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We present a time-resolved spectral analysis of 51 long and 11 short brightGRBs observed with the {\em Femri}/GBM, paying special attention to $E_{\rm p}$evolution within a same burst. Among 8 single-pulse long GRBs, 5 showhard-to-soft evolution, while 3 show intensity-tracking. The multi-pulse longGRBs have more complicated patterns. Among the GRBs whose time-resolvedspectrum is available for the first pulse, almost half (15/32 GRBs) show clearhard-to-soft evolution, and the other half (17/32 GRBs) show clearintensity-tracking. Later pulses typically show the tracking behavior, althougha hard-to-soft evolution pattern was identified in the 2nd pulse of 2 GRBswhose pulses are well separated. Statistically, the hard-to-soft evolutionpulses tend to be more asymmetric than the intensity-tracking ones, with asteeper rising wing than the falling wing. Short GRBs have $E_{\rm p}$ trackingintensity exclusively with the 16ms time resolution analysis. We performed asimulation analysis, and suggest that at least for some bursts, the lateintensity-tracking pulses could be a consequence of overlapping hard-to-softpulses. However, the fact that the intensity-tracking pattern exists in thefirst pulse of multi-pulse long GRBs and some single-pulse GRBs suggest thatintensity tracking is an independent component, which may operate in some latepulses as well. For the GRBs with measured redshifts, we present atime-resolved $E_{\rm p}-L_{\gamma, \rm iso}$ correlation analysis and showthat the scatter of the correlation is comparable to that of the globalAmati/Yonetoku relation. We discuss the predictions of various radiation modelsregarding $E_{\rm p}$ evolution, as well as the possibility of a precession jetin GRBs. It seems that the data pose great challenge to all these models, andhold the key to unveil the physics of GRB prompt emission.

astrophysical journal/the astrophysical journal

A Comprehensive Analysis of Fermi Gamma-ray Burst Data: II. $E_{\rm  p}$-Evolution Patterns and Implications for the Observed Spectrum-Luminosity  Relations