  We show that Gamma Ray Bursts (GRBs) of known redshift and rest frame optical
extinction detected by the Swift satellite fully confirm earlier results
concerning the distribution of the optical afterglow luminosity at 12 hours
after trigger (rest frame time). This distribution is bimodal and relatively
narrow, especially for the high luminosity branch. This is intriguing, given
that Swift GRBs have, on average, a redshift larger than pre-Swift ones, and is
unexpected in the common scenario explaining the GRB afterglow. We investigate
if the observed distribution can be the result of selection effects affecting a
unimodal parent luminosity distribution, and find that either the distribution
is intrinsically bimodal, or most (60 per cent) of the bursts are absorbed by a
substantial amount of grey dust. In both cases we suggest that most dark bursts
should belong to the underluminous optical family.
