Abstract
Gamma-ray
bursts are probably the most extreme cases of explosive phenomena
associated to the end of stellar evolution. Their huge radiated power
allows to detect them at cosmological distance. The observed emission
during a gamma-ray burst is produced by an ultra-relativistic outflow
ejected by a new born compact source. The first "prompt" phase of
emission, characterized by a short duration, a high variability and a
gamma-ray dominated spectrum, is due to a dissipative mechanism within
the jet itself. The second "afterglow" phase is associated to the
deceleration of the jet by the external medium : the flux is rapidly
decreasing, as well as the characteristic frequency which goes from
X-rays to the visible and radio domains. Gamma-ray bursts can be
separated in two classes according to their duration. These two classes
seem to differ mainly by the nature of the initial event responsible
for the formation of the central compact object. Long gamma-ray bursts
are associated to the gravitational collapse of some very massive
stars. The status of short gamma-ray bursts is less clear. The most
discussed scenario is the merger of a binary system of two neutron
stars or a neutron star and a black hole. For this reason, short
gamma-ray bursts appear as promising sources of gravitational waves.
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