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Paper: Properties of Dark and Optically Bright Gamma-Ray Bursts of BeppoSAX
Volume: 312, Third Rome Workshop on Gamma-Ray Bursts in the Afterglow Era
Page: 205
Authors: De Pasquale, M.; Piro, L.; Costa, E.; Feroci, M.; Gandolfi, G.; Perna, R.; in 't Zand, J.J.M.; Nicastro, L.; Frontera, F.; Antonelli, L.A.; Fiore, F.; Stratta, G.
Abstract: We examined the complete set of observation of GRB X-ray afterglows performed by BeppoSAX up to February 2001. We do not find increased X-ray absorption in GRBs without optical transient (dark GRBs) compared to GRB with optical transient (OTGRBs). Rather, we find that X-ray flux of dark GRB is ≈ 5 times lower than that of OTGRBs. This result, which is significant at 3σ level, could suggest that the dark GRBs are uncaught in the optical band just because they are faint sources. To test this hypothesis, we calculated the optical-to-X flux ratios of the GRBs of our sample. We find that, while 75% of dark GRBs have flux ratio upper limits still consistent with those of OTGRBs, the remaining 25% are ≈ 5 times weaker in the optical than X-rays. We discuss the possible causes of this behaviour, including a possible occurrence in high-density clouds or origin at very high redshift.
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