  Time-resolved spectroscopy is performed on eight bright, long gamma-ray
bursts (GRBs) dominated by single emission pulses that were observed with the
{\it Fermi Gamma-ray Space Telescope}. Fitting the prompt radiation of GRBs by
empirical spectral forms such as the Band function leads to ambiguous
conclusions about the physical model for the prompt radiation. Moreover, the
Band function is often inadequate to fit the data. The GRB spectrum is
therefore modeled with two emission components consisting of optically thin
nonthermal synchrotron radiation from relativistic electrons and, when
significant, thermal emission from a jet photosphere, which is represented by a
blackbody spectrum. To produce an acceptable fit, the addition of a blackbody
component is required in 5 out of the 8 cases. We also find that the low-energy
spectral index \alpha is consistent with a synchrotron component with \alpha =
-0.81\pm 0.1. This value lies between the limiting values of \alpha = -2/3 and
\alpha = -3/2 for electrons in the slow and fast-cooling regimes, respectively,
suggesting ongoing acceleration at the emission site. The blackbody component
can be more significant when using a physical synchrotron model instead of the
Band function, illustrating that the Band function does not serve as a good
proxy for a nonthermal synchrotron emission component. The temperature and
characteristic emission-region size of the blackbody component are found to,
respectively, decrease and increase as power laws with time during the prompt
phase. In addition, we find that the blackbody and nonthermal components have
separate temporal behaviors.
