Four years of monitoring blazar PKS 2155-304 with BeppoSAX:: Probing the dynamics of the jet

PKS 2155-304 is one of the brightest blazars in the X-ray band. It was repeatedly monitored with Beppo-SAX during three long campaigns of about 2 days each in November of 1996, 1997, and 1999. The source underwent different states of intensity and was clearly variable with successive flares detected. This paper presents temporal and spectral analysis to study the X-ray variability trends for a blazar. The variability shows larger amplitude and shorter timescale at higher energies. The power spectral densities have steep power-law slopes of similar to2-3, indicating shot-noise variability. Structure function analysis reveals the existence of a typical timescale characteristic of the half-duration of the flares. From the cross-correlation analysis we find that the values of soft lags, i.e., delays of soft (0.1-1.5 keV) photons with respect to hard (3.5-10 keV) ones, differ from are to are, ranging from a few hundred seconds to about 1 hr. There is a suggestion that the flares with shorter duration show smaller soft lags. The soft lags are also energy-dependent, with longer lags of lower energy emission with respect to the emission in the 4 10 keV range. The time-resolved X-ray spectral fits with a curved model show that peak energies of the synchrotron component are located in the very soft X-ray range or even below the BeppoSAX lower energy limit, 0.1 keV. A correlation between peak energies and fluxes is marginal. Spectral evolution during some flares shows clockwise loops in the spectral index flux plane, confirming the soft lags indicated by the cross-correlation analysis. Two flares, however, show evidence that spectral evolution follows opposite tracks in the soft- and hard-energy bands, respectively. The rich phenomenology is interpreted in the context of a model in which relativistic electrons are accelerated through internal shocks taking place in the jets. The most important parameter turns out to be the initial time interval between the two shells ejected from the central engine to produce the are, which may determine the structure of the shock and, in turn, the physical quantities of the emitting region used to reproduce the observed trends of the X-ray variability.