The heart of this thesis is the study of the multifrequency behaviour of a very special family of active galactic nuclei (AGNs) called blazars. The emission of these sources is generally dominated by non-thermal radiation from a plasma jet, making them the best candidates to investigate the properties of jets in AGNs.
We present an analysis of the flux and broad-band spectral variability of these objects from radio to the gamma-rays. The radio, optical and near-infrared data were mostly obtained thanks to the Whole Earth Blazar Telescope (WEBT) Collaboration and Steward Observatory blazar spectropolarimetric monitoring, while high-energy data come from space observatories. In particular, ultraviolet and X-ray data were taken from Swift and gamma-ray data from Fermi.
We produce multifrequency light curves that allow us to investigate the variability properties of these objects on different time scales, the correlation between flux changes in the different bands and possible time delays. From these results we infer which are the most plausible physical mechanisms for blazar emission and the jet structure, in particular where the emission is located and the jet possible curvature.
From this collection of multifrequency data we can determine the energy spectral distribution (SED). Observing simultaneously at different frequencies and in different brightness states, we can detect whether there are emission contributions that are not coming from the jet, but from the AGN nucleus. In some cases we are able to observe activity states in which emission might flow from the accretion disk, while in other ones it comes from the disk after reprocessing by fast-moving gas clouds around the accretion disk, known as broad line region (BLR). The detection of the accretion disk and/or BLR emission is very important because it helps us to understand the relation between the blazars and the other types of AGNs. This is a challenging topic since very high-quality data are needed, especially at ultraviolet frequencies, i.e. in a spectral region strongly affected by Galactic absorption.
Once we have built light curves and SEDs we use them to test different jet models proposed to describe blazar emission. We know that the blazar emission at low energies is synchrothron radiation produced by relativistic electrons moving in a magnetic field. The radiation at high energies is likely produced by an inverse Compton process, where soft photons are scattered by the same relativistic electrons. However, the origin of these photons is still under discussion. As for the variability mechanisms, particle injection, acceleration and cooling are likely at work, together with shocks propagating along the jet and orientation effects in curved and dynamic jets.
Recently, a lot of observing effort has been devoted in providing polarimetric data in order to obtain information on the behaviour of the magnetic field. We analyze a wide dataset of polarimetric data on several blazars to identify characteristic properties. We found a variety of behaviours, some of them ordered but the majority of them resulting in a chaotic trend, so that a simple and all-inclusive explanation is not possible. Indeed, it is very likely that turbulence affects the magnetic field most of the time. All the above issues are still matter of debate and are expected to provide a wide research field to explore for many years.