The UV spectra of Galactic and extragalactic sightlines often show O vi absorption lines at a range of redshifts , and from a variety of sources from the Galactic circumgalactic medium to AGN outflows .
Inner shell O vi absorption is also observed in X-ray spectra ( at \lambda = 22.03 Å ) , but the column density inferred from the X-ray line was consistently larger than that from the UV line .
Here we present a solution to this discrepancy for the z = 0 systems .
The O ii K \beta line ^ { 4 } S ^ { 0 } \rightarrow ( ^ { 3 } D ) 3 p ^ { 4 } P at 562.40 eV ( \equiv 22.04Å ) is blended with the O vi K \alpha line in X-ray spectra .
We estimate the strength of this O ii line in two different ways and show that in most cases the O ii line accounts for the entire blended line .
The small amount of O vi equivalent width present in some cases has column density entirely consistent with the UV value .
This solution to the O vi discrepancy , however , does not apply to the high column density systems like AGN outflows .
We discuss other possible causes to explain their UV/X-ray mismatch .
The O vi and O ii lines will be resolved by gratings on-board the proposed mission Arcus and the concept mission Lynx and would allow detection of weak O vi lines not just at z = 0 but also at higher redshift .