The prompt emission of GRB 990712 with BeppoSAX:: Evidence of a transient X-ray emission feature

We report on the prompt X- and gamma -ray observations of GRB 990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera 2. As a result of 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 spectrum: E-p 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 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) -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.