ar X iv : a st ro - p h / 05 11 05 2 v 1 1 N ov 2 00 5 1 NEW XMM - NEWTON SPECTROSCOPY OF THE MOST LUMINOUS AND DISTANT QUASARS

In the two parts of this contribution we describe two related XMM-Newton programs. The first part summarizes our study of the X-ray spectral properties and variability of z>4 quasars (Shemmer et al. 2005). The second part presents preliminary results from our ongoing XMM-Newton program to investigate the X-ray spectral properties and variability of luminous, high accretion-rate quasars at z∼2–3. We find that the X-ray photon index does not depend on luminosity or redshift, and there is suggestive evidence that it may depend on the accretion rate. None of our quasars is significantly absorbed, and none shows signatures of reflection. By jointly fitting high-quality spectra of eight radio-quiet z>4 quasars, including three from our XMM-Newton observations, we place tight constraints on the mean X-ray spectral properties of such sources. Most of our quasars are significantly X-ray variable on timescales of months–years, but none shows rapid (∼1 hr timescale) variations. Quasars at z>4 are valuable cosmological probes of the physical environment in the ∼1 Gyr old Universe. In particular , the most distant quasars known, at z∼6, have enabled tracing of the physical conditions in the Universe at the end of the re-ionization epoch with implications for large-scale structure formation (e.g., Fan et al. 2002). The study of z>4 quasars therefore has become one of the main themes in astrophysics during the past few years. One of the lines of research in this field is to determine whether the energy production mechanism of quasars is sensitive to the significant large-scale evolution the Universe has experienced over cosmic time. A central question in this context is whether black holes (BHs) in distant quasars feed and grow in the same way as BHs in local active galactic nuclei (AGN). Recent radio–optical observations of z>4 quasars have found that their spectral energy distributions (SEDs) are not significantly different from those of lower redshift sources implying no SED evolution, and hence no significant changes in the energy production mechanism of AGN are observed (e.g., see Carilli et al. 2001 and Petric et al. 2003 for radio observations ; Vanden Berk et al. 2001 and Pentericci et al. 2003 for UV–optical observations). X-rays from distant quasars are especially valuable for studying the energy production mechanism, since they provide information on the innermost regions of the central engine, where most of the nuclear energy is produced. Until fairly recently, only a handful of z>4 quasars were detected in …