  To detect a warm-hot intergalactic medium associated with the large-scale
structure of the universe, we observed a quasar behind the Virgo cluster with
XMM-Newton. With a net exposure time of 54 ks, we marginally detected an OVIII
K alpha absorption line at 650.9 +0.8/-1.9 eV in the RGS spectra, with a
statistical confidence of 96.4 %. The observed line center energy is consistent
with the redshift of M87, and hence the absorber is associated with the Virgo
cluster. From the curve of growth, the OVIII column density was estimated to be
> 7 x 10^16 cm^-2. In the EPIC spectra, excess emission was found after
evaluating the hot intracluster medium in the Virgo cluster and various
background components. We inspected the ROSAT All-Sky Survey map of the diffuse
soft X-ray background, and confirmed that the level of the north and west
regions just outside of the Virgo cluster is consistent with the background
model that we used, while that of the east side is significantly higher and the
enhancement is comparable with the excess emission found in the EPIC data. We
consider a significant portion of the excess emission to be associated with the
Virgo cluster, although a possible contribution from the North Polar Spur
cannot be excluded. Using the column density and the emission measure and
assuming an oxygen abundance of 0.1 and an ionization fraction of 0.4, we
estimate that the mean electron density and the line-of-sight distance of the
warm-hot gas are < 6 x 10^-5 cm^-3 and > 9 Mpc, respectively. These numbers
strongly suggest that we have detected a warm-hot intergalactic medium in a
filament associated with the Virgo cluster.
