The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
² INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere, 100, I-00133 Rome, Italy
³ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ Institute of Astrophysics, Foundation for Research and Technology-Hellas, Vasilika Vouton, GR-70013 Heraklion, Greece
⁵ Department of Astronomy, Yale University, PO Box 208101 New Haven, CT 06520-8101, USA
⁶ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁷ European Space Astronomy Centre (ESA/ESAC), 28691 Villanueva de la Canada, Madrid, Spain
⁸ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁹ Department of Physics, University of Crete, Voutes University Campus, 70013 Heraklion, Greece
¹⁰ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹¹ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
¹² Department of Physics and Astronomy, 20014 University of Turku, Turku, Finland
¹³ Finnish Centre for Astronomy with ESO, 20014 University of Turku, Turku, Finland
¹⁴ Instituto de Radioastronomía Millimétrica, Avenida Divina Pastora, 7, Local 20, E-18012 Granada, Spain
¹⁵ Orchideenweg 8, 53123 Bonn, Germany
¹⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
¹⁷ Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University Kagamiyama, 1-3-1 Higashi-Hiroshima, Hiroshima 739-8526, Japan
¹⁸ Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
¹⁹ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²⁰ Core Research for Energetic Universe (Core-U), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²¹ Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
²² Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²³ GFZ Helmholtz Centre for Geosciences, Telegrafenberg, 14476 Potsdam, Germany
²⁴ Julius-Maximilians-Universität Würzburg, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
²⁵ Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
²⁶ Dr. Karl-Remeis Sternwarte and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
²⁷ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Korea
²⁸ University of Science and Technology, Korea, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea

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Received: 22 December 2025
Accepted: 15 March 2026

Abstract

Multiwavelength polarimetry offers a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. Sources with synchrotron emission that spans a broad frequency range, from radio to X-rays, such as High Synchrotron Peak (HSP) type BL Lac objects, are particularly interesting. Previous measurements including radio, optical, and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical, and X-ray observations (Swift, Nustar, and IXPE) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232, we find a strong frequency dependence of the degree of polarization, with a ratio Π_X/Π_O ≃ 5.2. For RGB J0710+591, IXPE derives a 1σ upper limit Π_X < 11.6%, comparable to the measured optical degree of polarization (average Π_O ∼ 12%). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that reproduces the observed data of both sources.