Systematic analysis of X-ray GRB afterglows observed with XMM-Newton and Chandra

We present a sample of GRB afterglows observed in the X-ray and with XMM-Newton or Chandra, focusing on the brightest events. We have derived for this sample the temporal slope and the spectral index of the continuum, and used closure relationships to discriminate the afterglow environment for each burst. We show that jet features are excluded most of the time, while ISM and wind environments cannot be discriminated. We finally focus on GRB040106, whose environment can be constrained by the X-ray observations only. For this burst.. we present a broad-band analysis and derive some constraints on the positions of characteristic frequencies.