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.
