4 Conclusions

In the equipartition synchrotron scenario, the equality between the energy density of protons, leptons and magnetic field leads to a remarkable good agreement with the observed spectra. For this to be achieved the acceleration of electrons has to be impulsive, take place in a very limited volume of the interacting shell, and the e$^{\pm}$density has to be small enough, not to lower the mean lepton energy.

At the other extreme, when the particle acceleration occupies the entire shell volume and/or lasts for a shell light crossing time, the mean lepton energy is controlled by the balance between the heating and the cooling rate. This leads to mildly or sub relativistic lepton energies: the synchrotron emission is inhibited by self-absorption, and provides soft photons for the dominant inverse Compton scattering. The ratio of the observed multiple Compton scattering and the self-absorbed synchrotron luminosities is $\sim 10^5$, and determines the required Comptonization y parameter. As long as the compactness of the emission region is large, relativistic temperatures cannot be achieved, because in this case e$^{\pm}$ are copiously produced, increasing the optical depth and decreasing the temperature. If, on the other hand, the temperature is low and the optical depth is large, photons are trapped inside the shell, and part of the luminosity is used to expand it. As a result, the observed radiation is formed when the system adjusts to have $\tau_{\rm T}\sim$ a few and $kT\sim 50$ keV. This may be why GRBs are observed to emit MeV photons.