Confirmation of X-Ray Absorption by WHIM in the Sculptor Wall
  In a previous paper we reported a 3-sigma detection of an absorption line
from the Warm-Hot Intergalactic Medium (WHIM) using the Chandra and XMM X-ray
grating spectra of the blazar H2356-309, the sight-line of which intercepts the
Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify
our initial detection, we obtained a deep (500 ks), follow-up exposure of
H2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint
analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect
the redshifted OVII WHIM line at a significance level of 3.4-sigma, a
substantial improvement over the 1.7-sigma level reported previously when using
only the Cycle-8 data. The significance increases to 4.0-sigma when the
existing XMM grating data are included in the analysis, thus confirming at
higher significance the existence of the line at the redshift of the Sculptor
Wall with an equivalent width of 28.5+/-10.5 mA (90% confidence). We obtain a
90% lower limit on the OVII column density of 0.8 10^16 cm^-2 and a 90% upper
limit on the Doppler-b parameter of 460 km/s. Assuming the absorber is
uniformly distributed throughout the ~ 15 Mpc portion of the blazar's
sight-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 density
is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we
estimate a baryon over-density of ~ 30 for the WHIM, which is consistent with
the peak of the WHIM mass fraction predicted by cosmological simulations. The
clear detection of OVII absorption in the Sculptor Wall demonstrates the
viability of using current observatories to study WHIM in the X-ray absorption
spectra of blazars behind known large-scale structures.
