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Robotic Polarimetry of Blazars

Jermak, HE (2017) Robotic Polarimetry of Blazars. Doctoral thesis, Liverpool John Moores University.

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Abstract

The motivation of this thesis was the study of radio-loud, active galaxies. These galaxies house relativistic jets at their centres, powered by accretion onto a super massive black hole. The focus was on the optical flux and polarised emission produced by these powerful jets. An automated pipeline was developed to reduce data from the Liverpool Telescope Ringo2 and Ringo3 polarimeters. As part of this work, the Ringo3 instrumental polarisation and depolarisation were characterised by repeated observations of standard stars. The Ringo2 and Ringo3 optical polarimetry and photometry of a sample of 20 gamma-ray bright blazars were combined with Fermi gamma-ray space telescope data and were used to explore possible correlations and thus probe the emission sites in the jet. We found that optical and gamma-ray fluxes had strong, positive correlations. This suggests that the dominant source of optical and gamma-ray emission is from shared emission regions. If the Inverse Compton model is adopted to explain the gamma-ray emission (i.e. upscattering of photons by relativistic electrons), this correlation suggests that synchrotron self-Compton emission processes are occurring in the jet, along with inverse Compton upscattering from nearby electrons (rather than those outside the jet). The gamma-ray flux and optical degree of polarisation were not significantly correlated. The optical flux and degree of polarisation were weakly positively correlated (with correlations that did not improve with an introduced lag). Both of these results imply that there is no large scale highly ordered magnetic field in the region where the gamma-ray emission originates. We found that the maximum degree of polarisation differs depending on the location of the source's synchrotron-peak. This may be a result of the viewing angle of the observer with respect to the jet. This suggests that the majority of optical polarisation is produced in shocked regions within the jet, downstream of the main emission region. We found that the degree of polarisation was lower during a period of polarisation angle rotation compared with a period of non-rotation. This implies that the downstream magnetic field structure is either helical or compressed in a direction transverse to that of the jet. Consistent with other work, our Ringo3 colour analysis showed that, with the exception of one source, flat spectrum radio quasars had a `redder' when brighter property. This suggests that when the source is more luminous, the jet (i.e. non-thermal) emission dominates over the thermal emission from the accretion disk (which is powerful in FSRQs). We found that BL~Lacs had a `bluer' when brighter behaviour, suggesting that the brighter emission may come from more energetic photons within the jet. We presented data from our long-term, multi-colour, blazar monitoring campaign. We found that all but one source had a `redder' polarisation when the polarisation was higher. This implies that the highest polarisation is associated with higher densities of lower energy particles in the jet. Well-sampled, regular cadence data is very important for the effective study and interpretation of blazars. This is particularly crucial for the interpretation of the position angle rotations, which can afford information about the electric vector angle (and hence the magnetic field angle). In this work, we presented the design of a new multicolour polarimeter, MOPTOP. The optical components in MOPTOP allow as much of the light from the source to be exploited as possible by replacing the rotating Polaroid (from the Ringo polarimeter design) with a rotating half-wave plate and beam splitter. MOPTOP's design minimises exposure times, allowing more frequent observations and a better sampling of data. A densely sampled monitoring program that is not interrupted by periods of sunlight would be highly desirable for the study of blazar jets.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: polarimetry; astronomy; blazars
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
Date Deposited: 09 Feb 2017 11:28
Last Modified: 09 Feb 2017 11:28
Supervisors: Steele, IA
URI: http://researchonline.ljmu.ac.uk/id/eprint/5462

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