Based on constraints from Big Bang nucleosynthesis and the cosmic microwave background, the baryon content of the high-redshift Universe can be precisely determined. However, at low redshift, about one-third of the baryons remain unaccounted for, which poses the long-standing missing baryon problem. The missing baryons are believed to reside in large-scale filaments in the form of warm-hot intergalactic medium (WHIM). In this work, we employ a novel stacking approach to explore the hot phases of the WHIM. Specifically, we utilize the 470 ks Chandra LETG data of the luminous quasar, H1821+643, along with previous measurements of UV absorption line systems and spectroscopic redshift measurements of galaxies toward the quasars sightline. We repeatedly blueshift and stack the X-ray spectrum of the quasar corresponding to the redshifts of the 17 absorption line systems. Thus, we obtain a stacked spectrum with $8.0$ Ms total exposure, which allows us to probe X-ray absorption lines with unparalleled sensitivity. Based on the stacked data, we detect an OVII absorption line that exhibits a Gaussian line profile and is statistically significant at the $3.3 sigma$ level. Since the redshifts of the UV absorption line systems were known a priori, this is the first definitive detection of an X-ray absorption line originating from the WHIM. The equivalent width of the OVII line is $(4.1pm1.3) mathrm{mAA}$, which corresponds to an OVII column density of $(1.4pm0.4)times10^{15} mathrm{cm^{-2}}$. We constrain the absorbing gas to have a density of $n_{rm H} = (1-2)times10^{-6} rm{cm^{-3}}$ for a single WHIM filament. We derive $Omega_{rm b} rm(O,VII) = (0.0023 pm 0.0007) , left[ f_{O,VII} , {Z/Z_{odot}} right]^{-1}$ for the cosmological mass density of OVII, assuming that all 17 systems contribute equally.
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