  The observations of the prompt emission of gamma ray bursts (GRB) by GLAST
Burst Monitor (GBM), on board Fermi Gamma-ray Space Telescope, suggest the
presence of a significant thermal spectral component, whose origin is not well
understood. Recently, it has been shown that for long duration GRBs, the
spectral width as defined as the logarithm of the ratio of the energies at
which the spectrum falls to half its peak value, lie in the range of 0.84-1.3
with a median value of 1.07. Thus, while most of the GRB spectra are found to
be too narrow to be explained by synchrotron emission from an electron
distribution, they are also significantly broader than a blackbody spectrum
whose width should be 0.54. Here, we consider the possibility that an intrinsic
thermal spectrum from a fire-ball like model, may be observed to be broadened
if the system undergoes a rapid temperature evolution. We construct a toy-model
to show that for bursts with durations in the range 5-70 seconds, the widths of
their 1 second time-averaged spectra can be at the most $\lesssim 0.557$. Thus,
while rapid temperature variation can broaden the detected spectral shape, the
observed median value of $\sim 1.07$ requires that there must be significant
sub-photospheric emission and/or an anisotropic explosion to explain the
broadening for most GRB spectra.
