Short-term X-ray variability of the BL Lacertae object PKS 2155 - 304 - Power spectrum and cross-correlation analysis

We present a detailed power spectrum and cross-correlation analysis of the X-ray light curves of the BL Lac object PKS 2155 - 304, obtained with EXOSAT. The power spectra show a marked red noise-type variability, with the power increasing steeply toward low frequency. The variability is more pronounced in the 1-6 keV band than in the 0.1-2 keV band. We show that the presence of data gaps (though limited to <20% in the EXOSAT light curves) can flatten significantly the logarithmic slope of the power spectrum. A technique for bridging data gaps which reduces the power spectrum distortions is introduced. In this way an average power law slope of about - 2.5 is obtained for the power spectrum from the 1-6 keV data. The average power spectrum of the light curves after the removal of a linear trend, as required in this case, is characterized by a slope of - 1.9 +/- 0.4 over the 10^4^ to 3 x 10^-3^ Hz frequency range. Longer observations and alternative power spectrum estimation techniques are required to investigate further the red noise properties of PKS 2155-304. The 3-6 keV light curves are cross-correlated with the 0.1-2 keV and the 1-2 keV light curves, separately. We show that spurious time lags can be introduced by systematic uncertainties related to the swapping of the EXOSAT medium-energy detectors and the switching on and off of the instruments. If these effects are taken into account no evidence of lags is found, with upper limits of a few hundred seconds. The short-term X- ray variability of PKS 2155-304 is compared with that of Seyfert galaxies and is briefly discussed within the framework of BL Lac emission models.