The panchromatic polarization signatures of Active Galactic Nuclei

TL;DR

This paper reviews how multi-wavelength polarimetry reveals the complex emission mechanisms of active galactic nuclei, emphasizing the importance of panchromatic polarization measurements for understanding their formation, accretion, and impact.

Contribution

It provides a comprehensive review of the current state of panchromatic polarization studies of AGN and discusses future observational prospects with advanced telescopes and satellites.

Findings

Polarimetric signatures vary across wavelengths, revealing different physical processes.

Multi-wavelength polarimetry constrains AGN emission mechanisms and structure.

Future instruments will enable deeper understanding of AGN physics.

Abstract

Among all the astronomical sources investigated through the prism of polarimetry, active galactic nuclei (AGN) have proven to be the richest in terms of complex yet fundamental signatures that helped to understand their true nature. Indeed, AGN exhibit a wide range of wavelength-dependent polarimetric features that are intrinsically related to their multi-scale emission mechanisms. Each waveband is characterized by a different set of polarimetric signatures that can be related to various physical mechanisms that, from the radio band to the soft-$\gamma$ rays, probe increasingly smaller AGN regions. In fact, panchromatic polarization measurements are the key to understand how and when AGN form, accrete, and impact the host galaxy they reside in. In this chapter, I will first introduce AGN without focusing on a particular observational technique. I will then review the discoveries and constraints that spectro-, imaging and broadband polarimetry have achieved. Finally, I will highlight the important questions that remain unanswered and how they can be solved with future large millimeter and radio antennas, 30-m class optical and infrared telescopes, and high energy satellites equipped with state-of-the-art polarimeters.