Very high energy gamma rays from gamma ray bursts

Very high energy gamma rays from gamma ray bursts are explained by the expanding fireball model. The high energy gamma ray generation efficiency, the relativistic effect, and the absorption in cosmic background radiation are taken into account. It is shown that the ejection of gamma rays with the energies higher than 10GeV may happen at the very beginning of the fireball expansion similar to 10(-5) s with a fluence up to 10(-4)-10(-6)cm(-2) arriving at the earth.