Despite the enormous progress occurred in the last 10 years, the Gamma-Ray
Bursts (GRB) phenomenon is still far to be fully understood. One of the most
important open issues that have still to be settled is the afterglow emission
above 10 keV, which is almost completely unexplored. This is due to the lack of
sensitive enough detectors operating in this energy band. The only detection,
by the BeppoSAX/PDS instrument (15-200 keV), of hard X-ray emission from a GRB
(the very bright GRB 990123), combined with optical and radio observations,
seriously challenged the standard scenario in which the dominant mechanism is
synchrotron radiation produced in the shock of a ultra-relativistic fireball
with the ISM, showing the need of a substantial revision of present models. In
this respect, thanks to its unprecedented sensitivity in the 10-80 keV energy
band, Simbol-X, through follow-up observations of bright GRBs detected and
localized by GRB dedicated experiments that will fly in the >2010 time frame,
will provide an important breakthrough in the GRB field.Comment: 4 pages, 2 figures. Paper presented at "Simbol-X: the hard X-ray
universe in focus", held in Bologna, Italy, on 14-16 May 2007. To be
published in Memorie della Societa' Astronomica Italian
