Document Type

Article

Publication Date

5-7-2026

Department

Department of Physics

Abstract

Blazars are jetted active galactic nuclei, with the jet aligned along the observer’s line of sight. Their spectral energy distributions are dominated by non-thermal emission, with an underlying thermal component at optical/ultraviolet wavelengths. However, the underlying jet magnetic field structure and particle acceleration mechanisms requirements for the non-thermal emission are still under debate. Polarization measurements can provide critical insights, and we investigate the optical polarization properties of TeV-emitting blazars using long-term optical monitoring. We present results from the first 21 months of the Spectro-Polarimetric Observations of TeV Sources campaign, using the Southern African Large Telescope, of 14 blazars. Overall, observations of the sample during this campaign showed a low average optical polarization ((Formula presented) ). While some sources exhibited smooth polarization angle rotations on time-scales of days to weeks, others showed stochastic variations consistent with turbulent magnetic fields. The average ordering of the magnetic field was low ((Formula presented) ), consistent with emission arising in extended, turbulent regions of the jet. For individual sources, correlations between polarization and its frequency dependence were found, but were not found across the entire sample. The nature of the frequency dependence varied between observations, indicating that simple one-zone models are insufficient and that (Formula presented) must be wavelength dependent. This study highlights the complex nature of blazar jets and underscores the importance of long-term, multiwavelength polarimetric monitoring. This comprehensive dataset enables detailed modelling of individual sources, and provides valuable context for future X-ray polarimetry observations.

Publisher's Statement

© The Author(s) 2026. Publisher’s version of record: https://doi.org/10.1093/mnras/stag859

Publication Title

Monthly Notices of the Royal Astronomical Society

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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