The dependence of optical polarisation of blazars on the synchrotron component peak frequency

TL;DR

This study investigates how the optical polarisation of blazars varies with their synchrotron peak frequency, revealing differences between gamma-ray-loud and quiet sources and proposing a jet model to explain these phenomena.

Contribution

It provides the first systematic analysis of optical polarisation variability in blazars in relation to their synchrotron peak frequency and gamma-ray activity.

Findings

Gamma-ray-loud blazars show distinct polarisation behaviour compared to quiet ones.

The polarisation stability correlates with the synchrotron peak frequency.

A simple jet model explains the observed polarisation dichotomy.

Abstract

The RoboPol instrument and the relevant program was developed in order to conduct a systematic study of the optical polarisation variability of blazars. Driven by the discovery that long smooth rotations of the optical polarisation plane can be associated with the activity in other bands and especially in gamma rays, the program was meant to investigate the physical mechanisms causing them and quantify the optical polarisation behaviour in blazars. Over the first three nominal observing seasons (2013, 2014 and 2015) RoboPol detected 40 rotations in 24 blazars by observing a gamma-ray-loud and gamma-ray-quite unbiassed sample of blazars, providing a reliable set of events for exploring the phenomenon. The obtain datasets provided the ground for a systematic quantification of the variability of the optical polarisation in such systems. In the following after a brief review of the discoveries that relate to the gamma-ray loudness of the sources we move on to discuss a simple jet model that explains the observed dichotomy in terms of polarisation between gamma-ray-loud and quite sources and the dependence of polarisation and the stability of the polarisation angle on the synchrotron peak frequency.