An X-ray WHIM metal absorber from a Mpc-scale empty region of space
  We report a detection of an absorption line at ~44.8 {\AA} in a > 500 ks
Chandra HRC-S/LETG X-ray grating spectrum of the blazar H 2356-309. This line
can be identified as intervening CV-K{\alpha} absorption, at z\approx0.112,
produced by a warm (log T = 5.1 K) intergalactic absorber. The feature is
significant at a 2.9{\sigma} level (accounting for the number of independent
redshift trials). We estimate an equivalent hydrogen column density of log
N_H=19.05 (Z/Zsun)^-1 cm^-2. Unlike other previously reported FUV/X-ray metal
detections of warm-hot intergalactic medium (WHIM), this CV absorber lies in a
region with locally low galaxy density, at ~2.2 Mpc from the closest galaxy at
that redshift, and therefore is unlikely to be associated with an extended
galactic halo. We instead tentatively identify this absorber with an
intervening Warm-Hot Intergalactic Medium filament possibly permeating a
large-scale, 30 Mpc extended, structure of galaxies whose redshift centroid,
within a cylinder of 7.5 Mpc radius centered on the line of sight to H
2356-309, is marginally consistent (at a 1.8{\sigma} level) with the redshift
of the absorber.
