The X-ray afterglow of GRB 030329

We report on XMM-Newton and Rossi-XTE observations of the bright (fluence similar to10(-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 t - t(GRB) similar to 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-Newton observation, at t - t(GRB) similar to 37 days, shows a flux of 4 x 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 x 10(20) cm(-2), consistent with the Galactic value. A further XMM-Newton pointing at t - t(GRB) ∼ 61 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.