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In a previous paper we reported a 3-sigma detection of an absorption linefrom the Warm-Hot Intergalactic Medium (WHIM) using the Chandra and XMM X-raygrating spectra of the blazar H2356-309, the sight-line of which intercepts theSculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verifyour initial detection, we obtained a deep (500 ks), follow-up exposure ofH2356-309 as part of the Cycle-10 Chandra Large Project Program. From a jointanalysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detectthe redshifted OVII WHIM line at a significance level of 3.4-sigma, asubstantial improvement over the 1.7-sigma level reported previously when usingonly the Cycle-8 data. The significance increases to 4.0-sigma when theexisting XMM grating data are included in the analysis, thus confirming athigher significance the existence of the line at the redshift of the SculptorWall with an equivalent width of 28.5+/-10.5 mA (90% confidence). We obtain a90% lower limit on the OVII column density of 0.8 10^16 cm^-2 and a 90% upperlimit on the Doppler-b parameter of 460 km/s. Assuming the absorber isuniformly distributed throughout the ~ 15 Mpc portion of the blazar'ssight-line that intercepts the Sculptor Wall, that the OVII column density is ~2 10^16 cm^-2 (corresponding to b > 150 km/s where the inferred column densityis only weakly dependent on b), and that the oxygen abundance is 0.1 solar, weestimate a baryon over-density of ~ 30 for the WHIM, which is consistent withthe peak of the WHIM mass fraction predicted by cosmological simulations. Theclear detection of OVII absorption in the Sculptor Wall demonstrates theviability of using current observatories to study WHIM in the X-ray absorptionspectra of blazars behind known large-scale structures.

Confirmation of X-Ray Absorption by WHIM in the Sculptor Wall