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$\beta$
(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$\sigma$ (2.7$\sigma$ and 1.2$\sigma$ 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$\alpha$ 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$\beta$ 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$\sigma$ (3.7$\sigma$ if systematics are
considered), and thus the combined (HI+OVIII) statistical significance of the
detection is of 5.0$\sigma$.
  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.
