High Resolution Spectroscopy of X-ray Quasars: Searching for the X-ray   Absorption from the Warm-Hot Intergalactic Medium
  We present a survey of six low to moderate redshift quasars with Chandra and XMM-Newton. The primary goal is to search for the narrow X-ray absorption lines produced by highly ionized metals in the Warm-Hot Intergalactic Medium. All the X-ray spectra can be fitted by a power law with neutral hydrogen absorption method. The residuals that may caused by additional emission mechanisms or calibration uncertainties are taken account by polynomial in order to search for narrow absorption features. No real absorption line is detected at above 3-sigma level in all the spectra. We discuss the implications of the lack of absorption lines for our understanding of the baryon content of the universe and metallicity of the intergalactic medium (IGM). We find that the non-detection of X-ray absorption lines indicates that the metal abundance of the IGM should be smaller than ~0.3 solar abundance. We also discuss implications of the non-detection of any local (z ~ 0) X-ray absorption associated with the ISM, Galactic halo or local group, such as has been seen along several other lines of sight (LOS). By comparing a pair of LOSs we estimate a lower limit on the hydrogen number density for the (z ~ 0) 3C 273 absorber of n_H >= 4e-3 cm^-3. 
Taotao Fang (1), Claude R. Canizares (2), Herman L. Marshall (2) ((1)   UC Berkeley, (2) MIT)