摘要：The cosmological missing baryons atz < 1 most likely hide in the hot (T ≳ 105.5K) phase of the warm hot intergalactic medium (WHIM). While the hot WHIM is hard to detect due to its high ionisation level, the warm (T ≲ 105.5K) phase of the WHIM has been very robustly detected in the far-ultraviolet (FUV) band. We adopted the assumption that the hot and warm WHIM phases are co-located and therefore used the FUV-detected warm WHIM as a tracer for the cosmologically interesting hot WHIM. We performed an X-ray follow-up in the sight line of the blazar PKS 2155–304 at the redshifts where previous FUV measurements of O VI, SiIV,and broad Lyman-alpha (BLA) absorption have indicated the existence of the warm WHIM. We looked for the O VIIHeαand O VIIILyαabsorption lines, the most likely hot WHIM tracers. Despite the very large exposure time (≈1 Ms), the Reflection Grating Spectrometer unit 1 (RGS1) on-boardXMM-Newtondata yielded no significant detection which corresponds to upper limits of logN(OVII(cm−2)) ≤ 14.5−15.2 and logN(OVIII(cm−2)) ≤ 14.5−15.2. An analysis of the data obtained with the combination of the Low Energy Transmission Grating (LETG) and the High Resolution Camera (HRC) on-boardChandrayielded consistent results. However, the data obtained with the LETG, combined with the Advanced CCD Imaging Spectrometer (ACIS) lead to the detection of an feature resembling an absorption line atλ ≈ 20 Å at simple one-parameter confidence level of 3.7σ, consistent with several earlier LETG/ACIS reports. Given the high statistical quality of the RGS1 data, the possibility of RGS1 accidentally missing the true line atλ ∼ 20 Å is very low: 0.006%. Neglecting this, the LETG/ACIS detection can be interpreted as Lyαtransition of O VIIIat one of the redshifts (z ≈ 0.054) of FUV-detected warm WHIM. Given the very convincing X-ray spectral evidence for and against the existence of theλ ∼ 20 Å feature, we cannot conclude whether or not it is a true astrophysical absorption line. Considering cosmological simulations, the probability of the LETG/ACISλ ∼ 20 Å feature being due to the astrophysical O VIIIabsorber co-located with the FUV-detected O VIabsorber is at the very low level of ≲0.1%. We cannot completely rule out the very unlikely possibility that the LETG/ACIS 20 Å feature is due to a transient event located close to the blazar.
