Swift and XMM-Newton observations of the dark GRB 050326

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 $(7.7\pm0.9) \times 10^{-6}$ erg cm -2 (20–150 keV), and its spectrum was hard, with a power law photon index $\Gamma = 1.25 \pm 0.03$. The X-ray afterglow was quite bright, with a flux of $7 \times 10^{-11}$ 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 ${\sim} 1$ h and ${\sim} 6$ d after the burst, and decayed with a slope $\alpha = 1.70\pm0.05$. 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, $N_{{\rm H},z} \ga 4 \times 10^{21}$ 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 $\vartheta_{\rm j} \ga 7\degr$. Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was larger than $3 \times 10^{51}$ 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 $z \ga 4.5$ is required.