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 It includes the results of 26 still unpublished Chandra and XMM-Newton observations, which yield 25 (22) new X-ray detections in the 0.5-2 keV (2-10 keV) energy band.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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Abstract.A sample of nine bright Seyfert 1 and NELG type galaxies, observed with BeppoSAX, is analyzed to assess on a truly broad band basis (0.1-200 keV) the issue of the spectral contributions of Compton reflection and iron line fluorescence from circumnuclear gas. The empirical description adopted for the direct continuum is the commonly used power law with an exponential cut-off. The most direct test of the theoretical predictions, namely that the equivalent width of the line, Wα, and the strength R of the reflection relative to the direct continuum are closely related to each other, gives a substantially positive result, that is their mean ratio is very close to expectation, and only a modest spread in the iron abundance seems implied. The existence of a steep correlation between R and the slope Γ of the power law is not confirmed. A weak evidence is found that the existence of a very shallow trend to increase on average with Γ cannot be altogether excluded in both R and Wα, but needs to be tested with a larger sample. The energy E f in the exponential cut-off spans a range from about 80 to more than 300 keV. A possible correlation is found, with E f increasing on average with Γ: if ignored, for instance by keeping E f at a fixed value in a sample study, it could be cause of artificial steepening in a correlation between R and Γ.

We have used realistic Comptonization models to fit high quality BeppoSAX data of 6 Seyfert galaxies. Our main effort was to adopt a Comptonization model taking into account the anisotropy of the soft photon field. The most important consequence is a reduction of the first scattering order, which produces a break (the so-called anisotropy break) in the outgoing spectra. Thus anisotropic Comptonization models yield spectra with convex curvature. The physical parameters of the hot corona (i.e. the temperature and optical depth) obtained fitting this class of models to broad band X-ray spectra are substantially different from those derived fitting the same data with the power law + cut-off model commonly used in the literature. In particular, our best fits with Comptonization models in slab geometry give a temperature generally much larger and an optical depth much smaller than derived from the power law + cut-off fits, using standard Comptonization formulae. The estimate of the reflection normalization is also larger with the slab corona model. For most objects of our sample, both models give Compton parameter values larger than expected in a slab corona geometry, suggesting a more "photon starved" X-ray source configuration. Finally, the two models provide different trends and correlation between the physical parameters: for instance, with the power law + cut-off fits, we obtain a correlation between the reflection normalization and the corona temperature whereas we find an anti-correlation between these parameters with the slab geometry. These differences have major consequences for the physical interpretation of the data. In the framework of reprocessing models, the cut-off power law best fit results suggest the thermal corona to be dominated by electron-positron pairs. On the contrary, the slab corona model is in better agreement with a low pair density solution.

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10 −14 erg cm −2 s −1 . Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as "FIORE P3". The analysis of the broadband energy distribution suggests the presence of a heavily obscured active nucleus.