2005ApJ...629..686Z

Starting from XMM-Newton EPIC pn data, we present the X-ray variability characteristics of PKS 2155-304 using a simple analysis of the excess variance, σ²_XS, and of the fractional rms variability amplitude, F_var. The scatter in σ²_XSand F_var, calculated using 500 s long segments of the light curves, is smaller than the scatter expected for red-noise variability. This alone does not imply that the underlying process responsible for the variability of the source is stationary, since the real changes of the individual variance estimates are possibly smaller than the large scatters expected for a red-noise process. In fact, the averaged σ²_XSand F_var, which reduce the fluctuations of the individual variances, also change with time, indicating nonstationary variability. Moreover, both the averaged σ_XS(absolute rms variability amplitude) and the averaged F_varshow linear correlation with source flux, but in an opposite sense: σ_XScorrelates with flux, but F_varanticorrelates with flux. These correlations suggest that the variability process of the source is strongly nonstationary, as random scatters of variances should not yield any correlation. Spectra of F_varwere constructed to compare variability amplitudes in different energy bands. We found that the fractional rms variability amplitude of the source, when significant variability is observed, increases logarithmically with the photon energy, indicating significant spectral variability. The point-to-point variability amplitude may also track this trend, suggesting that the slopes of the power spectral density of the source are energy-independent. Using the normalized excess variance, the black hole mass of PKS 2155-304 was estimated to be about 1.45x10⁸ M_☉. This is compared and contrasted with the estimates derived from measurements of the host galaxies.