Statistical studies of optically dark gamma-ray bursts in the Swift era

We compare the properties of optically dark GRBs, defined by the optical-to-X-ray spectral index beta_OX<0.5, and normal ones discovered by the Swift satellite before the year 2008 in a statistical way, using data collected from the literature and online databases. Our sample include 200 long bursts, 19 short bursts, and 10 with measured high redshifts (z>=4). The ratio of dark bursts is found to be about 10-20%, and is similar between long bursts, short ones, and the high-z sub-sample. The result for long bursts is consistent with both the pre-Swift sample and studies by other authors on smaller Swift samples. The existence of dark short GRBs is pointed out for the first time. The X-ray derived hydrogen column densities of dark GRBs clearly prefer large values compared with those of normal bursts. This supports the dust extinction scenario as the main cause of dark GRBs. Other possibilities like very high redshifts and non-standard emission mechanisms are less likely although not fully excluded.