PhD Thesis Defence by Daniel Peter Lawther

Supervisor: Assoc. Prof. Marianne Westergaard

External examiners:Troels Haugbølle, Sarah Gallagher and Keith Horne

Title: Clues to the inner workings of active galactic nuclei from spectral energy distributions, photoionization modeling and the rare subset of FeLoBAL quasars.

Abstract: I will present the results of four studies relating to accretion phenomena in Active Galactic Nuclei (AGN). In the first study I examine how diffuse continuum emission from the AGN broad line emitting region causes a systematic bias in estimates of accretion disk sizes based on continuum reverberation mapping. Reprocessing of ionizing continuum variability in the broad-line region produces a reverberation signal that contributes significantly to the observed variability delay spectrum at all UV-optical wavelengths. The resulting lengthening of the observed time lags is to first order consistent with the factor ~2-3 discrepancy between disk sizes predicted by standard thin-disk models and those inferred by continuum reverberation mapping experiments. 

Secondly, I will present Swift UV and X-ray observations obtained during 2014-2020 that document the re-ignition of the changing-look AGN Mrk 590, which was in a dormant state for several years, and now displays flaring behavior in the UV and X-rays. The UV to X-ray spectral energy distribution during these flares is consistent with those of 'ordinary' AGN, and the Eddington luminosity ratio has returned to the regime for which an optically thick accretion disk is expected. Changing-look behavior in AGN is not well understood, as the observed variability timescales are much shorter than the theoretically predicted timescales for changes in the accretion flow. I will discuss one possible physical interpretation for the changing-look behavior, i.e. a transition from an advection-dominated accretion flow to a thermal disk, and examine the available observational evidence for such a transition in Mrk 590. I will also present X-ray to UV reverberation mapping results for this source. I find a strong correlation between the X-ray and UV variability, with the UV bands lagging the X-rays by ~3 days.

My third project is a catalog of rest-frame UV to X-ray spectral energy distributions for 143 quasars at redshifts z~2. This catalog contains roughly equal numbers of radio-loud and radio-quiet sources. As part of an on-going study of these sources, my aim is to determine to which degree the sample is representative of the broader population of z~2 quasars (as observed by the SDSS), and to verify whether the radio-loud and radio-quiet subsamples are well matched in terms of UV spectral energy distributions. I find that the sample quasars are brighter than the majority of SDSS quasars. The spectral energy distributions for the radio-loud and radio-quiet subsamples are statistically identical. The sample is therefore well-suited to investigations of the radio loudness dichotomy for bright quasars.

Finally, I will present a Hubble Space Telescope imaging study of the host galaxies of four Iron low-ionization broad absorption quasars (FeLoBALs). These rare sources are suggested to be either young quasars in the process of expelling a 'cocoon' of gas and dust, or intrinsically ordinary quasars viewed along a particular angle to the central source. For the young quasar scenario, if quasar activity is indeed merger-triggered, we would expect FeLoBAL host galaxies to have undergone recent mergers. I demonstrate that their host galaxies are consistent with either quiescent elliptical galaxies or dust-reddened starburst galaxies, given the available HST data. I will outline the additional observations required in order to settle this long-standing issue.

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