Involved people at OAB: Comastri, Vignali.
Several observational programs aimed to understand the high energy emission properties of different classes of AGN are carried out using data from ROSAT ASCA and BeppoSAX X--ray satellites.
The X--ray properties of high redshift radio--quiet have been investigated in some detail using ASCA. Relatively good quality X--ray spectra have been obtained for about a dozen of quasars in the redshift range 1.8--2.5. The first results (concerning only 5 objects in the sample) clearly indicate the lack of intrinsic absorption and of the characteristic reprocessing features (Compton hump and iron line) which are common among lower luminosity Seyfert 1 objects. In addition the average X--ray spectrum (extending up to 30 keV in the quasar frame) is significantly flatter than that of lower redshift quasars. Unfortunately the present data do not allow to distinguish between a redshift or a luminosity dependence. Further insights are expected from the analysis of the entire sample and from the comparison with literature data (currently in progress).
The BeppoSAX Core Program observations of a sample of Narrow Line Seyfert 1 Galaxies (NLS1) in the 0.1--10 keV band were recently completed. The capabilities of the BeppoSAX detectors, and especially the relatively large MECS effective area at high energy (> 5 keV), have been fully exploited to further investigate, with respect to previous ROSAT and ASCA observations, several of the distinctive properties of NLS1. More specifically, the main scientific objective was to test whether the 2--10 keV spectral indices distribution and the properties of the iron K--shell features in the 6--10 keV region were different from that of normal, broad--line Seyfert 1 and how the peculiar X--ray properties of NLS1 (steep X--ray spectra and rapid variability) are linked to the optical ones (narrow Balmer lines and strong Fe II emission). The most important results were the discovery, in a few objects, of spectral features due to highly ionized matter (iron line and edge) and the parameterization of the overall X--ray continuum in terms of a strong soft excess, best fitted with a thermal black--body like spectrum, plus a steep power law component. The program was complemented by several quasi-simultaneous optical and UV observations which allowed to perform broad band spectral fits with accretion disk models. The results favor a low mass highly accreting black hole.
AGN synthesis models for the X--ray background are also investigated. Even though there is a relatively good agreement with the present observational data a coherent self--consistent picture has yet to be reached. The major problem is the discrepancy between the predictions of those models computed assuming the most up--to--date results, and the high energy (> 2 keV) source counts. In particular the lack of highly obscured high luminosity objects (the so--called type 2 quasars) is difficult to explain. One obvious possibility is a substantial contribution from non--AGN, flat spectrum sources. Another interesting possibility is that the optical properties of X--ray obscured AGN are different from what expected (i.e. narrow lined AGN). The large variety in the optical classification of faint X--ray obscured HELLAS and Chandra sources would support the second hypothesis. As soon as sizeable samples of X--ray selected sources will be available thanks to the foreseen medium--deep and deep Chandra and XMM--Newton surveys it will be possible to better constrain the AGN models parameter space.