The X-ray afterglow of GRB 030329

We report on XMM-Newton and Rossi-XTE observations of the bright (fluence ~10 -4 erg cm -2) and nearby ( z=0.1685) Gamma-Ray Burst GRB 030329 associated to SN2003dh. The first Rossi-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 Rossi-XTE measurement (at ...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-Newton observation, at ...days, shows a flux of 4*10^-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*10^20 cm -2, consistent with the Galactic value. A further XMM-Newton pointing at ...days shows a flux fainter by a factor ~2. The combined Rossi-XTE and XMM-Newton 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-Newton spectra to optical frequencies lies a factor of ~10 below simultaneous measurements. This is likely due to the presence of SN2003dh.