Other
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006head....9.1850k&link_type=abstract
American Astronomical Society, HEAD meeting #9, #18.50; Bulletin of the American Astronomical Society, Vol. 38, p.390
Other
Scientific paper
In a series of papers, the first detection of highly ionized gas associated with two Warm-Hot Intergalactic Medium (WHIM) filaments has been reported by Nicastro et al. (2005). The evidence is based on absorption lines from O VII and other ions towards the bright blazar Mrk 421, measured by the Chandra/LETGS. This result was used to argue that the missing 45% of all baryons have at last been found and are located in the WHIM X-ray forest. We investigate the robustness of this detection by a re-analysis of the original LETGS data, using also more recent LETGS spectra and a thorough analysis of >955 ks of XMM-Newton/RGS data on Mrk 421. We confirm the apparent strength of several features in the original Chandra spectra, but demonstrate that they are statistically not significant. This is due to the number of redshift trials that are made in the search for features and that are not taken into account in the original paper. Moreover, both the RGS data that have a 30% higher sensitivity to line detections than the original LETGS observations as well as the new LETGS spectra do not show any significant feature at the relevant wavelengths. Our careful analysis of the RGS spectra shows that RGS has similar sensitivity to find weak absorption features as LETGS. Finally, we show that a possible association of one of the proposed filaments with a Lyman alpha absorption system lacks sufficient statistical evidence. We conclude that there is insufficient proof for the existence of the proposed WHIM filaments towards Mrk 421, the brightest blazar. Therefore, the highly ionized component of the WHIM still remains to be discovered. Moreover, unless redshifts are known a priori, the unambiguous proof for X-ray absorption lines towards Mrk 421 or any other source is harder than previously thought.
de Plaa Jelle
de Vries Cor P.
den Herder J. A.
Kaastra Jelle S.
Kahn Steve M.
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