Intra-night optical polarization monitoring of blazars

¹ Department of Physics, University of Crete, GR-70013 Heraklion, Greece
² Institute of Astrophysics, Foundation for Research and Technology-Hellas, Vasilika Vouton, GR-70013 Heraklion, Greece
³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁴ Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia
⁵ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁷ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden
⁸ Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁹ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
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, University of Turku FI-20014, Finland

^★ Corresponding authors: apolychronakis@physics.uoc.gr; liodakis@ia.forth.gr

Received: 4 October 2025
Accepted: 12 November 2025

Abstract

Blazars are known for their extreme variability across the electromagnetic spectrum. Variability at very short timescales can allow us to discriminate between competing models. This is particularly true for polarization variability, which allows us to probe particle acceleration and high-energy emission models in blazars. Here we present results from the first pilot study of intra-night optical polarization monitoring conducted using RoboPol at the Skinakas Observatory; these results are supplemented by observations from the Calar Alto, Perkins, and Sierra Nevada observatories. Our results show that while variability patterns can vary widely between sources, variability on timescales as short as minutes is prevalent in blazar jets. The amplitudes of the variations are typically small, a few percent for the polarization degree and less than 20° for the polarization angle, pointing to a significant contribution to the optical emission from a turbulent magnetic field component. The overall stability of the polarization angle over time points to a preferred magnetic field orientation.