We have examined the complete set of X-ray afterglow observations of dark and optically bright gamma-ray bursts (GRBs) performed by BeppoSAX through 2001 February. 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 increased 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 Kolmogorov-Smirnov test shows that this difference is significant at 99.8% probability. Under the assumption that dark and OTGRBs 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-ray spectral index = 0.794 ± 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 ≥2.6 σ. If this subpopulation 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 behavior, including a possible occurrence in high-density clouds or origin at very high redshift and a connection with ancient, Population III stars.
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