Gamma-Ray Bursts ( GRBs ) are the brightest objects observed .
They are also the most relativistic objects known so far .
GRBs occur when an ultrarelativisitic ejecta is slowed down by internal shocks within the flow .
Relativistic particles accelerated within these shocks emit the observed gamma-rays by a combination of synchrotron and inverse Compton emission .
External shocks with the circumstellar matter slow down further the ejecta and produce the afterglow , which lasts for months .
Comparison of the predictions of this fireball model with observations confirm a relativistic macroscopic motion with a Lorentz factor of \Gamma \geq 100 .
Breaks in the light curves of the afterglow indicate that GRBs are beamed with typical opening angles of a few degrees .
The temporal variability of the gamma-rays signal provide us with the best indirect evidence on the nature of the “ internal engine ” that powers the GRBs and accelerates the relativistic ejecta , suggesting accretion of a massive disk onto a newborn black hole : GRBs are the birth cries of these black holes .
Two of the most promising models : Neutron Star Mergers and Collapars lead naturally to this scenario .

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