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

TL;DR

This study investigates whether a turbulent, collisionally ionised interstellar medium can explain the soft X-ray absorption excess observed in gamma-ray burst afterglow spectra, potentially reducing the need for extreme additional absorbing components.

Contribution

It demonstrates that a dense, turbulent ISM can produce significant X-ray absorption, offering a plausible explanation for the observed excess in GRB spectra.

Findings

A dense, collisionally ionised ISM can produce a factor of 10 difference in UV/optical and X-ray absorption.

The modeled absorption levels are still 2-3 orders of magnitude lower than observed in GRB spectra.

Turbulent ISM contribution can lessen the need for extreme circumburst or intergalactic absorption models.

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 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.