This thesis is dealing with the X-ray variability characteristics of blazars, using data obtained mostly with BeppoSAX. First we performed a comprehensive variability analysis on two TeV sources, MKN 421 and PKS 2155-304, respectively. We then studied possible variability trend in the blazars class as a whole.
MKN 421 and PKS 2155-304, monitored with BeppoSAX between 1996 and 2000, showed very similar variability characteristics. The light curves are characterized by recurrent flares with durations of ~ 1 day. The quasi-symmetric structure of the flares is seen in most cases. In general, the variability amplitude is larger and time scale shorter with increasing energy, but do not correlate with the luminosity. The structure functions are characterized by a steep slope of ~ 1.2 at short time scales, with a clear turn-over at a characteristic time scale of ~ 0.5 days. A probable flare periodicity of ~ 1 day was present in the 2000 observations of MKN 421. Soft lags, hard lags, and zero lags between different X-ray energies were detected in MI<N 421, but in PKS 2155-304 only soft and zero lags were found. The energy-dependence of time lags is well determined when a flare showed pronounced time lags. Our analysis
shows that the relative changes of values in the dominant time scales involved in the emission region are necessary to account for complex energy-dependent variability behaviors observed in these two sources. The fact that both the quasi-symmetric structure and time lags ·were detected in some flares indicates that the sizes of the emission regions are energy-dependent. X-ray variability trends of blazars are qualitatively consistent with a scenario postulating continuity in the spectral energy distributions in blazars. The variability amplitude and photon indices in the 2-10 keV inversely correlate with the radio luminosities, and sources with steeper X-ray spectra show larger amplitude of variability.
