On the discrepancy between the X-ray and UV absorption measurements of O vi in the local ISM

Abstract

ABSTRACT The total amount of O vi present in the interstellar medium (ISM) obtained via absorption measurements in UV and X-ray spectra is currently in disagreement, with the latter being significantly larger (by a factor of 10 or more) than the former. Previous works have proposed that the blend of the O vi Kα line (22.032 Å) with the O ii Kβ-L12 line (22.04 Å) could account for the stronger absorption observed in the X-ray spectra. Here, we present a detailed study of the oxygen absorption in the local ISM, implementing our new model IGMabs which includes photoabsorption cross-sections of highly ionized species of abundant elements as well as turbulence broadening. By analysing high-resolution Chandra spectra of 13 low-mass X-ray binaries (LMXBs) and 29 extragalactic sources, we have estimated the column densities of O i – O iii and from O vi – O viii along multiple line-of-sights. We find that in most cases the O ii Kβ-L12 line accounts for <30${{\ \rm per\ cent}}$ of the total O vi K α + O ii K β. We conclude that the amount of O ii predicted by our model is still insufficient to explain the discrepancy between X-ray and UV measurements of O vi column densities.