γ-ray Variability Of Blazars And Its Correlation With Radio Variability

Active Galactic Nuclei(AGNs) are the strongly active centers of one particularkind of galaxies. When the relativisitic jets of raido-loud AGNs point close to us(often≤10), there will form a special kind of AGNs which is called Blazars. Blazars oftenexhibit huge apprent luminosities(up to1049erg s1), non-thermal emission domi-nance, irregular and rapid variable emission in entire electromagnetic band, strong ra-dio and optical polarization and superluminal motion. Since the emission from Blazarsmainly comes from the relativistic jets, they become the excellent lobarotories for s-tudying high-energy physics. In addition, the study of Blazars can also promote ourunderstanding of the black hole, magnetic field on large scale, extragalactic γ-raybackground, etc. Even though Blazars have been well studied at multiple wavelengthsand much insight has been gained into the nature of these extreme objects, there arestill numerous open issues remaining, such as the composition and formation of rela-tivistic jets, the radiative mechanism of the high-energy part of the SED, the locationof γ-ray emission region, etc.Variability studies, including the studies of variability time scale, light curve vari-ation, spectra changes and multiwavelength correlation, could reveal the location ofthe emission region, the changes in the condition and structure in the emission regionand the changes in the radiative processes and mechanisms, so it is essential to re-search the γ-ray variability. VLBI is the unique method to resolve Blazars on pc scale,thus the combination of γ-ray and radio studies would promote the localization of theγ-ray emission region. Particularly, the connections between γ-ray flares and the ejec-tions of radio components have been found in a number of Blazars, for example, OJ287, PKS1510-089, Mrk421,3c454.3, etc. However, there are also contradictions inprevious studies, such as whether γ-ray or radio leads, so further analyses and researchare required.There are three chapters in this thesis altoghther. Chapter one is an overview ofbasic AGN knowledge, such as historical background, properties, categories, modeland radiative mechanism. The chapter two introduces γ-ray instrumentation, the prop- erties of γ-ray from Blazars and the γ-radio connection. In chapter three we emphasisthe γ-ray variability of4C21.35and its correlation with the radio variability. TheFermi-LAT data from5August2008to23October2013has been used to investigatethe γ-ray properties of4C21.35between100MeV and300GeV. Specifically, the vari-ability properties with1-day time bins have been summaried, and11chosen markedbursts have been analysed in detail with3-hour time bins. The shortest variability timescale we have got is5.4hours. We also use the method of subtracting a linear increasefrom the original radio light curve, and then by correlation analysis conclude that theγ-ray variations lead the15GHz radio variations by359.3+81.05.8days. The radio opacityin the γ-ray emission region is used to interpret the delay, and the distance between theγ-ray emission region and the region becoming transparent in radio band r≈45.2pc has been obtained provided that the emission region is in uniform motion. Then bycomparing it with the radio core radius obtained by VLBA, a decelarating inner jet islikely to exist.