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 \sim 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 \alpha _ { X } = 1.46 \pm 0.04 ; the spectrum has a power-law shape with photon index \Gamma \sim 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 .