The X-ray afterglow of GRB 030329

We report on XMM and XTE observations of the bright (fluence ~ 10^-4 erg cm^-2) and nearby (z=0.1685) Gamma-Ray Burst \gr associated to SN2003dh. The first XTE observation, 5 hours after the burst, shows a flux decreasing with time as a power law with index 0.9+/-0.3. Such a decay law is only marginally consistent with a further XTE measurement (at t-t_GRB ~ 30 hr). Late time observations of this bright afterglow at X-ray wavelengths have the advantage, compared to optical observations, of not being affected by contributions from the supernova and host galaxy. A first XMM observation, at t-t_GRB ~ 37 days, shows a flux of 4*E^-14 erg cm^-2 s^-1 (0.2-10 keV). The spectrum is a power law with photon index Gamma =1.9 and absorption <2.5*E²⁰ cm^-2, consistent with the Galactic value. A further XMM pointing at t-t_GRB ~ 61 days shows a flux fainter by a factor ~ 2. The combined XTE and XMM measurements require a break at t ~ 0.5 days in the afterglow decay, with a power law index increasing from 0.9 to 1.9, similar to what is observed in the early part of the optical afterglow. The extrapolation of the XMM spectra to optical frequencies lies a factor of ~ 10 below simultaneous measurements. This is likely due to the presence of SN2003dh.