  The observational constraints on the baryon content of the WHIM rely almost
entirely on FUV measurements. However, cosmological, hydrodynamical simulations
predict strong correlations between the spatial distributions of FUV and X-ray
absorbing WHIM. In this work we investigate this prediction by analyzing
XMM-Newton X-ray counterparts of FUV-detected intergalactic OVI absorbers known
from FUSE and HST/STIS data, and compare this information to the predictions of
simulations. We study the X-ray absorption at the redshift of the only
significantly detected OVI absorber in the TonS180 sightline's FUV spectrum,
found at $z=0.04579\pm0.00001$. We characterize the spectral properties of the
OVI-OVIII absorbers and explore the ionization processes behind the measured
absorption. The observational results are compared to the predicted warm-hot
gas properties in the EAGLE simulation to infer the physical conditions of the
absorber. We detect both OVI and OVII absorption at a $5\sigma$ confidence
level, whereas OVIII absorption is not detected. CIE modeling constrains the
X-ray absorbing gas temperature to log$\,T_{CIE}$(K)$=6.22\pm0.05$ with a total
hydrogen column density $N_H=5.8_{-2.2}^{+3.0}\times
Z_{sun}/Z_{abs}\times10^{19}$ cm$^{-2}$. This model predicts an OVI column
density consistent with that measured in the FUV, but our limits on the OVI
line width indicate >90 % likelihood that the FUV-detected OVI arises from a
different, cooler phase. We find that the observed absorber lies about a factor
of two further away from the detected galaxies than is the case for similar
systems in EAGLE. Understanding the abundance of the systems similar to the one
considered in this work helps to define the landscape for WHIM searches with
future X-ray telescopes.
