The X-ray afterglow of GRB 030329

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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 $0.9\pm0.3$. Such a decay law is only marginally consistent with a further Rossi-XTE measurement (at $t-t_{\rm GRB}\sim 30$ 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 $t-t_{\rm GRB}\sim 37$ days, shows a flux of $4\times10^{-14}$ erg cm -2 s -1 (0.2–10 keV). The spectrum is a power law with photon index $\Gamma=1.9$ and absorption -2 , consistent with the Galactic value. A further XMM-Newton pointing at $t-t_{\rm GRB}\sim61$ days shows a flux fainter by a factor ~2. The combined Rossi-XTE and XMM-Newton measurements require a break at $t\sim 0.5$ 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.