Dust extinction bias in the column density distribution of gamma-ray
  bursts; high column density, low redshift GRBs are more heavily obscured
  The afterglows of gamma-ray bursts (GRBs) have more soft X-ray absorption
than expected from the foreground gas column in the Galaxy. While the redshift
of the absorption can in general not be constrained from current X-ray
observations, it has been assumed that the absorption is due to metals in the
host galaxy of the GRB. The large sample of X-ray afterglows and redshifts now
available allows the construction of statistically meaningful distributions of
the metal column densities. We construct such a sample and show, as found in
previous studies, that the typical absorbing column density (N_HX) increases
substantially with redshift, with few high column density objects found at low
to moderate redshifts. We show, however, that when highly extinguished bursts
are included in the sample, using redshifts from their host galaxies, high
column density sources are also found at low to moderate redshift. We infer
from individual objects in the sample and from observations of blazars, that
the increase in column density with redshift is unlikely to be related to
metals in the intergalactic medium or intervening absorbers. Instead we show
that the origin of the apparent increase with redshift is primarily due to dust
extinction bias: GRBs with high X-ray absorption column densities found at
$z\lesssim4$ typically have very high dust extinction column densities, while
those found at the highest redshifts do not. It is unclear how such a strongly
evolving N_HX/A_V ratio would arise, and based on current data, remains a
puzzle.
