The Study Of The Variability Of BL Lac Objects

BL Lacs are an extreme subclass of active galactic nuclei(AGNs),which have very rapid and strong variability throughout the entire electromagnetic wavelengths.The research of variability is an useful tool to understand the physical nature of BL Lacs.The results of variability analysis can shed light on the radiation mechanism and the physical processes in the emission regions.There are more than 100 years histories for the observations of BL Lacs.However,due to the random processes of BL Lacs variability and the complex behavior of optical spectrum,more observations are needed to constrain the physical features in it.In this thesis,the basic properties,classification and uniform model of the active galactic nuclei and blazars have been described in Section 1.The basic properties and spectra behavior are presented in Section 2.Section 3 introduces the variability behaviors on various timescales and variability mechanism of the BL Lacs.Section 4 presents the CCD photometry and the reduce process of aperture photometric for BL Lacs.Section 5 presents our variability analysis on a typical BL Lac OJ 287,based on the observations at Yunnan observatories.Finally,we summarize our thesis.The BL Lac object OJ 287(0851+203)at redshift z=0.306 is a typical BL Lac,which is the only object with confirmed period of 12 years.From 2006 to2012,we monitored this object with multi-colors in the optical band.We analyze the variability,time lags and spectral properties of OJ 287.Combining with V band and R band data from SMARTS and Steward Observatory,we analyze the long-term timescales spectral behaviors of OJ 287 and search for the possible periodic signals.Through the analysis of time lags,there are not significant time lags in any two bands for our light curves.The bluer-when-brighter trend is dominant for intraday,which supports the shock-in-jet model.There are also a bluer-when-brighter trend between V-R index and V magnitude for long-term timescales.Some possible period of 513,176,36,30,26,17 and 14 days are found in the whole time-series data sets from 2006 to 2017.The possible explanationsinclude precession of jet,a helical structure of the magnetic field at the base of the jet,the orbital motion of the accretion disk close to the central primary black hole.