A search for Warm-Hot Intergalactic Medium features in the X-ray spectra   of Mkn 421 with the XMM-Newton RGS
  We present the high-resolution X-ray spectra of Mkn 421 obtained in November 2003 with the RGS aboard the XMM-Newton satellite. This Target of Opportunity observation was triggered because the source was in a high state of activity in the X-ray band. These data are compared with three archival RGS observations of the same source performed in November and December 2002 and one in June 2003. We searched for the presence of absorption features due to warm-hot intergalactic medium (WHIM). We identify various spectral features, most of which are of instrumental origin. With the sensitivity provided by our spectra we were able to identify only two lines of astronomical origin, namely features at 23.5 A, probably due to interstellar neutral oxygen absorption, and at 21.6 A, which corresponds to a zero-redshift OVII K_alpha transition. For the latter, we derive an upper limit to the gas temperature, which is consistent with WHIM, of a few times 10^5 K, if the gas density has a value of n_e > 10^-5 cm^-3. 
M. Ravasio, G. Tagliaferri, A.M.T. Pollock, G. Ghisellini, F.   Tavecchio