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The prompt emission of GRB990712 with BeppoSAX: evidence of a transient X-ray emission feature

F. Frontera·L. Amati·M. Vietri·J. J. M. in 't Zand·E. Costa·M. Feroci·J. Heise·N. Masetti·L. Nicastro·M. Orlandini·E. Palazzi·E. Pian·L. Piro·P. Soffitta
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Abstract

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.

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F. Frontera, L. Amati, M. Vietri, J. J. M. in 't Zand, E. Costa, M. Feroci, J. Heise, N. Masetti, L. Nicastro, M. Orlandini, E. Palazzi, E. Pian, L. Piro, P. Soffitta. 2001-03-19. The prompt emission of GRB990712 with BeppoSAX: evidence of a transient X-ray emission feature. https://doi.org/10.1086/319480

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