The Fireball Model of GRB

This Fireball theory that explains how so much energy can be channelized out from a very compact source in such a short time. In eighties Goodman has set the basis for this model with his work on a radiation electron positron fireball. However, it was clear that this simple model can not truly explain GRBs. It led inevitable to a thermal spectrum, whereas GRBs clearly show a non-thermal emission. The next step took place by Tsvi Piran and his student Amotz Shemi in 1990. They modified the Goodman’s model to include protons. They shown that all the energy of the initial fireball would be converted to a kinetic energy of the protons. If the baryonic load were small enough this would result in an ultra-relativistic outflow in which the matter moves at nearly equal to or equal to speed of light.
Thus the Martin Rees from Cambridge and Peter Meszaros from Penn State completed the puzzle when they suggested in 1992 that the kinetic energy of the outflow can be converted back to radiation via collision-less shocks between the outflow and the surrounding matter. At roughly the same time with Bohdan Paczynski, Ramesh Narayan suggested internal shocks within the outflow as an alternative way to covert the kinetic energy to radiation. The basic idea was the collision-less shocks accelerate electrons and produce magnetic fields.
The electron moving within these magnetic fields produce the observed Gamma-rays via the synchrotron process. After that on Rees and Meszaros have shown that the continuous interaction of the relativistic outflow with the surrounding matter produce a long lasting emission an afterglow. This afterglow is weaker and at lower wavelength (X-rays, optical and then radio) but it can last days, weeks or even months after the burst. Tsvi Piran and his student Re’em Sari, they proceeded to show that the prompt Gamma-ray emission couldn’t arise from external shocks between the relativistic outflow and the surrounding material.
Internal shocks within the outflow remained the only viable way to produce the prompt emission. As internal shocks can not exhaust all the energy of the outflow some is inevitably left for later external shocks, and those produce an afterglow. This concept has led to the commonly accepted internal-external shocks model according to which the prompt GRB is produced via internal shocks while external shocks produce later the afterglow.

Figure 6.3: The internal-external shocks model. A compact source produces a relativistic outflow produce the prompt Gamma-ray emission while external shocks with the surrounding matter produce the lower energy and longer lasting afterglow.

Figure 6.3: The internal-external shocks model. A compact source produces a relativistic outflow produce the prompt Gamma-ray emission while external shocks with the surrounding matter produce the lower energy and longer lasting afterglow.