Exploring particle populations and magnetic field structures in the relativistic jets of blazars

McCall, C ORCID: 0000-0002-3375-3397 (2025) Exploring particle populations and magnetic field structures in the relativistic jets of blazars. Doctoral thesis, Liverpool John Moores University.

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Abstract

Well-sampled optical photo-polarimetric observations are paramount to understanding emission mechanisms and particle populations within the inner, highly collimated acceleration region of relativistic jets. This thesis presents the results of multiwaveband photo-polarimetric monitoring of a sample of γ-ray bright blazars. This includes the development and implementation of reduction pipelines used to calibrate and analyse data from the Liverpool Telescope RINGO3 and MOPTOP instruments.

The Python pipelines developed in this work incorporated standard deferential photometry techniques, alongside instrument-specific calibration methods such as determining non-standard waveband reference star magnitudes for RINGO3, accounting for telescope mirror degradation, and deriving polarimetric coefficients for MOPTOP. A novel approach was developed to correct for the 180° electric vector position angle (EVPA) ambiguity, reducing the need for manual intervention and accounting for measurement uncertainties.

Long-term optical and γ-ray photometric correlations were observed across most sources, with trends suggesting that low-synchrotron peak (LSP) blazars demonstrate more scattered relationships than high-synchrotron peak (HSP) sources. These results support a predominantly leptonic emission scenario in blazar jets while allowing for the possibility of additional hadronic contributions, particularly in LSP sources where greater scatter in optical–γ-ray flux-flux space was identified. Spectral analysis of optical variability revealed logarithmic colour evolution trends, implying the presence of spectral variability stabilisation during increased optical photometric activity. These trends, classified as bluer-stable-when-brighter and redder-stable-when-brighter, require a two-component emission model, with contributions from both thermal (disk) and non-thermal (jet) emission.

Photo-polarimetric analysis revealed significant correlations and anti-correlations between optical and γ-ray flux and polarisation degree, consistent with emission mechanisms such as relativistic shocks and magnetic field kink instabilities. Frequency-dependent polarisation variability was detected, with LSP sources dominating the sample of objects exhibiting bluer- and redder-when-brighter polarisation trends. Optical EVPA rotations were identified across multiple sources, primarily during heightened photometric states. By analysing the accompanying polarimetric states, distinct rotation mechanisms were inferred, further supporting shock propagation and kink instabilities within blazar jets in addition to stochastic variability.

The 2015 OJ287 binary black hole impact outburst was examined in detail, confirming a dual-peak flaring episode. The first flare was attributed to thermal bremsstrahlung originating from the primary accretion disk while the second flare was attributed to synchrotron emission from the jet. These data were corroborated with multi-frequency observations and supported the formation of a knot propagating along the jet’s helical magnetic field.

Intranight monitoring of PKS 0735+178 during heightened γ-ray activity uncovered an intranight, red-leading temporal lag in the optical wavebands, suggestive of second-order Fermi acceleration processes such as magnetic reconnection. Multiple epochs of significant intranight variability were detected across the sample, with statistical tests confirming variability in OJ287 and PKS 0735+178. Colour trends were also observed during some of the variable nights, with both redder- and bluer-when-brighter behaviours detected in PKS 0735+178.

Long-term photo-polarimetric monitoring of blazars allows one to determine the dominant emission mechanisms occurring within the jet, leading to a better understanding of the structure and evolution of the involved magnetic field structures. These results collectively contribute to the understanding of multiwaveband blazar variability and jet emission mechanisms, while providing new insights into the connections between photometric, polarimetric, and spectral behaviours.

Item Type:	Thesis (Doctoral)
Uncontrolled Keywords:	Blazar; Jets; BL Lac; FSRQ
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Date of acceptance:	25 June 2025
Date of first compliant Open Access:	31 July 2025
Date Deposited:	31 Jul 2025 15:20
Last Modified:	31 Jul 2025 15:21
DOI or ID number:	10.24377/LJMU.t.00026800
Supervisors:	Jermak, H and Steele, I
URI:	https://researchonline.ljmu.ac.uk/id/eprint/26800

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