Discovery of a multiphase OVI and OVII absorber in the circumgalactic/intergalactic transition region

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.