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Astron. Astrophys. 338, 781-794 (1998)

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2. The weak Sy2 BeppoSAX core program

We have undertaken a program of observations with BeppoSAX, the Italian-Dutch X-ray satellite, aimed at studying the hard X-ray properties of weak Sy2s and at assessing the "real" distribution of their absorbing column densities.

As described in the next section, BeppoSAX is an excellent tool to pursue this goal, since it combines high sensitivity (required to observe weak AGNs) and a wide spectral coverage (0.1-300 keV, required to identify and disentangle various spectral components).

Sy2s suitable for this study were drawn out of Maiolino & Rieke's (1995) sample. This sample is extracted from the Revised Shapley-Ames (RSA) catalog of galaxies (that is limited to magnitude [FORMULA], Sandage & Tammann 1987), and Seyfert galaxies are selected according to their optical lines. As discussed in Maiolino & Rieke, this sample is much less biased than others, both in terms of luminosity of the Seyfert nuclei and in terms of properties of their host galaxies. Out of the 54 Sy2s in the Maiolino & Rieke sample 22 have already been observed by ASCA. We selected 8 of the remaining 32 Sy2s, based on their [OIII]5007Å (narrow) line flux: to maximize the chances of detection we chose the sources showing the highest [OIII] flux. Maiolino & Rieke also show that, although the [OIII] line is emitted on scales much larger than the putative pc-scale torus, the [OIII] is not a completely isotropic indicator of the nuclear luminosity, for the host galaxy disk might obscure part of the NLR. However, once the [OIII] flux is corrected for the extinction deduced from the Balmer decrement, it should provide an indication of the nuclear activity that is independent of the pc-scale obscuration due to the torus. As a consequence, although our [OIII]-based selection criterion might introduce a bias in our sample for intrinsically luminous sources, it avoids biases against highly obscured Sy2 nuclei, thus overcoming limitations of former surveys.

Preliminary results of this survey were published in Salvati et al. (1997, 1998). In this paper we report and discuss results of BeppoSAX observations for all of the 8 Seyfert 2s in our [OIII] selected sample. A more thorough statistical analysis, obtained by merging our BeppoSAX data with data in the literature, is presented in Bassani et al. (in prep.).

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998