XMM-Newton view 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 3 years, we present a detailed spectral and temporal variability of the 0.2--10 keV X-ray emission from the X-ray bright BL Lac object PKS 2155-304. The spectral variability is examined with a model independent hardness ratio method. We find that the spectral evolution of the source follows the light curves well, indicating that the spectra harden when the fluxes increase. The plots of hardness ratios versus count rates show that the spectral changes are particularly significant during flares. The cross-correlation functions (CCFs) show that the light curves in the different energy bands are well correlated at different time lags. The CCF peaks (i.e., the maximum correlation coefficients) tend to become smaller with larger energy differences, and the variability in the different energy bands are more correlated for the flares than for the other cases. In most cases the higher energy band variations lead the lower energy band, but in two cases we observed the opposite behavior that the lower energy variability possibly leads the higher energy variability. The time lags increase with the energy differences between the two cross-correlated light curves. The maximum lag is found to be up to about one hour, in support with the findings obtained with previous low Earth orbit X-ray missions. We discuss our results in the context of the particle acceleration, cooling and light crossing timescales. 
Y.H. Zhang (Tsinghua University), A. Treves (Insubria University), L.   Maraschi (OAB), J.M. Bai (Yunnan Observatory), F.K. ...