  The observed intergalactic OVI absorbers at z>0 have been regarded as a
significant reservoir of the ``missing baryons''. However, to fully understand
how these absorbers contribute to the baryon inventory, it is crucial to
determine whether the systems are collisionally ionized or photoionized (or
both). Using the identified intergalactic OVI absorbers as tracers, we search
for the corresponding X-ray absorption lines, which are useful for finding the
missing baryons and for revealing the nature of the OVI absorbers. Stacking the
Chandra grating spectra along six AGN sight lines, we obtain three spectra with
signal-to-noise ratios of 32, 28, and 10 per 12.5 mA spectral bin around the
expected OVII Kalpha wavelength. These spectra correspond to OVI absorbers with
various dynamic properties. We find no detectable NeIX, OVII, OVIII, NVII, or
CVI absorption lines in the spectra, but the high counting statistics allows us
to obtain firm upper limits on the corresponding ionic column densities (in
particular N(OVII)<=10 N(OVI) on average at the 95% confidence level). Jointly
analyzing these non-detected X-ray lines with the averaged OVI column density,
we further limit the average temperature of the OVI-bearing gas to be
log[T(K)]<=5.7 in collisional ionization equilibrium. We discuss the
implications of these results for physical properties of the putative warm-hot
intergalactic medium and its detection in future X-ray observations.
