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Two-thirds of long duration gamma-ray bursts (GRBs) show soft X-rayabsorption in excess of the Milky Way. The column densities of metals inferredfrom UV and optical spectra differ from those derived from soft X-ray spectra,at times by an order of magnitude, with the latter being higher. The origin ofthe soft X-ray absorption excess observed in GRB X-ray afterglow spectraremains a heavily debated issue, which has resulted in numerous investigationson the effect of hot material both internal and external to the GRB host galaxyon our X-ray afterglow observations. Nevertheless, all models proposed so farhave either only been able to account for a subset of our observations (i.e. atz > 2), or they have required fairly extreme conditions to be present withinthe absorbing material. In this paper, we investigate the absorption of the GRBafterglow by a collisionally ionised and turbulent interstellar medium (ISM).We find that a dense (3 per cubic centimeters) collisionally ionised ISM couldproduce UV/optical and soft X-ray absorbing column densities that differ by afactor of 10, however the UV/optical and soft X-ray absorbing column densitiesfor such sightlines and are 2-3 orders of magnitude lower in comparison to theGRB afterglow spectra. For those GRBs with a larger soft X-ray excess of up toan order of magnitude, the contribution in absorption from a turbulent ISM asconsidered here would ease the required conditions of additional absorbingcomponents, such as the GRB circumburst medium and intergalactic medium.

Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM