Accretion disc reverberation mapping of the
narrow-line Seyfert 1 galaxy Mrk 1239

Abstract

Accretion allows the SMBHs that power AGN to grow across cosmic time and it plays a major role in the formation history of galaxies. Small angular sizes and large distances prevent us from studying AGN through direct imaging, however, inferring their geometry can be achieved through reverberation mapping. By substituting spatial resolution for time resolution, the variable nature of AGN emission can be exploited to study AGN beyond the reach of direct imaging techniques. Many recent investigations have revealed a discrepancy with theoretical accretion disc models, specifically the Shakura-Sunyaev thin disc model. These studies find that although the lag spectrum follows the expected relation the lags and therefore disc sizes are larger on average by a factor of , , and than the theoretical thin disc lags for Sour, Javelin and PyROA respectively when the band is set as the driver.

The first investigation in this thesis outlines a comprehensive comparison of Javelin and PyROA, which are modern reverberation mapping algorithms that extract time lags to a higher precision than the legacy cross-correlation methods. We find consistency between all three algorithms and with existing data after implementing qualitative parameter optimisations to reduce effects such as aliasing and contamination from unexpected emissions. We then applied this knowledge to optical light curves for a reverberation mapping study of the highly obscured AGN, Mrk 1239. In line with several other studies, we observe that the lag spectrum is consistent with the expected power law but that the observed lags are a factor of larger than the thin disc prediction. When the band was set as the driver, with an unfixed power law and for a fixed . These lag estimates are and from the theoretical disc size estimate at the driving wavelength of . A better understanding of the systematics of Sour, Javelin and PyROA must be undertaken because some values are inconsistent between the algorithms. Furthermore, we attempted to connect the unusually flat spectral shape, which according to the thin disc model should be of the disc component spectrum (reconstructed from the flux decomposition of the light curves). However, a rimmed disc model was not able to reconcile the flat spectral shape with the observed lags and we conclude that the spectral shape is a result of dust extinction that is non-disc in origin, most likely originating from a thick dusty torus that covers the disc emission. A thin disc model fits the unreddened disc spectrum well with an Eddington ratio of , resulting in a larger-than-expected disc size. Applying the rimmed disc model to the unreddened fluxes again with an Eddington ratio of was able to reconcile the spectral shape as well as the disc sizes in the , , and bands, we find that the observed lags are smaller than the rimmed disc model by a factor of for the , , and bands. This is the first reverberation mapping study of an obscured AGN and we sought to connect Mrk 1239's spectral properties with the scales of structures observed with reverberation mapping to further our understanding of accretion discs and the processes within AGN in the context of the unified model.