CONFIRMATION OF X-RAY ABSORPTION BY WARM-HOT INTERGALACTIC MEDIUM IN THE SCULPTOR WALL

Abstract

In a previous paper we reported a 3-sigma detection of an absorption line\nfrom the Warm-Hot Intergalactic Medium (WHIM) using the Chandra and XMM X-ray\ngrating spectra of the blazar H2356-309, the sight-line of which intercepts the\nSculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify\nour initial detection, we obtained a deep (500 ks), follow-up exposure of\nH2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint\nanalysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect\nthe redshifted OVII WHIM line at a significance level of 3.4-sigma, a\nsubstantial improvement over the 1.7-sigma level reported previously when using\nonly the Cycle-8 data. The significance increases to 4.0-sigma when the\nexisting XMM grating data are included in the analysis, thus confirming at\nhigher significance the existence of the line at the redshift of the Sculptor\nWall with an equivalent width of 28.5+/-10.5 mA (90% confidence). We obtain a\n90% lower limit on the OVII column density of 0.8 10^16 cm^-2 and a 90% upper\nlimit on the Doppler-b parameter of 460 km/s. Assuming the absorber is\nuniformly distributed throughout the ~ 15 Mpc portion of the blazar's\nsight-line that intercepts the Sculptor Wall, that the OVII column density is ~\n2 10^16 cm^-2 (corresponding to b > 150 km/s where the inferred column density\nis only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we\nestimate a baryon over-density of ~ 30 for the WHIM, which is consistent with\nthe peak of the WHIM mass fraction predicted by cosmological simulations. The\nclear detection of OVII absorption in the Sculptor Wall demonstrates the\nviability of using current observatories to study WHIM in the X-ray absorption\nspectra of blazars behind known large-scale structures.