  We investigated the rest frame spectral lags of two complete samples of
bright long (50) and short (6) gamma-ray bursts (GRB) detected by Swift. We
analysed the Swift/BAT data through a discrete cross-correlation function (CCF)
fitted with an asymmetric Gaussian function to estimate the lag and the
associated uncertainty. We find that half of the long GRBs have a positive lag
and half a lag consistent with zero. All short GRBs have lags consistent with
zero. The distributions of the spectral lags for short and long GRBs have
different average values. Limited by the small number of short GRBs, we cannot
exclude at more than 2 sigma significance level that the two distributions of
lags are drawn from the same parent population. If we consider the entire
sample of long GRBs, we do not find evidence for a lag-luminosity correlation,
rather the lag-luminosity plane appears filled on the left hand side, thus
suggesting that the lag-luminosity correlation could be a boundary. Short GRBs
are consistent with the long ones in the lag-luminosity plane.
