Optical Spectropolarimetry Observations of the BL Lac-type object PKS 0537-441 after a period of quiescence

Abstract

Blazars are a radio-loud subclass of active galactic nuclei with relativistic jets closely aligned with our line of sight. They are highly variable across all time-scales, and display rapid flares across multiple wavelength bands. At optical wavelengths, the observed emission is a superposition of non-thermal emission arising from the jet, and thermal emission that originates from the accretion disc, broad-line region, dust torus and host galaxy. Optical spectropolarimetry observations of blazars can be used to disentangle the thermal non-polarised and non-thermal polarised emission components in blazar emission during flaring and quiescent states. As part of a long-term monitoring campaign, spectropolarimetry observations have been taken with the Southern African Large Telescope of a selection of blazars during periods of increased activity. Here we focus on spectropolarimetry results and Fermi-LAT light curve of the BL-Lac type object PKS 0537-441 observed around a period of increased activity. It was observed to be active at both optical and gamma-ray energies after recovering from a faint state in late 2018. Spectropolarimetry observations were taken twice, with the degree of linear polarisation being between 35% and 40% during the first observation, and between 10% and 15% during the second, while there was no significant change in the strength of the emission line.