GRB990123: Multiwavelength afterglow study

Summary. — We report on the BeppoSAX and multiwavelength data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X–ray afterglow between ∼20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. The backwards extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus indicating that both are consistent with being produced by the same phenomenon. For the X–ray afterglow we found a power–law decay with index αX = 1.46 ± 0.04; the spectrum has a power-law shape with photon index Γ ∼ 2. An extensive set of multiwavelength observations on the afterglow, collected from the literature and made during the BeppoSAX pointing, allowed constructing a Spectral Flux Distribution. We performed an analysis of the GRB afterglow emission in the context of the “fireball ” model. PACS 95.75.Fg – Spectroscopy and spectrophotometry. PACS 95.85.Nv – X–ray. PACS 98.70.Rz – gamma–ray sources; gamma–ray bursts.