  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 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 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 equal to or higher than 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.
