D-53123 Bonn, Germany
²⁸ INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
²⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
³⁰ Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
³¹ Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375, USA
³² Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
³³ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
³⁴ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
³⁵ INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
³⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
³⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
³⁸ Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA
³⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴⁰ Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, California 94305, USA
⁴¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁴² Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
⁴³ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁴⁴ Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
⁴⁵ Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
⁴⁶ University of British Columbia, Vancouver BC V6T 1Z4, Canada
⁴⁷ International Center for Hadron Astrophysics, Chiba University, Chiba 263-8522, Japan
⁴⁸ Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
⁴⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Strada Comunale Cinthia, 80126 Napoli, Italy
⁵⁰ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁵¹ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁵² Graduate School of Science, Division of Particle and Astrophysical Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
⁵³ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁵⁴ Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong
⁵⁵ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
⁵⁶ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵⁷ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁵⁸ Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA
⁵⁹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁶⁰ Anton Pannekoek Institute for Astronomy & GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶¹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China

^⋆ Corresponding author; pouya.kouch@utu.fi

Received: 22 November 2024
Accepted: 31 January 2025

Abstract

The X-ray polarization observations, made possible with the Imaging X-ray Polarimetry Explorer (IXPE), offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here, we report the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength (MWL) campaign of the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array (VLBA). The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare, the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3 ± 4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3σ level) on the X-ray polarization degree; however, a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% (3σ). We modeled the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. Our combined MWL polarization observations and SED modeling tentatively disfavor the use of hadronic models for the X-ray emission in S4 0954+65.