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 the neutral hydrogen absorption method. The residuals that may be caused by additional emission mechanisms or calibration uncertainties are taken account by a polynomial in order to search for narrow absorption features. No real absorption line is detected at above the 3 sigma level in all of the spectra. We discuss the implications of the lack of absorption lines for our understanding of the baryon content of the universe and the metallicity of the intergalactic medium (IGM). We find that the nondetection of X-ray absorption lines indicates that the metal abundance of the IGM should be smaller than similar to 0.3 solar abundance. We also discuss implications of the nondetection of any local (z similar to 0) X-ray absorption associated with the interstellar medium, Galactic halo, or Local Group, such as has been seen along several other lines of sight ( LOSs). By comparing a pair of LOSs we estimate a lower limit on the hydrogen number density for the (z similar to 0) 3C 273 absorber of n(H) greater than or similar to 4 x 10(-3) cm(-3).