Re-born fireballs in Gamma-Ray Bursts

Abstract

We consider the interaction between a relativistic fireball and material assumed to be still located just outside the progenitor star. Only a small fraction of the expected mass is sufficient to efficiently decelerate the fireball, leading to dissipation of most of its kinetic energy. Since the scattering optical depths are still large at distances comparable to the progenitor radius, the dissipated energy is trapped in the system, accelerating it to relativistic velocities. The process resembles the birth of another fireball at radii R~1e11 cm, not far from the transparency radius, and with a starting bulk Lorentz factors Gamma_c~10. As seen in the observer frame, this "re--generated" fireball appears collimated within an angle theta_j=1/Gamma_c. If the central engine works intermittently, the funnel can, at least partially, refill and the process can repeat itself. We discuss how this idea can help solving some open issues of the more conventional internal shock scenario for interpreting the Gamma-Ray Burst properties.