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 O \emissiontype VIII 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 O \emissiontype VIII column density was estimated to be \gtrsim 7 \times 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 can not 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 \lesssim 6 \times 10 ^ { -5 } cm ^ { -3 } and \gtrsim 9 Mpc , respectively .
These numbers strongly suggest that we have detected a warm-hot intergalactic medium in a filament associated with the Virgo cluster .