Broad emission line variability in active galactic nuclei on long timescales
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Date
23/12/2019Author
Homan, David Sebastiaan
Metadata
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.