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 and the X-ray afterglow beginning 54 min and 8.5 hr after the burst , respectively .
The prompt GRB 050326 fluence was ( 7.7 \pm 0.9 ) \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 hr after the burst .
Its light curve did not show any break nor flares between \sim 1 hr and \sim 6 d after the burst , and decayed with a slope \alpha = 1.70 \pm 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 _ { { 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 _ { 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 .