On the discrepancy between the X-ray and UV absorption measurements of O VI in the local ISM
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 VIKα 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 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.
Additional Information© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2018 November 23. Received 2018 November 22; in original form 2018 October 8. Published: 27 November 2018. This research was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG) cluster of excellence 'Origin and Structure of the Universe'. E.G. thank Amit Pathak, Ken Sembach, and Bart Wakker for helpful comments on the UV absorption line measurements. J.A.G. acknowledges support from the National Aeronautics and Space Administration (NASA) grant 80NSSC17K0345 and from the Alexander von Humboldt Foundation.
Published - sly223.pdf
Accepted Version - 1811.09280.pdf