The optical spectropolarimetric behaviour of a selection of high-energy blazars

TL;DR

This study uses optical spectropolarimetry of 18 blazars to analyze their polarization behavior, aiming to disentangle thermal and non-thermal emission components and understand jet physics during different activity states.

Contribution

It provides new spectropolarimetric data for a diverse sample of blazars, revealing complex polarization behaviors and highlighting the need for ongoing observations and modeling.

Findings

Partial correlation between polarization and frequency dependence in some sources

No overall correlation across the entire blazar sample

Spectropolarimetric variability observed during flaring and quiescent states

Abstract

At optical/ultraviolet energies, blazars display an underlying thermal (unpolarized) contribution from the accretion disc, torus and line emitting regions, diluting the polarized emission from the jet-component. Optical polarimetry can be used to disentangle the thermal and non-thermal components, and place constraints on the particle populations and acceleration mechanisms responsible for the non-thermal emission. We present the results of a linear optical spectropolarimetric observing campaign of 18 blazars (6 BLLs and 12 FSRQs) undertaken with the Southern African Large Telescope between 2016 and 2022. This was done to observe these systems during flaring states, as well as long term monitoring of PKS1510-089, AP Lib and PKS 1034-293. The observations traced the frequency dependence of the degree and angle of polarization, as well as changes in the spectral line strengths. We investigated possible correlations between the polarization and other observed characteristics for the sources. While an indication of correlation was found between the frequency dependence and the average level of polarization for some sources, a correlation was not found for the population as a whole. These results highlight that continuous observations and in-depth modelling of polarization and its frequency dependence is required to obtain a more holistic view of TeV blazars.