Polarimetric Properties of Blazars Caught by the WEBT

Claudia M Raiteri,Massimo Villata

doi:10.3390/galaxies9020042

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 γ-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.

Highlights

According to the commonly accepted scenario for active galactic nuclei (AGN), they are powered by a supermassive black hole (SMBH), which is fed by an accretion disc
If we want to understand what happens in the jet, the contribution of the unpolarised disc must be subtracted from the total flux, and the polarisation degree must be corrected for its “dilution” effect as
Because of the smooth trend of the rotation, a stochastic effect was ruled out, and the event was interpreted as the signature of a compressive feature moving along a helical magnetic field

Summary

Introduction

According to the commonly accepted scenario for active galactic nuclei (AGN), they are powered by a supermassive black hole (SMBH), which is fed by an accretion disc. A fraction of AGN, around 10–20%, are radio-loud, which classically means that they have a large ratio of the radio flux at 5 GHz over the optical one in the B band F5 GHz/FB ≥ 10, e.g., [1] These objects show radio jets coming out from the nuclear region, in a direction presumed to be perpendicular to the accretion disc or along the SMBH rotation axis, which might not be aligned with the normal to the accretion disc. The variability time-scales shorten inversely to the Doppler factor The objects where this situation occurs are called “blazars” and show unpredictable strong variability at all wavelengths, from the radio to the γ-ray band, on a continuous range of time-scales. The data analysis often involves supporting high-energy observations by satellites at UV+X-ray frequencies like XMM-Newton and Swift, at γ-ray energies like Fermi, or by ground-based Cherenkov telescopes like MAGIC, observing in the TeV energy domain

Polarimetric Signatures in Blazars

Polarimetric Results by the WEBT

Conclusions