Upper panel: β_OX for the short GRBs of the SBAT4 sample. Upper limits are shown with a down-pointing arrow. The horizontal line marks the limit of GRB darkness. Medium panel: β_OX as a function of redshift. GRBs with unknown redshift are shown at z = 0. Lower panel: β_OX as a function of the intrinsic column density. Down-pointing arrows indicate upper limits on β. Right-pointing arrows indicate GRBs with unknown redshift for which the column density has been computed at z = 0.