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F. Nicastro, M. Elvis, A. Fruscione (Smithsonian Astophysical Observatory)
Mkn~421 was observed by Chandra twice, on 1999 November 5, as part of the Chandra calibration program and with the ACIS-HETG configuration, and on 2000 May 29, following our Target Of Opportunity request aimed to catch the source in a ultra-high state, with both the ACIS-HETG and the HRC-LETG configurations. The two observations lasted for 26 ks and 40 (20+20) ks respectively. The 2-10 keV mkn421 flux was of 0.2 and 13 mCrab during the two observations, respectively. The Chandra grating spectra of Mkn~421 show that the beamed X-ray continuum of this source is absorbed and reprocessed by a number of varying systems of ionized gas along our line of sight or in the AGN environment. None of these X-ray systems is identified with the intervening matter imprinting strong Ly\alpha absorption in the UV spectrum of MKn~421. This allows us to put strong limits on the ionization degree, column density, density and temperature of this IGM cloud. In this contribute we present our comparative analysis of both the Chandra observations of Mkn~421, and discuss the results.