The X-ray afterglow of GRB 030329

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica – CNR, Sezione di Milano “G.Occhialini”, Via Bassini 15, 20133 Milano, Italy
² Università degli Studi di Milano, Dipartimento di Fisica, v. Celoria 16, 20133 Milano, Italy
³ INAF-Osservatorio Astronomico di Brera, v. Bianchi 46, 23907 Merate (LC), Italy
⁴ Institute of Astronomy, Madingley Road, Cambridge CB3 OHA, UK
⁵ XMM-Newton Science Operation Center, ESA, Vilspa, Apartado 50727, 28080 Madrid, Spain

Corresponding author: A. Tiengo, tiengo@mi.iasf.cnr.it

Received: 30 May 2003
Accepted: 23 July 2003

Abstract

We report on XMM-Newton and Rossi-XTE observations of the bright (fluence ~10^-4 erg cm^-2) and nearby () 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 . 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  erg cm^-2 s^-1 (0.2–10 keV). The spectrum is a power law with photon index and absorption < 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  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.