The z>4 Quasar Population Observed by Chandra and XMM-Newton

The current status of our Chandra and XMM-Newton project on high-redshift (z>4) quasars is briefly reviewed. We report the main results obtained in the last few years for the detected quasars, along with a few (~10%) intriguing cases where no detection has been obtained with Chandra snapshot observations.Comment: 4 pages, to appear in the proceedings of 'Multiwavelength AGN surveys' (Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujic

X-ray Lighthouses of the High-Redshift Universe. II. Further Snapshot Observations of the Most Luminous z>4 Quasars with Chandra

We report on Chandra observations of a sample of 11 optically luminous (Mb<-28.5) quasars at z=3.96-4.55 selected from the Palomar Digital Sky Survey and the Automatic Plate Measuring Facility Survey. These are among the most luminous z>4 quasars known and hence represent ideal witnesses of the end of the "dark age ''. Nine quasars are detected by Chandra, with ~2-57 counts in the observed 0.5-8 keV band. These detections increase the number of X-ray detected AGN at z>4 to ~90; overall, Chandra has detected ~85% of the high-redshift quasars observed with snapshot (few kilosecond) observations. PSS 1506+5220, one of the two X-ray undetected quasars, displays a number of notable features in its rest-frame ultraviolet spectrum, the most prominent being broad, deep SiIV and CIV absorption lines. The average optical-to-X-ray spectral index for the present sample (=-1.88+/-0.05) is steeper than that typically found for z>4 quasars but consistent with the expected value from the known dependence of this spectral index on quasar luminosity. We present joint X-ray spectral fitting for a sample of 48 radio-quiet quasars in the redshift range 3.99-6.28 for which Chandra observations are available. The X-ray spectrum (~870 counts) is well parameterized by a power law with Gamma=1.93+0.10/-0.09 in the rest-frame ~2-40 keV band, and a tight upper limit of N_H~5x10^21 cm^-2 is obtained on any average intrinsic X-ray absorption. There is no indication of any significant evolution in the X-ray properties of quasars between redshifts zero and six, suggesting that the physical processes of accretion onto massive black holes have not changed over the bulk of cosmic time.Comment: 15 pages, 7 figures, accepted for publication in A

The evolution of obscured accretion

Our current understanding of the evolution of obscured accretion onto supermassive black holes is reviewed. We consider the literature results on the relation between the fraction of moderately obscured, Compton-thin AGN and redshift, and discuss the biases which possibly affect the various measurements. Then, we discuss a number of methods - from ultradeep X-ray observations to the detection of high-ionization optical emission lines - to select the population of the most heavily obscured, Compton-thick AGN, whose cosmological evolution is basically unknown. The space density of heavily obscured AGN measured through different techniques is discussed and compared with the predictions by current synthesis models of the X-ray background. Preliminary results from the first half of the 3 Ms XMM observation of the Chandra Deep Field South (CDFS) are also presented. The prospects for population studies of heavily obscured AGN with future planned or proposed X-ray missions are finally discussed.Comment: 6 pages, 2 figures. Invited talk at the conference "X-ray Astronomy 2009: Present status, multiwavelength approach and future perspectives", September 2009, Bologna. To appear in AIP Conf. Proc. (editors: A. Comastri, M. Cappi, L. Angelini)

The high-redshift Universe with the International X-ray Observatory

We discuss some of the main open issues related to the light-up and evolution of the first accreting sources powering high redshift luminous quasars. We discuss the perspectives of future deep X-ray surveys with the International X-ray Observatory and possible synergies with the Wide Field X-ray Telescope.Comment: 6 pages, 6 figures. Proceedings of "The Wide Field X-ray Telescope Workshop", held in Bologna, Italy, Nov. 25-26 2009. To appear in Memorie della Societ\`a Astronomica Italiana 2010 (arXiv:1010.5889