Unveiling blazar synchrotron emission: A multiwavelength polarimetric study of high-synchrotron and low-synchrotron peaked populations

¹ Institute of Astrophysics, Foundation for Research and Technology-Hellas, GR-71110 Heraklion, Greece
² Department of Physics, University of Crete, 70013 Heraklion, Greece
³ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁴ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
⁵ Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁷ INAF, Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere, 100, I-00133 Rome, Italy
⁸ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁹ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁰ Max-Planck-Institut für Physik, D-85748 Garching, Germany
¹¹ Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA
¹² Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, MO 63130, USA
¹³ Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
¹⁴ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
¹⁵ St. Petersburg State University, 7/9, Universitetskaya nab., 199034 St. Petersburg, Russia
¹⁶ Department of Physics and Astronomy, 20014 University of Turku, Finland
¹⁷ Finnish Centre for Astronomy with ESO, 20014 University of Turku, Finland
¹⁸ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
¹⁹ Instituto de Radioastronomía Millimétrica, Avenida Divina Pastora, 7, Local 20, E-18012 Granada, Spain
²⁰ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
²¹ Interdisziplinäres Zentrum für wissenschaftliches Rechnen (IWR), Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
²² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
²³ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
²⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
²⁵ Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Crimea
²⁶ Pulkovo Observatory, St.Petersburg 196140, Russia
²⁷ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park IC2, 146 Brownlow Hill, Liverpool, UK
²⁸ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²⁹ Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University Kagamiyama, 1-3-1 Higashi-Hiroshima, Hiroshima 739-8526, Japan
³⁰ Institute of Integrated Research, Institute of Science Tokyo, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
³¹ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³² Core Research for Energetic Universe (Core-U), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³³ Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
³⁴ Graduate School of Sciences, Tohoku University, Aoba-ku, 980-8578 Sendai, Japan
³⁵ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Korea
³⁶ University of Science and Technology, Korea, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea
³⁷ Section of Astrophysics, Astronomy & Mechanics, Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis Zografos 15784, Greece
³⁸ GFZ Helmholtz Centre for Geosciences, Telegrafenberg, 14476 Potsdam, Germany
³⁹ Julius-Maximilians-Universität Würzburg, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
⁴⁰ Dr. Karl-Remeis Sternwarte and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany

^⋆ Corresponding author.

Received: 5 May 2025
Accepted: 12 August 2025

Abstract

The polarimetric properties of blazars enable us to place constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high-synchrotron peaked (HSP) and low-synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, along with optical polarisation observations of LSP from RoboPol and other instruments. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centred around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region. These results offer strong support for the emerging energy stratified scenario of particle acceleration followed by energy loss in blazar jets.