Observing Mkn 421 with XMM--Newton: the EPIC--PN point of view
  We present three observations (four exposures) of Mkn 421 performed by XMM--Newton in the Autumn of 2002, concentrating on the EPIC--PN camera data. The X-ray spectra were soft and steepening toward high energies. The source was highly variable and the hardness ratio plots displayed a clear harder--when--stronger correlation. During two complete flares the source showed strong spectral evolution: a hardness ratio and a time resolved spectral analysis revealed both a clockwise and a counterclockwise rotating loop patterns, suggesting the presence of temporal lags between different energy bands variations. We confirmed this result and estimated the delay amounts with a cross--correlation analysis performed on the single flares, discussing also variability patterns that could reproduce the asymmetry seen in the cross-correlation function. We verified our findings reproducing the two flares with analytical models. We obtained consistent results: during one flare, Mkn 421 displayed soft lags, while in the other case it showed hard lags. In both cases, the entity of the delays increases with the energy difference between the compared light curves. Finally, we discussed the presence and the frequency dependence of the temporal lags as an effect of particle acceleration, cooling and escape timescales, showing that our data are consistent with this picture. 
M. Ravasio, G. Tagliaferri, G. Ghisellini, F. Tavecchio