On the Exotic Hard X-ray Source Populations in the Hellas2XMM survey

Recent hard X-ray surveys have proven to be effective in discovering large numbers of X-ray sources that, despite the likely association with active nuclei, appear to be characterized by "peculiar" properties. Among these "exotic" source populations, we will focus on the nature of two classes of hard X-ray sources: those characterized by high X-ray-to-optical flux ratios -- a fraction of these are associated with the rather elusive Type 2 quasars -- and the X-ray bright optically normal galaxies, also known as XBONGs.Comment: On behalf of the HELLAS2XMM Collaboration, 6 pages, 4 figures, contribution to the Proceedings of the COSPAR Scientific Assembly, E1.3 "High-Energy Radiation from Black Holes: from Supermassive Black Holes to Galactic Solar Mass Black Holes", Paris (France), July 18-25, 2004, accepted for publication in Advances in Space Researc

Restless Quasar Activity: From BeppoSAX to Chandra and XMM-Newton

We briefly review some of the progress made in the last decade in the study of the X-ray properties of the quasar population from the luminous, local objects observed by BeppoSAX to the large, rapidly increasing population of z>4 quasars detected by Chandra and XMM-Newton in recent years.Comment: 4 pages, 4 figures. To appear in the Proceedings of the Symposium "The Restless High-Energy Universe", 5-8 May 2003, Amsterdam, The Netherlands, E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers Ed

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

Compton-thick AGN and the Synthesis of the Cosmic X-ray Background: the Suzaku Perspective

We discuss the abundance of Compton-thick AGN as estimated by the most recent population synthesis models of the cosmic X-ray background. Only a small fraction of these elusive objects have been detected so far, in line with the model expectations. The advances expected by the broad band detectors on board Suzaku are briefly reviewed.Comment: proceedings of "The Extreme Universe in the Suzaku Era", Kyoto 4-8 December 2006, to be published in Progress of Theoretical Physics, Supplemen