New technologies to understand active galactic nuclei

Abstract

Optical polarization is a powerful diagnostic of the magnetic-field structure and emission mechanisms within blazar jets. Rapid changes in polarization degree and angle are often linked to high-energy variability, making optical polarimetry an important complementary tool for the Cherenkov Telescope Array (CTA). This the- sis presents the development, validation, and application of a complete analysis pipeline for extracting linear polarization from FITS images acquired using a rotating half-wave-plate polarimetric system.

The pipeline incorporates automated star detection, aperture photometry, angle association, and sinusoidal modulation fitting to determine the polarization degree and angle. Its performance was assessed through three complementary datasets: real polarimetric observations from the PICSARR instrument on the 0.6 m Penrith Observatory telescope, synthetic simulations with fully controlled input polarization, and an independent CTApol–Marana dataset consisting of four intrinsically unpolarised stars used to test the correct recovery of a zero–polarisation signal.

The simulation framework verified the mathematical correctness of the modulation and fitting procedures, with accurate recovery of the true polarization parameters in low-noise conditions. Noise-stress experiments revealed three operational regimes: a signal-dominated regime with reliable recovery, a transitional regime where uncertainties grow rapidly with noise, and a noise-dominated regime where meaningful extraction is no longer possible. These results establish practical signal-to-noise constraints for future polarimetric observations.

Application to real PICSARR data of HD 147084 resulted in clean modulation curves and a recovered polarization of ∼ 4%, consistent with published values for this weakly polarized standard. A second validation using CTApol observations of four unpolarised stars confirmed that the pipeline does not introduce spurious polarization: all four targets yielded fractional polarisation values below 0.3%, consistent with instrumental zero.

Together, these results demonstrate that the pipeline performs robustly in both non-zero and zero-polarization regimes and is suitable for operational use in optical polarimetric monitoring. This work provides a validated framework and performance benchmarks that support CTA-related multi-wavelength campaigns and future optical studies of blazar polarization variability.

Date of Thesis submission	2025
Original language	English
Awarding Institution	Western Sydney University
Sponsors	Australian Government Research Training Program Scholarship
Supervisor	Nick Tothill (Supervisor), Ain De Horta (Supervisor) & Miroslav Filipovic (Supervisor)