  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 40s 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 Ep of the E F(E) spectrum: Ep is above our energy passband
during the first pulse and goes down to ~10 keV during the second pulse.
Another peculiarity is noted in this event for the first time: the possible
evidence of a 2s 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 kTbb ~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.
