The X-shooter GRB afterglow legacy sample (XS-GRB)

In this work we present spectra of all $\gamma$-ray burst (GRB) afterglowsthat have been promptly observed with the X-shooter spectrograph until 31-03-2017. In total, we obtained spectroscopic observations of 103 individual GRBs observed within 48 hours of the GRB trigger. Redshifts have been measured for 97 per cent of these, covering a redshift range from 0.059 to 7.84. Based on a set of observational selection criteria that minimize biases with regards to intrinsic properties of the GRBs, the follow-up effort has been focused on producing a homogeneous sample of 93 afterglowspectra for GRBs discovered by the Swiftsatellite. We here provide a public release of all the reduced spectra, including continuum estimates and telluricabsorption corrections. For completeness, we also provide reductions for the 18 late-time observations of the underlying host galaxies. We provide an assessment of the degree of completeness with respect to the parent GRB population, in terms of the X-ray properties of the bursts in the sample and find that the sample presented here is representative of the full Swiftsample. We constrain the fraction of dark bursts to be < 28 per cent and we confirm previous results that higher optical darkness is correlated with increased X-ray absorption. For the 42 bursts for which it is possible, we provide a measurement of the neutral hydrogen column density, increasing the total number of published HI column density measurements by $\sim$ 33 per cent. This dataset provides a unique resource to study the ISM across cosmic time, from the local progenitor surroundings to the intervening universe.