XMM‐NewtonView of PKS 2155−304: Hardness Ratio and Cross‐Correlation Analysis of EPIC pn Observations

With currently available XMM-Newton EPIC pn observations spanned over about 3years, we present a detailed spectral and temporal variability of the 0.2--10keV X-ray emission from the X-ray bright BL Lac object PKS 2155-304. Thespectral variability is examined with a model independent hardness ratiomethod. We find that the spectral evolution of the source follows the lightcurves well, indicating that the spectra harden when the fluxes increase. Theplots of hardness ratios versus count rates show that the spectral changes areparticularly significant during flares. The cross-correlation functions (CCFs)show that the light curves in the different energy bands are well correlated atdifferent time lags. The CCF peaks (i.e., the maximum correlation coefficients)tend to become smaller with larger energy differences, and the variability inthe different energy bands are more correlated for the flares than for theother cases. In most cases the higher energy band variations lead the lowerenergy band, but in two cases we observed the opposite behavior that the lowerenergy variability possibly leads the higher energy variability. The time lagsincrease with the energy differences between the two cross-correlated lightcurves. The maximum lag is found to be up to about one hour, in support withthe findings obtained with previous low Earth orbit X-ray missions. We discussour results in the context of the particle acceleration, cooling and lightcrossing timescales.Comment: Accepted by Ap