Highly-ionized metals as probes of the circumburst gas in the natal
  regions of gamma-ray bursts
  We present here a survey of high-ionization absorption lines in the afterglow
spectra of long-duration gamma-ray bursts (GRBs) obtained with the
VLT/X-shooter spectrograph. Our main goal is to investigate the circumburst
medium in the natal regions of GRBs. Our primary focus is on the NV 1238,1242
line transitions, but we also discuss other high-ionization lines such as OVI,
CIV and SiIV. We find no correlation between the column density of NV and the
neutral gas properties such as metallicity, HI column density and dust
depletion, however the relative velocity of NV, typically a blueshift with
respect to the neutral gas, is found to be correlated with the column density
of HI. This may be explained if the NV gas is part of an HII region hosting the
GRB, where the region's expansion is confined by dense, neutral gas in the
GRB's host galaxy. We find tentative evidence (at 2-sigma significance) that
the X-ray derived column density, N_H,X, may be correlated with the column
density of NV, which would indicate that both measurements are sensitive to the
column density of the gas located in the vicinity of the GRB. We investigate
the scenario where NV (and also OVI) is produced by recombination after the
corresponding atoms have been stripped entirely of their electrons by the
initial prompt emission, in contrast to previous models where highly-ionized
gas is produced by photoionization from the GRB afterglow.
