2 Discussion

Oxygen and iron edges are the most prominent absorption features in the spectra of X-ray sources. This individuates two almost distinct accessible part of the redshift-column density plane for GRB: one characterized by a moderate $N_{\rm H}\sim 10^{21}-10^{22}$ cm^-2 and $z\sim 0.1-0.5$ and the other one by a column larger than 10²³ cm^-2. Note that for a standard dust to gas ratio, a column of $N_{\rm H} =10^{22}$cm^-2 corresponds to an optical extinction $A_V\sim 6$ mag, precluding the possibility to detect the optical afterglow. If, furthermore, it were not possible to perform spectroscopic optical observation of the host galaxy (either because a precise position is lacking, or because it is too faint), then the X-rays could be the only mean to determine the redshift, especially for strong bursts located at z > 2-3, for which the iron edge lies in the most sensitive energy range of X-ray detectors.