  We study the time-resolved spectra of eight GRBs observed by Fermi GBM in its
first five years of mission, with 1 keV - 1 MeV fluence $f>1.0\times10^{-4}$
erg cm$^{-2}$ and signal-to-noise level $\text{S/N}\geq10.0$ above 900 keV. We
aim to constrain in detail the spectral properties of GRB prompt emission on a
time-resolved basis and to discuss the theoretical implications of the fitting
results in the context of various prompt emission models. We perform
time-resolved spectral analysis using a variable temporal binning technique
according to optimal S/N criteria, resulting in a total of 299 time-resolved
spectra. We fit the Band function to all spectra and obtain the distributions
for the low-energy power-law index $\alpha$, the high-energy power-law index
$\beta$, the peak energy in the observed $\nu F_\nu$ spectrum $E_\text{p}$, and
the difference between the low- and high-energy power-law indices $\Delta
s=\alpha-\beta$. Using the distributions of $\Delta s$ and $\beta$, the
electron population index $p$ is found to be consistent with the "moderately
fast" scenario which fast- and slow-cooling scenarios cannot be distinguished.
We also apply a physically motivated synchrotron model, which is a triple
power-law with constrained power-law indices and a blackbody component, to test
for consistency with a synchrotron origin for the prompt emission and obtain
the distributions for the two break energies $E_\text{b,1}$ and $E_\text{b,2}$,
the middle segment power-law index $\beta$, and the Planck function temperature
$kT$. A synchrotron model is found consistent with the majority of
time-resolved spectra for these eight energetic Fermi GBM bursts with good
high-energy photon statistics, as long as both the cooling and injection break
are included and the leftmost spectral slope is lifted either by inclusion of a
thermal component or when an evolving magnetic field is accounted for.

High Energy Astrophysical Phenomena