We present a sample of type 2 Seyfert galaxies which behave differently than classical type 2 objects. We have shown that there are strong indications in favour of the "true'' Compton thin nature of such sources.
We estimate the fraction of Seyfert 2s with low absorption to be in the range of 10%-30% and this fraction is likely to increase at low luminosities. The study of a complete sample of LLAGNs will provide a more reliable estimate of their incidence in the Seyfert galaxies population.
The unabsorbed Seyfert 2 galaxies cannot be described by the Unified Model in its simple formulation. The standard BLR of Seyfert galaxies is either obscured by something other than the torus or it is weak, absent or has just faded away.
As a concluding remark, it is worth noting that the results presented in this paper add further complications to the classification scheme of AGNs in general and of Seyfert galaxies in particular.
Acknowledgements
We thank Massimo Cappi and Luigi Foschini for useful comments and John Stephen for the help in the final revision. We thank the referee for the helpful comments that improved the manuscript. This work has made use of XMM-Newton data. This research has made use of the NASA/IPAC extragalactic database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, the High Energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA's Goddard Space Flight Center, the BeppoSAX Science Data Center, the ASCA archive and the TARTARUS database.
Copyright ESO 2002