Broad emission line variability in active galactic nuclei on long timescales

Abstract

Active Galactic Nuclei are among the most powerful sources of radiation in the Universe and are known to show variability across the EM spectrum. This thesis will focus on the variability in the optical and UV range, which is dominated by emission from the central accretion disc and the Broad Line Region (BLR). All AGN are powered by the accretion of matter onto a supermassive black hole. The BLR reprocesses part of the continuum generated by the accretion disc, re-emitting the energy in atomic emission lines. Variations in the continuum lead to variations in the broad emission lines, however the manner in which the broad lines respond is complex. Neither the accretion disc nor the BLR are currently fully understood. Over the past years the availability of large data-sets from long term spectroscopic observations, such as the Sloan Digital Sky Survey (SDSS), has greatly improved our ability to study the variability of AGN, expanding the observed timescales to decades. These timescales match the dynamical timescales associated with the BLR, implying that we could be able to track structural changes in this region. The research presented in this thesis will use the response of the broad lines to the continuum to investigate the properties of the BLR, with an emphasis on long timescales. The research is split over two independent, but closely linked studies. The first is a case study of the highly variable active galaxy Markarian 110. The second study focusses on one particular emission line: MgIIλ2798. Both studies rely on optical spectroscopic data. A pipeline was written to fit archival and new spectra included in the study. Mrk 110 is a local Seyfert I, at z=0.035, with available spectroscopic data going back decades. Combining archival data with new observations from the William Herschel and Tillinghast Telescopes, it is possible to track Mrk 110 through dips and peaks in the continuum at a relatively high cadence. Of particular note is the behaviour of HeIIλ4686, which varies significantly more than the other emission lines. This line can be used as a proxy for the ionising (FUV) part of the continuum. Comparing Hα, Hβ, and HeIλ5876 with the FUV flux we note that the response of the line fluxes to continuum changes evolves considerably on the timescale of years. For the highest continuum fluxes it is possible to detect a saturation level of the line responses, correlated with the Reverberation Mapping lags associated with the lines. Analysing the line profiles, we can detect an offset between the narrow and broad components of the Hβ and HeIIλ4686 lines, likely associated with a radial flow in the BLR. The Magnesium line at 2798Å is a prominent feature in most AGN spectra. Its response to continuum changes has been shown to be limited in previous studies. The study presented here will make use of two data-sets. The first is a sample of 43 extremely variable quasars. This sample combines SDSS spectra with new observations from the William Herschel, Magellan, and MTT Telescopes. The second data-set consists of approximately 16,000 SDSS quasars with repeat spectroscopy. The data from both samples indicate that there is in fact a broad range in MgII behaviour. We observe that the line tracks the continuum on average, but also note considerable scatter in this correlation. Combining the results from the Mrk 110 and MgII studies we find that there is evidence in all samples for broad line responsivity that evolves over time. There is also considerable evidence to suggest that the dynamics of both the HeIIλ4686 and the MgII line are not only set by their positions in the gravitational potential.