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 ToO observation was triggered since 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 Autumn 2002 and one in Summer 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. We find that only two of them are most likely of astronomical origin, namely features at 23.5 Å, probably due to interstellar neutral oxygen absorption, and at 21.6 Å, which corresponds to a zero-redshift OVII Kα transition. For the latter, we derive an upper limit to the gas temperature, which is consistent with WHIM, of temperature of few times 10 K, only if the gas density has a value of ne & 10 cm.