We report on the prompt X– and \gamma –ray observations of GRB990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera No .
2 .
Due to Sun constraints , we could not perform a follow-up observation with the BeppoSAX Narrow Field Instruments .
The light curve of the prompt emission shows two pulses and a total duration of about 40 s in X-rays .
In gamma–rays the event is even shorter .
The 2–700 keV spectral emission with time shows a discontinuity in the peak energy E _ { p } of the EF ( E ) spectrum : E _ { p } is above our energy passband during the first pulse and goes down to \sim 10 keV during the second pulse .
Another peculiarity is noted in this event for the first time : the evidence of a 2 s duration emission feature during the tail of the first pulse .
The feature is consistent with either a Gaussian profile with centroid energy of 4.5 keV or a blackbody spectrum with kT _ { bb } \sim 1.3 keV .
We discuss the possible origin of the feature .
The most attractive possibility is that we are observing the thermal emission of a baryon-loaded expanding fireball , when it becomes optically thin .