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

Abstract

In a series of papers, Nicastro et al. have reported the detection of z > 0O vii absorption features in the spectrum of Mrk421obtainedwiththeChandraLowEnergyTransmissionGratingSpectrometer(LETGS).Weevaluatethisresult in the context of a high-quality spectrum of the same source obtained with the Reflection Grating Spectrometer (RGS) onXMM-Newton.Thedatacompriseover955ksof usableexposuretimeandmorethan2:6 ; 10 4 countsper50m8at 21.68.Weconcentrateonthespectrallycleanregion(21:3 < k < 22:5),wheresharpfeaturesduetotheastrophysically abundant O vii may reveal an intervening, warm-hot intergalactic medium (WHIM). We do not confirm detection of any of the intervening systems claimed to date. Rather, we detect only three unsurprising, astrophysically expected features down to the log(Ni) � 14:6(3� ) sensitivity level. Each of the two purported WHIM features is rejected with a statistical confidence that exceeds that reported for its initial detection. While we cannot rule out the existence of fainter, WHIM related features in these spectra, we suggest that previous discovery claims were premature. A more recentpaperbyWilliamsetal.claimstohavedemonstratedthattheRGSdataweanalyzeheredonothavetheresolution or statistical quality required to confirm or deny the LETGS detections. We show that our analysis resolves the issues encountered by Williams et al. and recovers the full resolution and statistical quality of the RGS data. We highlight the differences between our analysis and those published by Williams et al. as this may explain our disparate conclusions. Subject headingg intergalactic medium — large-scale structure of universe — methods: data analysis — techniques: spectroscopic — telescopes — X-rays: diffuse background