Probing physics of particle acceleration in relativistic jets : PKS2155-304

We propose to observe the Xray bright blazar PKS 2155304 for a full XMM orbitin order to study its time variability with unprecedented time/energyresolution. Long ASCA and SAX observations suggest that soft Xray photonslag the harder ones by 4 to 0.5 hours. The unique capabilities of XMM will allowto unambiguously confirm these lags and measure their energy dependence withinthe Xray band and in the UV. The length is dictated by the need to measureshort Xray lags with high precision as well as long lags expected in the UVArrangements to combine the observations with GTO time are in progress. The data will allow to set new constraints on the mechanisms of particle acceleration and cooling in jets. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]