Polarimetric Properties of Blazars Caught by the WEBT

TL;DR

This paper reviews the polarimetric properties of blazars observed by the WEBT, highlighting their variability and the insights gained into blazar emission mechanisms through extensive multi-wavelength and polarimetric monitoring.

Contribution

It provides a comprehensive review of WEBT's polarimetric observations of blazars, emphasizing the role of polarization variability in understanding blazar physics.

Findings

Strong variability in polarization degree and angle observed.

Evidence supporting a geometrical interpretation of blazar variability.

Enhanced understanding of blazar emission mechanisms through polarimetric data.

Abstract

Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called "blazars". The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the gamma-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies.