Analysis of Fermi GRB data. IV. Spectral lags

These pulses can be classified into hard-to-soft pulses (H2S; 64/92), H2S-dominated-tracking pulses (21/92), and other tracking pulses (7/92). We focus on the relationship between spectral evolution and spectral lags of H2S and H2S-dominated-tracking pulses.

The main trend of spectral evolution (lag behavior) is estimated with logE_p_{propto}k_E_log(t+t_0_) (^{tau}{propto}k_^{tau}logE), where E_p_ is the peak photon energy in the radiation spectrum, t+t_0_ is the observer time relative to the beginning of pulse -t0, and ^{tau} is the spectral lag of photons with energy E with respect to the energy band 8-25keV. For H2S and H2S-dominated-tracking pulses, a weak correlation between k_^{tau}/W and kE is found, where W is the pulse width.