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XMM-{\em Newton} and FUSE Tentative Evidence for a WHIM filament along the Line of Sight to PKS~0558-504

We present a possible OVIII X-ray absorption line at $z=0.117 \pm 0.001$ which, if confirmed, will be the first one associated with a broad HI Ly$β$ (BLB: FWHM=$160^{+50}_{-30}$ km s$^{-1}$) absorber. The absorber lies along the line of sight to the nearby ($z=0.1372$) Seyfert 1 galaxy PKS~0558-504, consistent with being a WHIM filament. The X-ray absorber is marginally detected in two independent XMM-Newton spectra of PKS~0558-504, a long $\sim 600$ ks Guest-Observer observation and a shorter, $\sim 300$ ks total, calibration observation, with a combined single line statistical significance of 2.8$σ$ (2.7$σ$ and 1.2$σ$ in the two spectra, respectively). When fitted with our self-consistent hybrid-photoionization WHIM models, the combined XMM-{\em Newton} spectrum is consistent with the presence of OVIII K$α$ at $z=(0.117 \pm 0.001)$. This model gives best fitting temperature and equivalent H column density of the absorber of log$T=6.56_{-0.17}^{+0.19}$ K, and logN$_H=(21.5 \pm 0.3) (Z/Z_{0.01\odot})^{-1}$ cm$^{-2}$. The statistical sigificance of this single X-ray detection is increased by the detection of broad and complex HI Ly$β$ absorption in archival FUSE spectra of PKS~0558-504, at redshifts $z=0.1183 \pm 0.0001$ consistent with the best-fitting redshift of the X-ray absorber. The single line statistical significance of this line is 4.1$σ$ (3.7$σ$ if systematics are considered), and thus the combined (HI+OVIII) statistical significance of the detection is of 5.0$σ$. The detection of both metal and H lines at a consistent redshift, in this hot absorbing system, allows us to speculate on its metallicity. By associating the bulk of the X-ray absorber with the BLB line detected in the FUSE spectrum at $z_{BLB}=0.1183 \pm 0.0001$, we obtain a metallicity of 1-4\% Solar.