TOWARD AN UNDERSTANDING OF GRB PROMPT EMISSION MECHANISM: II. PATTERNS OF PEAK ENERGY EVOLUTION AND THEIR CONNECTION TO SPECTRAL LAGS.

The prompt emission phase of gamma-ray bursts (GRBs) exhibits two distinct patterns of the peak-energy ( ) evolution; i.e., time-resolved spectral analyses of spectra of broad pulses reveal (1) "hard-to-soft" and (2) "flux-tracking" patterns of evolution in time, the physical origin of which still remains not well understood.