  We present a catalog of XMM-Newton and Chandra observations of gamma-ray
burst (GRB) afterglows, reduced in a common way using the most up-to-date
calibration files and software. We focus on the continuum properties of the
afterglows. We derive the spectral and temporal decay indices for 16 bursts. We
place constraints on the burst environment and geometry. A comparison of the
fast XMM-Newton follow-up and the late Chandra observations shows a significant
difference in those parameters, likely produced by a transition from jet
expansion taking place between two and ten days after the burst. We do not
observe a significant shrinking of the luminosity distribution when we correct
for beaming; more burst observations are needed to confirm this result. We also
compare our results with those obtained by BeppoSAX and SWIFT; there is no
strong discrepancy between the afterglow fluxes observed with these satellites
when we carefully take into account the different median observation time of
each observatory.
