2018ApJ...865..153L

The spectral evolution and spectral lag behavior of 92 bright pulses from 84 gamma-ray bursts observed by the Fermi Gamma-ray Burst Monitor (GBM) telescope are studied. 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∝k_Elog(t+t₀) (1∝k_1logE), where E_p is the peak photon energy in the radiation spectrum, t + t₀ is the observer time relative to the beginning of pulse -t₀, and 1is the spectral lag of photons with energy E with respect to the energy band 8-25 keV. For H2S and H2S-dominated-tracking pulses, a weak correlation between k_1/W and k_E is found, where W is the pulse width. We also study the spectral lag behavior with peak time t_pE_ of pulses for 30 well-shaped pulses and estimate the main trend of the spectral lag behavior with logt_pE_∝k_tp_logE. It is found that k_tp_ is correlated with k_E. We perform simulations under a phenomenological model of spectral evolution, and find that these correlations are reproduced. We then conclude that spectral lags are closely related to spectral evolution within the pulse. The most natural explanation of these observations is that the emission is from the electrons in the same fluid unit at an emission site moving away from the central engine, as expected in the models invoking magnetic dissipation in a moderately high-σ outflow.