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2.5. Dual Absorbers

A simple, absorbed power law does not provide an adequate description of the observed X-ray spectrum for some of the Compton thin Sy2s. In these cases there is evidence for a second photoelectric cutoff at higher energies due to a medium that only partially covers the X-ray source (Malaguti et al. 1999, Hayashi et al. 1996, Weaver et al. 1994, Vignali et al. 1998). While the photoelectric cutoff at low energies is very likely due to the torus, the partial covering observed at higher energies can only be obtained if the second absorbing medium is close to the X-ray source and has a similar size. A most likely candidate is the BLR. Generally, the column density of the partial covering medium is of the order of ~ 1023 cm-2, in agreement with estimates for the NH of broad line clouds. The covering factor of the partial absorber is generally found to be larger than ~ 30%, that is significantly higher than the covering factor of the broad line clouds which, based on the broad lines equivalent width and on the absence of the Ly-edge cutoff in the UV spectrum of QSOs, is expected to be about 10%. However, I do not consider this a major caveat since the dual absorbers discovered so far are not representative of the true distribution of the partial covering absorption systems, but only sample the tail with high covering factor. Partial covering systems with a covering factor of 10% probably remained undetected in the past observations. I expect Chandra and XMM to discover a large number of dual absorbers with low partial covering.

Finally, thanks to the extended spectral coverage of BeppoSAX some cases of dual absorber with partial covering characterized by an absorbing column as high as NH ~ 1024 cm-2 are being found (eg. Turner et al. 2000). Such a high NH is still consistent with that expected for the broad line clouds. Indeed, the estimated column of 1022-1023 cm-2 for the broad line clouds is only a lower limit that is required to produce the low ionization broad emission lines (MgII, FeII, etc...).