Variability in the BL Lac Objects

Large and rapid variability is one of the defining properties of BL Lac objects.Variability in BL Lac objects has always served as a powerful and important tool for constraining the physics of these unusual objects.This thesis present a review for observational and theoretical studies of variability in BL Lac objects and relevant studies such as theories and models for BL Lac objects,variability in other Active Galactic Nuclei(AGN),and some of our research work in this field. There are seven chapters in this thesis altogether.Chapter One is an overview of variability study.The importance of variability study is discussed,and the most significant new progresses in the studies of variability in BL Lac objects are simply presented,together with a review of the methods and approaches for variability study.Chapter two presents some elemental knowledge of AGNs which is relevant to BL Lac objects.Since the observational study on variability aims at testing theories and models for BL Lac objects,Chapter Three mainly present theoretical study for BL Lac objects,especially,the relativistic jet model and the unify scheme for BL Lac objects.We point out that to unify BL Lac objects itself(say,to unify RBL and XBL,or other schemes)is as important as to unify BL Lac objects and its parent population(say FRI radio galaxies)for testing relativistic jet model,because the viewing angles should show a continuous distribution.In Chapter Four we review the studies of short timescale variability in BL Lac objects,the observations from radio to γ-rays,the multirange observation,the connections to other properties of BL Lac objects,the differences with the variability in other AGNs and the theoretic models,in particularly,shocks-in-jet model.We point out that the abrupt short-term drops which are also common in BL Lac objects,though to be not as frequent as flares,are a powerful challenge to shocks-in-jet model,and that the investigation for variability in radio-quiet AGNs may help to reveal the mechanisms for those short timescale variations which do not originate in the relativistic jet of BL Lac objects.Chapter Five reviews long timescale variability in BL Lac objects,especially the periodicity and the confirming observation during OJ 94 program,and the theoretic models for long timescale variability.We point out that the fact that almost all BL Lac objects which have light curve with long time span show periodicity for long timescale variability which indicates that long timescale variability is somewhat different from short timescale variability which is irregular,and that it is difficult for shocks-in-jet model to explain periodicity in long term variability,even though the model has been revised to have a lighthouse effect.New models proposed to explain periodicity are summarized at this Chapter.We point out that studies of long timescale variability may reveal the large scale structure of AGNs,such as the tramsportation of energy and matter from the central region to relativistic jet,the existence of accretion disk.In Chapter Six the behaviour of short timescale variability in radio-selected and X-ray-selected BL Lac objects is compared to test the unify scheme.The results of our work do not support different viewing angle scenario.In Chapter Seven we present our studies of difference of short timescale variability between the high states and low states of BL Lac objects.It is found that the BL Lac objects in our sample show stronger activity with smaller flares and flickers in high state than that in low state.It is suggested that the orientation of the relativistic jet of a BL Lac object changes with the time,and thus cause the variability on long timescale.When the jet is close to the line of the sight,a BL Lac object is in the high state and vice versa.In this scenario we explained the difference between variability on short timescale in the high state and low state of BL Lac objects respectively,and the new phenomena in OJ 287 discovered in OJ 94 program by Takalo et al.