We have examined the complete set of X-ray afterglow observations of dark and optically bright GRBs performed by BeppoSAX until February 2001 .
X-ray afterglows are detected in \sim 90 \% of the cases .
We do not find significant differences in the X-ray spectral shape , in particular no higher X-ray absorption in GRBs without optical transient ( dark GRBs ) compared to GRBs with optical transient ( OTGRBs ) .
Rather , we find that the 1.6-10 keV flux of OTGRBs is on average about 5 times larger than that of the dark GRBs .
A K-S test shows that this difference is significant at 99.8 % probability .
Under the assumption that dark and OTGRB have similar spectra , this could suggest that the first are uncaught in the optical band because they are just faint sources .
In order to test this hypothesis , we have determined the optical-to-X ray flux ratios of the sample .
OTGRBs show a remarkably narrow distribution of flux ratios , which corresponds to an average optical-to-x spectral index \overline { \alpha } _ { OX } ^ { OT } = 0.794 \pm 0.054 .
We find that , while 75 % of dark GRBs have flux ratio upper limits still consistent with those of OT GRBs , the remaining 25 % are 4 - 10 times weaker in optical than in X-rays .
The significance of this result is \geq 2.6 \sigma .
If this sub-population of dark GRBs were constituted by objects assimilable to OTGRBs , they should have shown optical fluxes higher than upper limits actually found .
We discuss the possible causes of their behaviour , including a possible occurrence in high density clouds or origin at very high redshift and a connection with ancient , Population III stars .