1 The "equipartition" scenario

The main radiation mechanism assumed to give rise to both the burst event and the afterglow is synchrotron emission, by electrons of random Lorentz factor $\gamma\sim m_{\rm p}/m_{\rm e}$, emitting in a region at a distance $R\sim10^{13}$ cm from the origin (e.g. [Rees & Mészáros 1994]). At these distances the typical scattering optical depth of the entire shell, of width $\Delta R^\prime$, is $\tau_{\rm T}\sim 1$, and the equipartition magnetic field is $B\sim 10^5$ G.

This picture implicitly requires that:

1) The acceleration of the electrons is impulsive (i.e. on timescales much shorter than the cooling timescales), in order for their final random energy to be controlled by the equipartition condition.

2) The inverse Compton power must be at most of the same order of the synchrotron one. This requires that only a small fraction of the electrons are, at any moment, relativistic, to avoid excessive inverse Compton emission: only a very small part of shell volume must be active.

3) There cannot be copious pair production: if the density of electrons is increased by e$^{\pm}$ pairs, the mean energy per lepton is less than $m_{\rm p}/m_{\rm e}$.

We consider these constraints quite demanding. The emitting region cannot be very compact, not to have a significant amount of pair production and it must have a very narrow width, to avoid to emit too much by the inverse Compton process. A more detailed discussion of the ideas contained in this paper can be found in [Ghisellini & Celotti (1999)].