The missing gas problem in GRB host galaxies: evidence for a highly ionised component

TL;DR

This study investigates the discrepancy between X-ray and optical measurements of gas in GRB host galaxies, suggesting the presence of highly ionised, dense gas near the GRB as the cause of the missing gas problem.

Contribution

It provides evidence that most of the gas along GRB sight lines is highly ionised and not detectable via optical absorption, explaining the X-ray absorption excess.

Findings

Neutral gas is less than 10% of total gas in GRB hosts.

Most X-ray absorbing gas is ultra-highly-ionised and undetectable in UV lines.

The ionised gas is likely dense and located near the GRB.

Abstract

There is considerable discrepancy between the amount of X-ray absorption and that inferred from optical (rest frame UV) as measured along gamma-ray burst (GRB) sight lines, with the former being typically an order of magnitude higher than what would be expected from the measurement of neutral element species via optical absorption line spectroscopy. We explore this "missing gas problem" by using X-ray and optical measurements in a sample of 29 z=0.7-6.3 GRBs from both spectroscopic data and the afterglow broadband spectral energy distributions. The low ionisation species detected in the UV are associated with the neutral interstellar medium in the GRB host galaxy, while soft X-ray absorption, which is weakly dependent on the ionisation state of the gas, provides a probe of the total column of gas along the sight line. After careful consideration of any systematic effects, we find that the neutral gas consists of less than ~10% of the total gas, and this limit decreases with the more ionised that the X-ray absorbing gas is, which in our spectral fits is assumed to be neutral. Only a very small fraction of this ionised gas, however, is detected in UV absorption lines with ionisation potentials up to ~200eV (i.e. SiIV, CIV, NV, OVI), which leaves us to postulate that the X-ray excess is due to ultra-highly-ionised, dense gas in the GRB vicinity.