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PhD Thesis Abstract

Publications

What made the quasar blush? Emission mechanisms in optically red quasars.

Matthew T. Whiting, 2000; Ph.D. Thesis, University of Melbourne.

Quasars selected at radio frequencies have been shown to exhibit a much broader range of optical colours than the canonical blue colours of optically-selected quasars. This thesis sets out to explain this observation by the presence of optical non-thermal synchrotron emission, connected with the relativistic jet that powers the radio emission in these quasars.

In the first part of this thesis, we investigate the properties of synchrotron emission. This is firstly done from the perspective of observed optical synchrotron jets. The measured spectral indices of these jets show good evidence that the synchrotron spectrum is turning over at optical frequencies, indicating that the energy distribution of the emitting particles has a cutoff at some maximum energy. Secondly, we investigate analytic solutions for synchrotron emission from both ordered and tangled magnetic fields, and find that the emitted flux in the ordered field case exhibits a strong dependence on the viewing angle. This is compared with the expected dependence due to Doppler boosting caused by some bulk flow that would occur in a relativistic jet, and the implications for real jets are discussed.

The second part of the thesis is concerned with flat-spectrum radio quasars taken from the Parkes Half-Jansky Flat-spectrum Sample. We present an extensive data-set of optical and near-infrared photometry for >100 objects, and fit theoretical models to these observations. We find that about 40% of the sources have power-law SEDs, while a similar number show evidence for two primary components: a blue power law and optical synchrotron emission. The blue power law is similar to the dominant component observed in the spectra of optically-selected QSOs. There is strong evidence that the synchrotron component has a turnover in the rest frame UV-optical region of the spectrum. In the remaining sources it is likely that the synchrotron peaks at longer wavelengths, while peaks at shorter wavelengths appear to be ruled out. The relative strengths of these two components show variations of more than four orders of magnitude. The sources with power law SEDs show evidence for an excess number of red power law slopes compared to optically-selected quasars.

Analysis of polarisation data, both optical (from the literature) and near-infrared (new observations), supports the presence of an optically red, highly polarised component, which is consistent with the fitted synchrotron component.

The properties of the BL Lacertae objects in the sample are examined. It is found that, aside from having much lower emission line equivalent widths (by definition), the BL Lacs are indistinguishable from the other synchrotron-dominated sources in the sample. This suggests that the BL Lac phenomenon is caused primarily by a relative lack of emission line gas, rather than an excessively boosted synchrotron component.

Finally, observations of variability on timescales of hours in a source with no optical synchrotron emission demonstrates that the blue power law emission can exhibit short-timescale variations, a property which is often ascribed to synchrotron emission. This indicates that both components are important for modelling the optical emission from flat-spectrum radio quasars.

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