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In a series of papers, Nicastro et al. have reported the detection of z>0OVII absorption features in the spectrum of Mrk421 obtained with the ChandraLow Energy Transmission Grating Spectrometer (LETGS). We evaluate this resultin the context of a high quality spectrum of the same source obtained with theReflection Grating Spectrometer (RGS) on XMM-Newton. The data comprise over955ks of usable exposure time and more than 26000 counts per 50 milliAngstromat 21.6 Angstroms. We concentrate on the spectrally clean region (21.3 < lambda< 22.5 Angstroms) where sharp features due to the astrophysically abundant OVIImay reveal an intervening, warm--hot intergalactic medium (WHIM). We do notconfirm detection of any of the intervening systems claimed to date. Rather, wedetect only three unsurprising, astrophysically expected features down to theLog(N_i)~14.6 (3 sigma) sensitivity level. Each of the two purported WHIMfeatures is rejected with a statistical confidence that exceeds that reportedfor its initial detection. While we can not rule out the existence of fainter,WHIM related features in these spectra, we suggest that previous discoveryclaims were premature. A more recent paper by Williams et al. claims to havedemonstrated that the RGS data we analyze here do not have the resolution orstatistical quality required to confirm or deny the LETGS detections. We showthat our careful analysis resolves the issues encountered by Williams et al.and recovers the full resolution and statistical quality of the RGS data. Wehighlight the differences between our analysis and those published by Williamset al. as this may explain our disparate conclusions.Comment: 19 pages/7 figures/4 tables. 060424 submitted to ApJ 060522  re-submitted following ApJ reques

On the Putative Detection ofz> 0 X‐Ray Absorption Features in the Spectrum of Mrk 421