The OVI mystery: mismatch between X-ray and UV column densities
  The UV spectra of Galactic and extragalactic sightlines often show OVI
absorption lines at a range of redshifts, and from a variety of sources from
the Galactic circumgalactic medium to AGN outflows. Inner shell OVI absorption
is also observed in X-ray spectra (at lambda=22.03 AA), 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
OII K-beta line ^4S^0 --> (^3D)3p ^4P at 562.40 eV (==22.04 AA) is blended with
the OVI K-alpha line in X-ray spectra. We estimate the strength of this OII
line in two different ways and show that in most cases the OII line accounts
for the entire blended line. The small amount of OVI equivalent width present
in some cases has column density entirely consistent with the UV value. This
solution to the OVI 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 OVI and OII lines will be resolved by gratings
on-board the proposed mission Arcus and the concept mission Lynx and would
allow detection of weak OVI lines not just at z=0 but also at higher redshift.
