  Gamma-ray burst (GRB) progenitors are observationally linked to the death of
massive stars. X-ray studies of the GRB afterglows can deepen our knowledge of
the ionization status and metal abundances of the matter in the GRB
environment. Moreover, the presence of local matter can be inferred through its
fingerprints in the X-ray spectrum, i.e. the presence of absorption higher than
the Galactic value. A few studies based on BeppoSAX and XMM-Newton found
evidence of higher than Galactic values for the column density in a number of
GRB afterglows. Here we report on a systematic analysis of 17 GRBs observed by
Swift up to April 15, 2005. We observed a large number of GRBs with an excess
of column density. Our sample, together with previous determinations of the
intrinsic column densities for GRBs with known redshift, provides evidence for
a distribution of absorption consistent with that predicted for randomly
occurring GRB within molecular clouds.
