  The spectra of the majority of bursts exhibit a low-energy power law index,
alpha, that is either a constant or becomes softer with time. However, in the
burst of 30 May 1996 alpha becomes harder. Here we show that this behavior can
be explained by a hybrid model consisting of a thermal and a non-thermal
component. In this burst the power-law index of the non-thermal component
changes drastically from s ~ -1.5 to s ~ -0.67 at approximately 5 seconds after
the trigger, thereby revealing, at low energies, the thermal component with its
hard Rayleigh-Jeans tail. This leads to the large alpha-values that are found
if the Band function is fitted to the spectra. We suggest that the change in s
could be due to a transition from fast to slow cooling of the electrons
emitting in the BATSE range. This could be due to the fact that the magnetic
field strength becomes weaker.
