Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line
  Detections and Identifications (Paper I)
  We present the first results from our pilot 500 ks Chandra-LETG Large Program
observation of the soft X-ray brightest source in the z>=0.4 sky, the blazar
1ES 1553+113, aimed to secure the first uncontroversial detections of the
missing baryons in the X-rays.
  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 single-line statistical 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
Far-Ultraviolet (FUV) signposts. In particular, five of these possible
intervening absorption lines, are identified as CV and CVI K\alpha absorbers
belonging to three WHIM systems at z_X = 0.312, z_X = 0.237 and <z_X> = 0.133,
which also produce broad HI (and OVI for the z_X = 0.312 system) absorption in
the FUV. For two of these systems (z_X = 0.312 and 0.237), the Chandra X-ray
data led the a-posteriori discovery of physically consistent broad HI
associations in the FUV, so confirming the power of the X-ray-FUV synergy for
WHIM studies. The true statistical significances of these three X-ray
absorption systems, after properly accounting for the number of redshift
trials, are 5.8 sigma (z_X = 0.312; 6.3 sigma if the low-significance OV and CV
K-beta associations are considered), 3.9 sigma (z_X = 0.237), and 3.8 sigma
(\langle z_X \rangle = 0.133), respectively.
