The Ultraviolet And Optical Variability Of Active Galactic Nuclei

Since they were discovered,Active Galactic Nuclei(AGNs)have been found to show aperiodical variability,which covers wavelength ranging from radio waves all the way to X-ray and gamma-ray bands.Recently,with the rapid advancing of time-domain astronomy,the study of the variability of AGNs is starting to gain more attention because it holds otherwise inaccessible information on AGNs.This kind of study can improve our comprehension of AGNs in many aspects,including the temperature profile and the size of the accretion disk,the physical properties and geometry distribution of the gas in the Broad Line Region,the relationship between the presumed X-ray corona and the accretion disk,the phyical properties of infrared radiation,and so on.In addition,a better understanding of the variability of AGNs can help with the AGNs selection technique via variability properties.We will give a brief description of AGNs,and their basic properties,their radiation mechanisms,and their physical components in the first chapter.Next,we will focus on their variability,and we will give more details on the mathematical description and physical models on the variability of AGNs.These will serve as the background and foundation for the two major works of this thesis.The main theme of this thesis is the color variability of AGNs-more specifically,the timescale dependency of color variations.It was found that for optical variations,the color variabilamplitude was different on different timescales,with shorter timescales displaying more prominent color variability.In the third chapter,we intent to extend this research down to the center of AGNs and check for the timescale dependency with ultraviolet lightcurves.Using the lightcurves of quasars recorded by GALEX satellite,we first work out the structure functions for the two ultraviolet bands of NUV and FUV in the oberver's frame.With these two structure functions,we confirm the bluer-when-brighter trend in rest-frame EUV bands using two methods.We then confirmed that the color variations are indeed timescale dependent.In the first method,the ratios of structure function values on different timescales are used to mark the color variability amplitude.And for the second one,we check for the timescale dependency using the? description for color variation.Simulations are introduced to rule out the influence of measurement uncertainties on color variation calculations.Based on these results,we further discover that the amplitude of color variation is also dependent on redshift,while its dependence on bolometric luminosity is negligible.And in the end,we also find evidence that the light curves of ultraviolet bands,especially for the FUV band,cannot be well-described within the damped random walk model.Having checked the existence of timescale dependency of color variations for the samples of distant quasars,we move to focus on a single nearby AGN-NGC 5548.In this work,we make use of well-sampled high-quality lightcurves of six bands recor(?)ded by the Neil Gehrels Swift Observatory.First,we find that any two lightcurves of the six bands do display timescale dependent color variability.On top of this,we check for the validity of the reprocessing model.We show that while a reprocessing model with strict assumptions on the driving source and the disk size can apparently match the observed light curves and inter-band lags,it fails to reproduce the observed timescale dependency in the color variation.Such discrepancy raises a severe challenge to,and can hardly be reconciled under the widely accepted reprocessing diagram.It also demonstrates that the timescale dependency of the color variation is uniquely powerful in probing the physics behind AGN variations.