Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line Detections and Identifications (Paper I). (arXiv:1210.7177v1 [astro-ph.CO])

Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line Detections and Identifications (Paper I). (arXiv:1210.7177v1 [astro-ph.CO]):
About 30-40 percent of the expected number of baryons is still missing in the
local Universe (z \lesssim 0.4). They are predicted to be hiding in a web of
intergalactic gas at temperatures of about 10^5-10^7 K (the WHIM). Detecting
this matter has had limited success so far, because of its low-density and high
temperature, which makes it difficult to detect with current far-ultraviolet
and X-ray instrumentation.

Here we present the first results from our pilot 500 ks Chandra-LETG
observation of the soft X-ray brightest source in the z > 0.4 sky, the blazar
1ES 1553+113. We identify a total of 11 possible absorption lines, with
single-line statistical significances between 2.2-4.1 sigma. Six of these lines
are detected at high significance (3.6 < \sigma < 4.1), while the remaining
five are regarded as marginal detections in association with either other X-ray
lines detected at higher significance and/or FUV signposts. Three of these
lines are consistent with metal absorption at z~0. The remaining 8 lines may be
imprinted by intervening absorbers and are all consistent with being
high-ionization counterparts of FUV HI and/or OVI IGM signposts. In particular,
four of these eight absorption lines (4.1\sigma, 4.1\sigma, 3.8\sigma and
2.7\sigma), are identified as CV and CVI absorbers belonging to two WHIM
systems at z_X = 0.312 and z_X = 0.133, which also produce broad HI and OVI
absorption in the FUV. The true statistical significances of these two X-ray
absorption systems, after properly accounting for the number of redshift
trials, are 5.8\sigma and 3.8\sigma.