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
e-mail: mohit@mpe.mpg.de
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

Received: 14 December 2015
Accepted: 8 February 2016

Abstract

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.