Swift and XMM-Newton observations of the dark GRB 050326

Abstract

We present Swift and XMM-Newton observations of the bright gamma-ray burst GRB 050326, detected by the Swift Burst Alert Telescope. The Swift X-Ray Telescope (XRT) and XMM-Newton discovered the X-ray afterglow beginning 54 min and 8.5 h after the burst, respectively. The prompt GRB 050326 fluence was (7.7 ± 0.9)× 10^-6 erg cm^-2 (20-150 keV), and its spectrum was hard, with a power law photon index γ = 1.25 ± 0.03. The X-ray afterglow was quite bright, with a flux of 7 × 10^-11 erg cm^-2 s^-1 (0.3-8 keV), 1 h after the burst. Its light curve did not show any break nor flares between ∼ 1 h and ∼6 d after the burst, and decayed with a slope α = 1.70 ± 0.05. The afterglow spectrum is well fitted by a power-law model, suffering absorption both in the Milky Way and in the host galaxy. The rest-frame hydrogen column density is significant, N<inf>H.z</inf> k 4 ≳ 10²¹ cm^-2, and the redshift of the absorber was constrained to be z > 1.5. There was good agreement between the spatial, temporal, and spectral parameters as derived by Swift-XRT and XMM-Newton. By comparing the prompt and afterglow fluxes, we found that an early break probably occurred before the beginning of the XRT observation, similarly to many other cases observed by Swift. However, the properties of the GRB 050326 afterglow are well described by a spherical fireball expanding in a uniform external medium, so a further steepening is expected at later times. The lack of such a break allowed us to constrain the jet half-opening angle <inf>j</inf> ≳ 7°. Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was larger than 3 × 10⁵¹ erg, at the high end of GRB energies. Despite the brightness in X rays, only deep limits could be placed by Swift-UVOT at optical and ultraviolet wavelengths. Thus, this GRB was a "truly dark" event, with the optical-to-X-ray spectrum violating the synchrotron limit. The optical and X-ray observations are therefore consistent either with an absorbed event or with a high-redshift one. To obey the Ghirlanda relation, a moderate/large redshift z ≳ 4.5 is required. © ESO 2006.