The Multi-band Variability Of Active Galactic Nucleus

Active galactic nucleus?AGN?is one of the most important research objects in astrophysics,which involves several extreme physical processes such as black hole accretion,material ejection,the acceleration and radiation of relativistic high-energy particles.Multi-band variability is one of the most unique observational characteristics of AGNs.Based on the variability of AGNs,theoretically,any type of AGNs can be identified without any bias in the sky surveys.Moreover,the variability analysis of AGNs is the most effective way to reveal the physical structure and physical process of AGNs.In this thesis,based on the multi-band variability,two special types of AGNs?low-luminosity AGNs?LLAGNs?and blazars?are studied to explore the basic fea-tures and variability characteristics of high-redshift LLAGNs and the physical origins of multi-band activities of jets.In Chapter 1,we introduce the classification,basic model,multi-band radiation characteristics and related physical process of AGNs,and we summarize the physical mechanisms and research progress of the variability of AGNs.Finally,the background and significance of the research works in this thesis are expounded.In Chapter 2,we utilize an X-ray variability selection technique to search for the“missing”LLAGNs in the 7 Ms Chandra Deep Field-South?CDF-S?survey,which has the longest observation time-span??17 yr?and the deepest flux limit so far.We find 12high-redshift LLAGN candidates from 110 unclassified CDF-S X-ray sources.Based on Monte Carlo simulations,we discuss the efficiency of searching AGNs using X-ray variability,and which provides an important reference for future X-ray time-domain sky surveys.Besides,we also investigate the basic properties of these LLAGNs,the fraction of variable AGNs in the CDF-S field,and the luminosity-variability relation of the long-term X-ray variability of AGNs.The main results are as follows:?1?there may be two subclasses of LLAGNs.One tends to host in high-mass galaxies,and they have the comparable black hole masses as those of high-luminosity AGNs,but their accretion rates are lower;the other tends to host in low-mass dwarf galaxies,and they have the lower black hole masses,but their accretion rates are the same as those of high-luminosity AGNs.?2?The fraction of variable AGNs in the CDF-S field is basi-cally consistent with the expected detection efficiency of AGN variability obtained by simulations,which demonstrates that X-ray variability is a near-ubiquitous property of AGNs with a wide range of luminosities.?3?There is an anti-correlation between X-ray luminosity and variability amplitude for high-luminosity AGNs,but as the luminosity drops to?10⁴²erg s^-1,the variability amplitude no longer appears dependent on the luminosity.The entire observed luminosity-variability trend can be roughly reproduced by an empirical AGN variability model based on a broken power-law power spectral density function.In Chapter 3,we present an exploration of the origins of multi-band activities for a high-redshift?z=1.8385?flat-spectrum radio quasar PKS 1502+106 with an extreme jet.Utilizing multi-band data from radio to?-ray and optical polarization observa-tions,we investigate two dramatic activities in detail:a?-ray dominated outburst in2015 and an optical dominated outburst with the highest optical polarization observed to date for blazars in 2017.A fast?-ray flare with a flux-doubling time-scale as short as 1-hr is discovered in 2015.At the peak of the flare,the?-ray spectrum hardens to?_?=1.82±0.04 and exhibits an obvious curvature/break characteristic that is caused by the typical”cooling break”.Modelings of multi-band SEDs reveal a very hard elec-tronic energy spectrum with the electronic spectral index of 1.07±0.53.This result suggests that this fast?-ray flare may be triggered by magnetic reconnection.During the outburst in 2017,the optical polarization degree and optical fluxes show a very tight correlation.By analyzing Stokes parameters of polarization observations,our results show that this outburst could be triggered by a transverse shock with a compression ratio of?>2.2.These two dramatic activities in PKS 1502+106 are rare cases that can be analyzed in detail based on the simultaneous multi-band observation data in high-redshift blazars so far.Our analyses reveal that the physical origins of the two dramatic activities are different,which provides an important reference for understanding the activities of the jets of high-redshift blazars.In the last chapter,we summarize the thesis and present a prospect for the research of AGNs based on the next-generation telescopes.