The X - ray afterglow of GRB 030329

the date of receipt and acceptance should be inserted later Abstract. 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 GRB030329 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-tGRB ∼30 hr). The XMM-Newton data, at t-tGRB ∼37 days, show a flux of 4×10 −14 erg cm −2 s −1 (0.2-10 keV). The spectrum is a power law with photon index Γ=1.9 and absorption <2.5×10 20 cm −2 , consistent with the Galactic value. 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. From the extrapolation of the XMM-Newton spectrum to optical frequencies we find an optical excess of a factor of ∼ 10 in flux, likely due to the presence of SN2003dh.