generated at the wind front is very high, $\langle
\tilde{\varepsilon}_\gamma\rangle\gtrsim 100$ MeV. This is because the bulk of these photons is generated in the thin vicinity of the wind front where there are both a strong magnetic field, $B\simeq B_0$, and high-energy electrons. The mean time that high-energy electrons spend in this vicinity and generate synchrotron radiation is very short, $\sim$ a few $\times T_p\sim
10^{-6}(B_{\rm dec}/10^5\,\,{\rm G})^{-1}(\Gamma_0/10^2)$ s. To get a high efficiency of synchrotron radiation at the wind front, $\tilde{\xi}_\gamma > 10^{-2}$, it is necessary to assume that the magnetic field $B_{\rm dec}$ is about its maximum value (see § 2). From equation (\[epsLab\]), we can see that in this case the mean energy of synchrotron photons is of the order of or higher than $10^2$ MeV.

For a typical value of $B_{\rm dec}$, that is $\sim 10^5$ G (see § 2), and $\Gamma_0\sim 10^2-10^3$, from equation (\[ksiLab\]) we have $\tilde \xi_\gamma\sim 10^{-3}-10^{-2}\ll 1$. Hence, high-energy electrons are accelerated at the wind front and injected into the region ahead of the front practically without energy losses. In our model, these high-energy electrons are the best candidates to be responsible for the X-ray and $\gamma$-ray emission of GRBs in 