Observational difference between gamma and X-ray properties of optically
  dark and bright GRBs
  Using the discriminant analysis of the multivariate statistical analysis we
compared the distribution of the physical quantities of the optically dark and
bright GRBs, detected by the BAT and XRT on board of the Swift Satellite. We
found that the GRBs having detected optical transients (OT) have systematically
higher peak fluxes and lower HI column densities than those without OT.
