F. Nicastro, M. Elvis, Y. Krongold, S. Mathur, A. Gupta, C. Danforth, X. Barcons, S. Borgani, E. Branchini, R. Cen, R. Davé, J. Kaastra, F. Paerels, L. Piro, J. M. Shull, Y. Takei, L. Zappacosta
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.
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http://arxiv.org/abs/1210.7177
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