  We present a high resolution (R~500) X-ray spectrum of the bright quasar
H1821+643 (z=0.297), obtained in a 470 ksec Chandra observation. We search for
X-ray absorption by highly ionized metal species, OVII and OVIII in particular,
at the redshifts of the six intervening OVI absorption systems known from UV
studies. We detect features with >~2-sigma significance at the predicted OVII
and OVIII wavelengths of one OVI system, at the OVII wavelength of a second,
and at the NeIX wavelength of a third. We find two additional features of
comparable strength (one OVII and one OVIII) within 1000 km/s of OVI redshifts.
The 1-sigma constraints in the two detected OVI systems imply gas overdensities
lower than the values delta>100 expected in virialized systems, suggesting that
the absorption arises in lower density, filamentary structures. At the 2-sigma
level, however, the physical constraints are weak. If we treat our 2-sigma
detections of known OVI systems as real, but assume minimal OVII and OVIII in
the other systems, we estimate [f(OVI)+f(OVII)+f(OVIII)]/f(OVI) = 32 +/- 9 for
the average ratio of all highly ionized oxygen species to OVI. Combined with
estimates of the total column density of OVI absorption per unit redshift, this
ratio implies a total baryon fraction associated with detected OVI absorbers
Omega_b(OVI)~0.03/h_70, a substantial fraction of the baryon density predicted
by BBN, and larger than that in known low redshift components. Because of the
limited S/N of the detections, these results must be treated with caution.
Nonetheless, the combination of the OVI data with these X-ray forest
measurements provides the most direct evidence to date for the pervasive,
moderate density, shock-heated intergalactic medium predicted by leading
cosmological scenarios. (Abridged.)
