2003A&A...409..983T

We report on XMM-Newton and Rossi-XTE observations of the bright (fluence∼10^–4erg/cm²) 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∼30hr). 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∼37-days, shows a flux of 4x10^–14erg/cm²/s (0.2-10keV). The spectrum is a power law with photon index Γ=1.9 and absorption <2.5x10²⁰cm^–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.