The quasi-thermal components found in many Fermi gamma-ray bursts ( GRBs ) imply that the photosphere emission indeed contributes to the prompt emission of many GRBs .
But whether the observed spectra empirically fitted by the Band function or cutoff power law , especially the spectral and peak energy ( E _ { p } ) evolutions can be explained by the photosphere emission model alone needs further discussion .
In this work , we investigate in detail the time-resolved spectra and E _ { p } evolutions of photospheric emission from a structured jet , with an inner-constant and outer-decreasing angular Lorentz factor profile .
Also , a continuous wind with a time-dependent wind luminosity has been considered .
We show that the photosphere spectrum near the peak luminosity is similar to the cutoff power-law spectrum .
The spectrum can have the observed average low-energy spectral index \alpha \sim - 1 , and the distribution of the low-energy spectral index in our photosphere model is similar to that observed ( -2 \lesssim \alpha \lesssim 0 ) .
Furthermore , the two kinds of spectral evolutions during the decay phase , separated by the width of the core ( \theta _ { c } ) , are consistent with the time-resolved spectral analysis results of several Fermi multi-pulse GRBs and single-pulse GRBs , respectively .
Also , for this photosphere model we can reproduce the two kinds of observed E _ { p } evolution patterns rather well .
Thus , by considering the photospheric emission from a structured jet , we reproduce the observations well for the GRBs best fitted by the cutoff power-law model for the peak-flux spectrum or the time-integrated spectrum .