The effect of an intrinsic column on GRB spectra

We present results of a study to ascertain if material surrounding a GRB site would prevent a measurement of the neutral hydrogen column density to the site and thus an estimate of the distance scale. Using a 1‐D photo‐ionization structure model in conjunction with simple input spectra, it is found that unionized matter densities near the burst site cannot exceed ∼2×1022 cm−2 otherwise the resulting roll‐over in the spectrum would have been observed. Furthermore, by varying the matter density and its ionization state, we find that the spectrum in the ∼0.5 to few keV region experiences considerable distortion long before the cut‐off due to neutral hydrogen is affected, no matter how low the column to the source is. When viewed in the light of existing measurements, the data indicate that the column to the source can be derived for any scenario.