| Abstract
| - The emission features observed in the X-ray afterglow of Gamma Ray Bursts are extremely powerful. Since they last at least for several hours, they imply energies of the order of 10 49 ergs. This in turn implies that the energy contained in the illuminating continuum thought to be responsible of the line production must exceed 10 51 ergs. This is a strong lower limit to the energy reservoir of Gamma Ray Bursts, which is independent of collimation and beaming, and bears important consequences on the possible collimation of the fireball radiation and the density of the medium surrounding the burst.
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