High resolution X-ray spectra of quasars

Past X-ray observations by ASCA suggest that warm absorbers (Ovii and Oviii edges) are apparently rare in high luminosity AGNs (quasars) while they are more common in low luminosity AGNs (Seyferts). However, this could be a selection effect if high luminosity AGNs have mostly narrow absorption lines (with no strong bound free edges), which escaped detection by the low resolution of ASCA. To check this hypothesis we are studying the highresolution X-ray spectra of quasars from grating spectrometers on board Chandra and XMMNewton in search for absorption lines. In this contribution we present spectra of three quasars. The spectra show narrow (several hundred kms) absorption and emission X-ray lines from H-like and He-like ions of O, Ne, Mg, and other abundant elements. We also detect absorption from iron L-shell lines and iron M-shell unresolved transition array. We present the analysis of MR2251-178 where we find that at least two, and probably three, distinct warm absorbers are needed to explain the high resolution spectrum of this object. We re-analyze the highresolution X-ray spectrum of PG 1211+143 and suggest that an outflow velocity of ∼ 3000 km s provides an adequate explanation to these data. We also present preliminary results form the Chandra/HETGS observation of the quasar 4C74.26.