Multi-wavelength Variability And Spectral Evolution Of Changing-look Active Galactic Nuclei

Active galactic nucleus（AGN）is a kind of luminous astrophysical compact source at the center of a galaxy,which shows much higher luminosity than a normal galaxy.The large diversity properties of observed AGNs can be successfully described with a few parame-ters（e.g.,inclination,jet）in the AGN unified model.However,the discovery of changing-look active galactic nuclei（CLAGNs）in recent years with the change of optical broad emis-sion lines（optical CLAGNs）and/or the variation of line-of-sight column densities（X-ray CLAGNs）within a few years challenges the orientation-based AGN unified model.The mechanism of changing-look phenomena is still unclear.In chapter 1,we briefly introduce the history,properties,classification of AGNs and the AGN unified model.We then introduce the discovery,observational properties,and the possible theoretical explanations for CLAGNs.In chapter 2,we study a nearby Seyfert galaxy Mrk 1018,which has undergone multiple times of changing-look events with low polarization degree and hydrogen column density based on the multi-wavelength observations during 2005-2019.We find a time lag of around20 days of optical/UV behind X-ray variation during the type 1.9 phase,which is far beyond the timescale predicted by the X-ray reprocessing scenario of thin disk.We find that both the X-ray photon index（Γ）and the optical-to-X-ray spectral index(α_ox)are anti-correlated with the Eddington-scaled 2–10 ke V X-ray luminosity(L_X/L_Edd)in the type 1.9 phase.However,the type 1 phase deviates from the anti-correlation.Therefore,the change of broad emission lines might be regulated by the evolution of the accretion disc.In chapter 3,we explore the mid-infrared（mid-IR）properties for a sample of 57 optical CLAGNs and 11 X-ray CLAGNs based on the Wide-field Infrared Survey Explorer（WISE）data.The mid-IR emission is less influenced by the dust extinction and has the potential to bring us better clues about the physical mechanism of CLAGNs.The optical CLAGNs show stronger variabilities compared to those of LLAGNs and QSOs.We find that Eddington-scaled mid-IR luminosities of CLAGNs stay between low-luminosity AGNs（LLAGNs）and luminous QSOs.The mean L_W1/L_Edd⁰.5 percent for CLAGNs,which suggest CLAGNs stay in a transitional stage,and they easily suffer the luminosity variation due to the transition of accretion modes with accretion rate at several percent of Eddington ratio.The X-ray CLAGNs show similar Eddington-scaled mid-IR luminosities with optical CLAGNs,but similar mid-IR variabilities and color with QSO.The X-ray CLAGNs might be also triggered by the variation of accretion rate,and the change of absorption is strongly correlated with the variable disk wind observed at a moderate inclination angle in the SSD scenario.We estimate the mid-IR variation time-lags behind the optical band for 13 bright CLAGNs with strong mid-IR variability.The tight MIR time-lag and the bolometric luminosity（τ-L）correlation is consistent with that in luminous QSOs.In chapter 4,we explore the variabilities of a flaring blazar（PKS0736+017）on timescales of minutes to days based on the data of the low-frequency radio array of the Murchison Wide-field Array（MWA）.As a SKA pathfinder,MWA with a large field-of-view can be used to not only study the low-frequency radio variability of flaring AGNs and other special astro-physical sources but also search for new transients.The radio emission of PKS0736+017 is not significantly variable compared to theγ-ray emission after the flare.We also search for the variability of sources within 5 degrees of central field-of-view.At the significance of 3σ,we do not find bright point sources with short-term variability.In chapter 5,we summarize the thesis and present the prospects for future works.