A possible Chandra and Hubble Space Telescope detection of extragalactic
  WHIM towards PG 1116+215
  (Abridged) We have analyzed Chandra LETG and XMM-Newton RGS spectra towards
the z=0.177 quasar PG 1116+215, a sightline that is rendered particularly
interesting by the HST detection of several OVI and HI broad Lyman-alpha
absorption lines that may be associated with the warm-hot intergalactic medium.
We performed a search for resonance K-alpha absorption lines from OVII and
OVIII at the redshifts of the detected far-ultraviolet lines. We detected an
absorption line in the Chandra spectra at 5.2 sigma confidence level at
wavelengths corresponding to OVIII K-alpha at z=0.0911+-0.0004+-0.0005
(statistical followed by systematic error). This redshift is within 3 sigma of
that of a HI broad Lyman-alpha of b=130 km/s at z=0.09279+-0.00005. We have
also analyzed the available XMM-Newton RGS data towards PG 1116+215.
Unfortunately, the XMM-Newton data are not suitable to investigate this line
because of instrumental features at the wavelengths of interest. At the same
redshift, the Chandra and XMM-Newton spectra have OVII K-alpha absorption line
features of significance 1.5 sigma and 1.8 sigma, respectively. We also
analyzed the available SDSS spectroscopic galaxy survey data towards PG
1116+215 in the redshift range of interest. We found evidence for a galaxy
filament that intersects the PG 1116+215 sightline and additional galaxy
structures that may host WHIM. The combination of HST, Chandra, XMM-Newton and
SDSS data indicates that we have likely detected a multi-temperature WHIM at
z=0.091-0.093 towards PG 1116+215.
