The Prompt Emission of GRB 990712 with [ITAL]B[/ITAL][CSC][ITAL]eppo[/ITAL][/CSC][ITAL]SAX[/ITAL]: Evidence of a Transient X-Ray Emission Feature

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Abstract
We report on the prompt X- and γ-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 γ-rays, the event is even shorter. The 2-700 keV spectral emission with time shows a discontinuity in the peak energy Ep of the EF(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 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 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.

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