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Browsing Physics by Author "Barnard, Joleen"

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    Spectropolarimetric behaviour of a selection of high-energy blazars
    (University of the Free State, 2022) Barnard, Joleen; Van Soelen, B.
    Blazars form part of the jetted class of Active Galactic Nuclei (AGN), with a relativistic jet closely aligning to the observer’s line of sight. Due, in part, to this viewing angle, these sources are characterised by Doppler-boosted emission across the entire electromagnetic spectrum that is variable on timescales from years down to minutes. These are some of the most numerous extragalactic γ-ray sources, dominated by polarised non-thermal emission produced by relativistic leptons and/or hadrons moving within the jet- component. Blazars are subdivided into two categories: BL Lac-type objects (BLLs), and Flat-Spectrum Radio Quasars (FSRQs). They are classified based on the presence/absence and equivalent widths of optical spectral features, as well as the location of their synchrotron peak frequencies. Blazar SEDs are characterised by two broad, non-thermal components, along with an underlying thermal contribution at optical/ultraviolet wavelengths due to the accretion disc, host galaxy, and broad-line region of the blazar. However, the nature and origin of the non-thermal, high-energy emission remains unclear, as it can be reproduced by two different models, each assuming different particle populations and emission regions that produce the emission. Polarimetry provides a unique diagnostic tool to probe the polarisation signatures of the emission at optical wavelengths. The investigation of the polarisation of blazar emission aides in constraining the particle populations and acceleration mechanisms responsible for the non-thermal emission at lower energies, and constraining the high-energy emission and polarisation. It also places constraints on the accretion disc component and mass of the supermassive black hole, and sheds light on the structure of the jet’s magnetic field. Here, an optical spectropolarimetric study of a selection of high-energy blazars was undertaken. The aims of this project was to: first, investigate the change in the degree of polarisation from high to low states, second, monitor the evolution of polarisation in BLLs and FSRQs over a long period of time, and last, investigate overall trends in the behaviour of the sample of blazars, and compare it to what has been found in other population studies. To achieve these goals, optical spectropolarimetric observations were taken of eighteen blazars (six BLLs and twelve FSRQs) using the Southern African Large Telescope (SALT), complemented by optical photometric data from the Las Cumbres Observatory (LCO), and γ- ray data from the Fermi Large Area Telescope (LAT). For the population of blazars investigated, it was found that FSRQs tend to reach higher degrees of polarisation than BLLs during high states. More regular occurrences of high states and a wider range of polarisation levels were observed for FSRQs. For both BLLs and FSRQs, the average degree of polarisation was higher while the γ-ray fluxes were still on the rise than while it was decreasing to a lower state. In agreement with what was found by the RoboPol monitoring campaign, the degree of polarisation showed no clear correlation with the γ-ray luminosity or redshift of the source, but showed a significant anti-correlation with the location of the synchrotron peak frequency (νsy) of the sources. Two sources with noteworthy results were AP Lib and PKS 1510–089. For AP Lib, long-term variations in the optical and γ-ray fluxes were detected, with the same behaviour mirrored in the degree of polarisation, but lagging by ∼ 54 days. PKS 1510–089 was observed multiple times in 2022, during an unprecedented low-state in which the non-thermal emission from the jet diminished completely. This project serves as a good basis for further investigation of blazar polarisation, both in optical/ultraviolet and X-ray/γ-ray regimes.