  Spectral lag, the time difference between the arrival of high-energy and
low-energy photons, is a common feature in Gamma-ray Bursts (GRBs). Norris et
al. 2000 reported a correlation between the spectral lag and the isotropic peak
luminosity of GRBs based on a limited sample. More recently, a number of
authors have provided further support for this correlation using arbitrary
energy bands of various instruments. In this paper we report on a systematic
extraction of spectral lags based on the largest Swift sample to date of 31
GRBs with measured redshifts. We extracted the spectral lags for all
combinations of the standard Swift hard x-ray energy bands: 15-25 keV, 25-50
keV, 50-100 keV and 100-200 keV and plotted the time dilation corrected lag as
a function of isotropic peak luminosity. The mean value of the correlation
coefficient for various channel combinations is -0.68 with a chance probability
of ~ 0.7 x 10^{-3}. In addition, the mean value of the power-law index is 1.4
+/- 0.3. Hence, our study lends support for the existence of a lag-luminosity
correlation, albeit with large scatter.
