The Comprehensive Study On The Variation Phenomena Of Blazars

Blazar is a subclass of radio loud active galactic nuclei(AGN)whose jet direction is close to our line of sight.Their general observed characteristics include the violent and rapid variation.The radiation at radio and optical bands are usually highly polarized.Their spectral energy distributions show a typical double-bump partern.First,the low-energy component(peak in the infrared to X-ray band)is interpreted as the synchrotron radiation of relativistic electrons.Second,the high-energy component(?-ray photons with peak in TeV)is explained by the inverse Compton scattering or the hadronic process.The launch of the Fermi Gamma-ray Space Telescope heralded a new era of blazar research.The study of multi-wavelength variations is helpful to understand the variation mechanism of blazars,the origin of cosmic high-energy ?-ray and high-energy neutrinos,and the formation of AGN jets.In this thesis,we take a comprehensive study on the cross-correlation among multi-wavelength light curves,optical color index,?-ray spectral index and optical polarization variation of blazars,and discuss the locations of ?-ray and optical emitting regions,the variation mechanism of optical,and the radiation mechanism of ?-ray by making use of the multi-wavelength observation data.This thesis is organized as follows.In chapter 1,we discuss the observational properties of AGN,as well as the taxonomy that has been developed to describe subclasses through the decades.We also reviewed the latest observations and theoretical studies;In chapter 2,we mainly introduce the ?-ray data reduction,the analysis technique of LCCF and the theoretical framework of emitting regions;In chapter 3,the correlation of multi-wavelength light curves and the location of emitting regions of two blazars AO 0235+164 and B2 1633+382 are discussed;In chapter 4,the two-component model is proposed based on the optical spectrum analysis.The correlation between the 7-ray spectral index and the polarization degree and the flux is quantitatively studied.The correlation between Stokes parameter,the optical flux and the polarization degree are discussed by using the helical jet model and the two-component model;Chapter 5 is the summary of the whole article and prospect.In Chapter 2,we introduce the multi-wavelength data reduction and analysis technique.The data reduction and analysis pipelines of Fermi-LAT are introduced.In the following section,we introduce the optical infrared monitoring programs of Small and Moderate Aperture Research Telescope System(SMARTS),Katzman Automatic Imaging Telescope(KAIT)and Steward Observatories(SO)as the multi-wavelength support program of Fermi mission,as well as the radio monitoring program of Owens Valley Radio Observatory(OVRO).In aspect of analysis technology,we discuss two methods for correlation analysis of light curves:the discrete cross-correlation function(DCF)and the local crosscorrelation function(LCCF).We use the LCCF to analyze the significance estimation technology of correlation,and compare it with previous results.For model fitting,we introduce Markov chain Monte Carlo method(MCMC),and use the reduced Chi-square and K-S test to evaluate the corresponding fitting results.In Chapter 3,the emitting regions of two blazars AO 0235+164 and B21633+382 are studied by using LCCF.For present optical telescopes and ?-ray detectors,most of blazars could only be identified as point-like objects,and the location of the optical and ?-ray emitting regions of them could not be resolved directly by images.To solve this difficulty,in this article,we collected long term multi-wavelength observation data of two blazars,and analyzed the correlations and time lags between them by using the LCCF.In the study of AO 0235+164,we found that both ?-ray and the optical V-band light curves are correlated with the radio light curve at exceed 3? significance level.The emitting regions of?-ray and optical coincide within the error range,which is about 6 pc upstream of the radio 15GHz core,and is far beyond the broad line region(BLR).In the study of B2 1633+382,we used the LCCF to analyze the multi-wavelength light curves,and found that both the ?-ray and the optical V-band are correlated with 15GHz radio and the correlation exceeds the 3? significance level.The ?-ray and optical emitting regions are also located outside of the BLR with 14 pc,which are upstream of the radio 15GHz core.In Chapter 4,we deeply analyzed the color index variation behaviours of these two blazars.With the results of emitting regions,we comprehensive studied the color index variation behaviour by using the two-component model including accretion disk and jet,the geometric model of jet precession and shock model.The color index variation of AO 0235+164 shows the redder when brighter(RWB)trend at the low flux state,but turns to the bluer when brighter(BWB)trend at the high flux state.While,the ?-ray spectral index always shows the softer when brighter(SWB)trend.We propose that such complex variation trends can be explained by the increasing jet component with the constant component.The variation of polarization and its correlation with optical flux can also constrain the variation mechanism.The optical PD flares and optical flux flares are not synchronous.It seems that one flux peak is sandwiched by two PD peaks,which have inverse rotation trajectories in the qu plane.The helical jet model can schematically show these characteristics of polarization with fine tuned parameters.The change of viewing angle is suggested to be the primary variable which lead all these variations,although other possibilities like the shock in jet model or the hadronic model are not excluded completely.The color index variation of B21633+382 shows an RWB trend,which is common in flat spectrum radio quasars(FSRQs).This behaviour is better fitted with the non-linear pattern.The broad line in its spectrum indicates the existence of the accretion disk component in the radiation.Thus,we consider the two-component(TC)model,which includes the relative constant background component and the varying jet component to study the variation behaviours.The Markov Chain Monte Carlo(MCMC)procedure is adopted to study the physical parameters of the jet and the background components.The study of normalized residuals indicates that the TC model fits better than the linear fitting model to some extent.The jet with helical magnetic field is hopeful to explain the variation,and the shock in jet model is not completely ruled out.These investigations present us with a quantitative method to disentangle the parameters of the jet from that of the background,and present a benchmark study on the variation mechanism of FSRQ targets.In summary,the correlation and time lag between multi-wavelength light curves are analyzed by using LCCF.The variation behaviour of blazars is explained by helical jet model and two-component model.Through these works,we have a better understanding about the emitting regions and variation mechanism of blazars,especially the polarization variation.By model analysis,we can provide a more scientific method for determining the fundamental physical parameters of the variation.In the future work,we will to make statistical study on larger samples and further explore the variation behaviours at short time scale.