Gamma-ray Variability Characteristics Of Blazars

In the first chapter of this thesis,the basic characteristics and the observational progress of active galactic nucleus in recent years are presented.Subsequent chapters focus on introducing my graduate study about the variability characteristics of blazars at GeV band.We systematically analyze about 8-year Fermi/LAT observation data for 26 blazar sources.For the analysis on 3C 454.3,it is found that:(1)the behavior of spectral evolution at gamma-ray band is "harder when brighter",(2)the typical variability timescale at gamma-ray band is several days,(3)the variability at 15 GHz differs from that at gamma-ray band,which may be caused by their emitting regions being different.For the analysis of other 25 sources,their spectral evolutions all behave "harder when brighter",similar to that of 3C 454.3.We also investigate the origin of their light curves and find that their light curves should be caused by both the different statistical noise processes and the possible systematic variations for all 26 cases.In addition,we also use the Lomb-Scargle method to analyze the gamma-ray light curves of S5 0716 + 714 and PMN J0948 + 0022,and find that there are possible periods about 350 days for S5 0716 + 714 and 480 days for PMN J0948 + 0022.Furthermore,we study the Lorentz factor(?)evolution patterns of relativistic jets of 3C 454.3,GRB 090168 and GRB 140508A and find that their ? evolution patterns are different from each other.For 3C 454.3,a pattern of r-tracking-gamma-ray flux is clearly found,likely indicating that the observed gamma-ray flares are being due to the Doppler boosting effect to the jet emission.The clear decrease evolution pattern of ? in GRB 140508A suggests the decay of the fireball central engine.And the increasing-to-coasting evolution pattern of ? in GRB 090618 may indicate that the relativistic fireball is still being accelerated at the place of photonsphere and then coming into the coasting phase.