摘要：Two-thirds of long duration gamma-ray bursts (GRBs) show soft X-ray absorption in excess
of the Milky Way. The column densities of metals inferred from 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 of the soft X-ray absorption excess observed in GRB
X-ray afterglow spectra remains a heavily debated issue, which has resulted in numerous
investigations on the effect of hot material both internal and external to the GRB host
galaxy on our X-ray afterglow observations. Nevertheless, all models proposed so far have
either only been able to account for a subset of our observations (i.e. at z> 2),
or they have required fairly extreme conditions to be present within the absorbing
material. In this paper, we investigate the absorption of the GRB afterglow by a
collisionally ionised and turbulent interstellar medium (ISM). We find that a dense (3
cm-3)
collisionally ionised ISM could produce UV/optical and soft X-ray absorbing column
densities that differ by a factor of 10. However the UV/optical and soft X-ray absorbing
column densities for such sightlines are 2−3 orders of magnitude lower in comparison to the GRB afterglow
spectra. For those GRBs with a larger soft X-ray excess by up to an order of magnitude,
the contribution in absorption from a turbulent ISM as considered here would ease the
required conditions of additional absorbing components, such as the GRB circumburst medium
and intergalactic medium.
