Soft X-ray absorption excess in gamma-ray burst afterglow spectra:
  Absorption by turbulent ISM
  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 per cubic centimeters) 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 and are 2-3 orders of magnitude lower in comparison to the
GRB afterglow spectra. For those GRBs with a larger soft X-ray excess of 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.
