The O VII X-Ray Forest Toward Markarian 421: Consistency between XMM-Newton and Chandra

Abstract

Recently, the first detections of highly ionized gas associated with two warm-hot intergalactic medium (WHIM ) filaments have been reported. The evidence is based on X-ray absorption lines due to O vii and other ions observed by Chandra toward the bright blazar Mrk 421. We investigate the robustness of this detection by a reanalysis of the original Chandra LETGS spectra, the analysis of a large set of XMM-Newton RGS spectra of Mrk 421, and additional Chandra observations. We address the reliability of individual spectral features belonging to the absorption components, and assess the significance of the detection of these components. We also use Monte Carlo simulations of spectra. We confirm the apparent strength of several features in the Chandra spectra, but demonstrate that they are statistically not significant. This decreased significance is due to the number of redshift trials that are made and that are not taken into account in the original discovery paper. Therefore, these features must be attributed to statistical fluctuations. This is confirmed by the RGS spectra, which have a higher signal-to-noise ratio than the Chandra spectra, but do not show features at the same wavelengths. Finally, we show that the possible association with a Ly absorption system also lacks sufficient statistical evidence. We conclude that there is insufficient observational proof for the existence of the two proposed WHIM filaments toward Mrk 421, the brightest X-ray blazar in the sky. Therefore, the highly ionized component of the WHIM still remains to be discovered. Subject headingg s: BL Lacertae objects: individual ( Mrk 421) -large-scale structure of universequasars: absorption lines -X-rays: ISM

The Nonisothermality and Extent of Galactic Diffuse Hot Gas toward Markarian 421

“…We find that, while our measured equivalent widths (EWs) for other oxygen lines are consistent with those reported by Williams et al (2005), the EW of O VII Kα line [11.4(10.20, 12.68) mA] is about ∼ 20% higher. Note that our value is consistent with that obtained by Kaastra et al (2006). We believe that this discrepancy is mainly due to their improper weighted instrumental response file 13 for the final co-added spectrum; the higher (> 1) order confusion of the HRC observation (if not properly being taken into account), the different placement of the continuum, and the different software packages utilized in the data analysis may also contribute.…”

Warm‐Hot Intergalactic Medium Associated with the Coma Cluster

“…It is possible to detect the average density WHIM in the X-ray spectra of extraordinarily bright background sources such as blazars, particularly when they are in outburst, (Nicastro et al 2002;Fang et al 2002;Mathur et al 2003;Fang et al 2003;Rasmussen et al 2003;McKernan et al 2004;Nicastro et al 2005). However, even the most convincing detection (Nicastro et al 2005) is controversial (Rasmussen et al 2006;Kaastra et al 2006). The best case of a bright blazar behind a cluster is so far unknown.…”

Fifteen years of experience with the Reflection Grating Spectrometer on XMM-Newton

“…in the case of the possible detection of the WHIM. [1][2][3] Experiences gained with the RGS since launch in December 1999, have led to a very good understanding of the instrument. This has led to a highly accurate calibration and quantization of instrument sensitivity, and has defined the optimum operating modes for various types of observations, enabling more challenging astronomical observations and discoveries in the years to come.…”