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The prompt emission phase of gamma-ray bursts (GRBs) exhibits two distinctpatterns of the peak-energy ($E_p$) evolution, i.e., time-resolved spectralanalyses of $\nu F_{\nu}$ spectra of broad pulses reveal (1) "hard-to-soft" and(2) "flux-tracking" patterns of $E_p$ evolution in time, the physical origin ofwhich still remains not well understood. We show here that these two patternscan be successfully reproduced within a simple physical model invokingsynchrotron radiation in a bulk-accelerating emission region. We show furtherthat the evolution patterns of the peak energy have, in fact, directconnections to the existence of two different (positive or negative) types ofspectral lags, seen in the broad pulses. In particular, we predict that (1)only the positive type of spectral lags is possible for the hard-to-softevolution of the peak energy, (2) both the positive and negative type ofspectral lags can occur in the case of flux-tracking pattern of the peakenergy, (3) for the flux-tracking pattern, the peak location of the flux lightcurve slightly lags behind the peak of the $E_p$ evolution with time if thespectral lags are positive, and (4) in the case of flux-tracking pattern,double-peaked broad pulses can appear in the light curves, the shape of whichis energy-dependent.

Toward an understanding of GRB prompt emission mechanism: II. Patterns of peak energy evolution and their connection to spectral lags