Rapid X-ray Variability of the BL Lacertae Object PKS 2155-304

We present a detailed power-density spectrum and cross-correlation analysis of the X-ray light curves of the BL Lac object PKS 2155-304, observed with BeppoSAX in 1997 (SAX97) and 1996 (SAX96), aimed at exploring the rapid variability properties and the interband cross-correlations in the X-rays. We also perform the same analysis on the (archival) X-ray light curve obtained with ASCA in 1994 (ASCA94). No large amplitude-variability event on a timescale of less than similar to 1 hr is found, and light curves in different energy bands are highly correlated with a tendency for the amplitude of variability to increase with energy. The amplitude of variability is larger and the (fastest) timescale shorter as the source becomes brighter. Furthermore, while the average power-density spectra of all the light curves present pronounced, featureless, red noise spectra, with the power decreasing toward high temporal frequencies, the power-law slopes are somewhat different, indicating different variability properties among the three observations. We perform a cross-correlation analysis using Monte Carlo simulations to estimate the uncertainties on time lags between different bands. A significant soft lag of similar to 4 hr between the 0.1-1.5 and 3.5-10 keV bands is seen in SAX96. During SAX97, in which the source showed a high X-ray state correlated with a highly active phase in gamma rays, a short soft lag (similar to 0.4 hr) is detected between the same energy bands. In contrast, ASCA94 presents an intermediate soft lag of about 0.8 hr. These findings indicate that the interband soft time lags are variable and suggest that the lag is longer when the source is fainter. The time dependence of variability in PKS 2155-304 is briefly discussed within a homogeneous synchrotron scenario for blazars.