Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line Detections and Identifications (Paper I)

TL;DR

This study reports the first unambiguous detection of the warm-hot intergalactic medium (WHIM) in X-ray absorption lines toward a bright blazar, demonstrating the synergy between X-ray and FUV observations for missing baryon detection.

Contribution

It presents the first high-significance detections of WHIM absorption lines in X-ray spectra, confirming the presence of missing baryons and showcasing the effectiveness of combined X-ray and FUV data analysis.

Findings

Detection of 11 possible absorption lines with 2.2-4.1 sigma significance.

Identification of three WHIM systems at specific redshifts with high statistical confidence.

Confirmation of the power of X-ray-FUV synergy in WHIM studies.

Abstract

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.