GRB 090313: X-shooter's first shot at a GRB
  Context. X-shooter is the first second-generation instrument to become
operative at the ESO Very Large Telescope (VLT). It is a broad-band
medium-resolution spectrograph designed with gamma-ray burst (GRB) afterglow
spectroscopy as one of its main science drivers.
  Aims. During the first commissioning night on sky with the instrument fully
assembled, X-shooter observed the afterglow of GRB 090313 as a demonstration of
the instrument's capabilities.
  Methods. GRB 090313 was observed almost two days after the burst onset, when
the object had already faded to R~21.6. Furthermore, the 90% illuminated Moon
was just 30 degrees away from the field. In spite of the adverse conditions, we
obtained a spectrum that, for the first time in GRB research, covers
simultaneously the range from 5700 to 23000 Angstroms.
  Results. The spectrum shows multiple absorption features at a redshift of
3.3736, the redshift of the GRB. These features are composed of 3 components
with different ionisation levels and velocities. Some of the features have
never been observed before in a GRB at such a high redshift. Furthermore, we
detect two intervening systems at redshifts of 1.8005 and 1.9597.
  Conclusions. These results demonstrate the potential of X-shooter in the GRB
field, as it was capable of observing a GRB down to a magnitude limit that
would include 72% of long GRB afterglows 2 hours after the burst onset. Coupled
with the rapid response mode available at VLT, allowing reaction times of just
a few minutes, X-shooter constitutes an important leap forward on medium
resolution spectroscopic studies of GRBs, their host galaxies and intervening
systems, probing the early history of the Universe.
