Investigating the variability of low-mass active galactic nuclei in the X-ray, UV, and optical regimes.

Abstract

Active galactic nuclei (AGN) display variability in their emission of radiation that can be used to probe the geometry and physical processes that drive them. In this thesis I examine the variability found in the emissions of X-ray, UV, and Optical radiation from AGN to constrain the properties of these extreme systems. To begin with I present optical interband time lags of the low-mass low-accretion rate AGN NGC 4395 using multiwaveband Liverpool Telescope and HiPERCAM Gran Telescopio Canarias data. The lags found here are consistent with X-ray reprocessing and suggest the first ever detection of the edge of the accretion disc in an AGN. I then present a study of X-ray/UV observations taken from XMM-Newton with simultaneous ground-based optical observations to measure the X-ray/UV/Optical lags of NGC 4395. These support the previous observations and gather further evidence for the existence of a truncated accretion disc in this object. Next I examine the long-term optical power spectrum of NGC 4395 using multiple light curves observed across different timescales. In addition I use simulation techniques to extend the previous X-ray observations and use reverberation modelling to create synthetic optical light curves from these. Comparison of the model fits of the data and the simulations suggest the existence of a secondary long-term source of variability in addition to X-ray reprocessing. Finally I investigate the X-ray to UV time lags of a different object, NGC 4593, using simultaneous XMM-Newton, Swift, and AstroSat observations. Here I find evidence of timescale-dependent lags. When filtered for short timescales the lags are consistent with X-ray reprocessing, but this suggests other long-term processes also exist within the object. I also note the presence of previously described contributions from the Broad Line Region’s Balmer and Paschen continua.

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