A complete sample of bright Swift Gamma–ray Bursts ( GRBs ) has been recently selected by Salvaterra et al .
( 2011 ) .
The sample has a high level of completeness in redshift ( 90 \% ) .
We derive here the intrinsic absorbing X–ray column densities of these GRBs making use of the Swift X–ray Telescope data .
This distribution has a mean value of \log ( N _ { H } / { cm ^ { -2 } } ) = 21.7 \pm 0.5 .
This value is consistent with the distribution of the column densities derived from the total sample of GRBs with redshift .
We find a mild increase of the intrinsic column density with redshift .
This can be interpreted as due to the contribution of intervening systems along the line of sight .
Making use of the spectral index connecting optical and X–ray fluxes at 11 hr ( \beta _ { OX } ) , we investigate the relation of the intrinsic column density and the GRB ‘ darkness ’ .
We find that there is a very tight correlation between dark GRBs and high X–ray column densities .
This clearly indicates that the dark GRBs are formed in a metal-rich environment where dust must be present .