Volume 595, November 2016
|Number of page(s)||10|
|Published online||24 October 2016|
Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM
1 Max-Planck-Institut für
Extraterrestrische Physik, PO Box
1312, Giessenbachstr. 1, 85741 Garching, Germany
2 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
3 Universitäts-Sternwarte München, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
4 Excellence Cluster Universe, Technische Universität München, Boltzmannstrasse 2, 85748 Garching, Germany
5 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
6 Physics Dept., University of Calabria, via P. Bucci, 87036 Arcavacata di Rende, Italy
7 Physikalisches Institut, Universität Köln, Zülpicher Strasse 77, 50937 Köln, Germany
Accepted: 8 February 2016
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.
Key words: gamma-ray burst: general / X-rays: ISM / ultraviolet: ISM / galaxies: irregular
© ESO 2016
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