We have determined the cosmological evolution of the density of active galactic nuclei (AGN) and of their N H distribution as a function of the un-absorbed 2-10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGN. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low luminosity (L X ∼10 43 erg s −1 ) AGN peak at z∼0.7, while high luminosity AGN (L X >10 45 erg s −1 ) peak at z∼2.0. A pure luminosity evolution model (PLE) can instead be rejected.There is evidence that the fraction of absorbed (N H >10 22 cm −2 ) AGN decreases with the intrinsic X-ray luminosity, and increases with the redshift.Our best fit solution provides a good fit to the observed counts, the cosmic X-ray background, and to the observed fraction of absorbed AGN as a function of the flux in the 10 −15 10 44 erg s −1 ) AGN have a density of 267 deg −2 at fluxes S 2−10 >10 −15 erg cm −2 s −1 . Using these results, we estimate a density of supermassive black holes in the local Universe of ρ BH = 3.2 h 2 70 × 10 5 M ⊙ Mpc −3 , which is consistent with the recent measurements of the black hole mass function in the local galaxies.

Abstract. The X-ray satellite SAX, a major program of the Italian Space Agency with participation of the Netherlands Agency for Aereospace Programs, was launched on April 30 1996 from Cape Canaveral. After launch it was renamed BeppoSAX in honour of Giuseppe (Beppo) Occhialini. The payload is characterized by a very wide spectral coverage from 0.1 to 300 keV, with well balanced performances both from its low and high energy instrumentation. Its sensitivity will allow the exploitation of the full band for weak sources (1/20 of 3C 273), opening new perspectives in the study of spectral shape and variability of several classes of objects. Furthermore, the presence of wide field cameras will allow monitoring of the long term variability of sources down to 1 mCrab and the discovery of X-ray transient phenomena. In this paper we describe the main aspects of the mission, the operations, the scientific capabilities of the instruments and the scientific objectives.

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We discuss the X-ray properties of 49 local (z < 0.035) Seyfert 2 galaxies with HST/WFC2 high-resolution optical coverage. It includes the results of 26 still unpublished Chandra and XMM-Newton observations, which yield 25 (22) new X-ray detections in the 0.5-2 keV (2-10 keV) energy band. Our sample covers a range in the 2-10 keV observed flux, F 2−10 , from 3 × 10 −11 to 6 × 10 −15 erg cm −2 s −1 . The percentage of objects that are likely obscured by Compton-thick matter (column density, N H ≥ σ −1 t 1.6 × 10 24 cm −2 ) is 50%, and reaches 80% for log (F 2−10 ) < 12.3. Hence, K α fluorescent iron lines with large Equivalent Width (EW > 0.6 keV) are common in our sample (6 new detections at a confidence level ≥2σ). They are explained as due to reflection off the illuminated side of optically thick material. We confirm a correlation between the presence of a ∼100-pc scale nuclear dust in the WFC2 images and Compton-thin obscuration. We interpret this correlation as due to the large covering fraction of gas associated with the dust lanes. The X-ray spectra of highly obscured AGN invariably present a prominent soft excess emission above the extrapolation of the hard X-ray component. This soft component can account for a very large fraction of the overall X-ray energy budget. As this component is generally unobscured -and therefore likely produced in extended gas structures -it may lead to a severe underestimation of the nuclear obscuration in z ∼ 1 absorbed AGN, if standard X-ray colors are used to classify them. As a by-product of our study, we report the discovery of a soft X-ray, luminous ( 7 × 10 40 erg s −1 ) halo embedding the interacting galaxy pair Mkn 266.

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