CHANDRA VIEW OF THE WARM-HOT INTERGALACTIC MEDIUM TOWARD 1ES 1553+113: ABSORPTION-LINE DETECTIONS AND IDENTIFICATIONS. I.

We present the first results from our pilot 500 ks Chandra Low Energy Transmission Grating Large Program observation of the soft X-ray brightest source in the z greater than or similar to 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 sigma and 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. Three of these lines are consistent with metal absorption at z similar or equal to 0, and we identify them with Galactic OI and CII. The remaining eight lines may be imprinted by intervening absorbers and are all consistent with being high-ionization counterparts of FUV Hi and/or OVI intergalactic medium signposts. In particular, five of these eight possible intervening absorption lines (single-line statistical significances of 4.1 sigma, 4.1 sigma, 3.9 sigma, 3.8 sigma, and 2.7 sigma), are identified as Cv and C vi K sigma 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 (for the third system the opposite applies), 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 (< z(X)> = 0.133), respectively.