1 Introduction

Seyfert galaxies belong to the class of Active Galactic Nuclei (AGN). According to the standard model, in active galaxies an accretion disk around a massive black hole produces a hard X-ray continuum, which photoionizes the Broad Line Region (BLR, where broad emission lines originate) and the Narrow Line Region (NLR, where narrow emission lines originate) located at <1 pc and at <100 pc from the nuclear engine respectively. Seyfert galaxies are classified as type 1 or type 2. Type 1 have both narrow forbidden lines ( $FWHM \leq 10^{3}$ km s^-1) and broad Balmer lines ( $FWHM \sim 10^{4}$ km s^-1) in their optical spectrum, while type 2 have only narrow lines. Actually they are the same object: type 2 Seyferts harbour a BLR, but this is obscured from view in some directions by a molecular torus (Unification Model; Antonucci 1993).

Optical spectropolarimetry measurements of scattered broad permitted lines provide strong evidence in favour of the unified model (Antonucci & Miller 1985). At least 35% of Seyfert 2 galaxies have broad emission lines seen in polarized light (Tran 2001; Moran et al. 2000), therefore a good fraction of Seyfert 2 galaxies seem to host a hidden Seyfert 1 nucleus.

More evidence in favour of the unified model comes from the X-ray spectra: the column density of neutral hydrogen in type 2 Seyferts is significantly higher than in type 1 objects as would be expected if, for the type 2 sources, the nucleus is observed through the torus (Turner et al. 1997; Smith & Done 1996). Observed column densities range from 10²² cm^-2 to higher than 10²⁴ cm^-2 for $\sim$96% of the objects (Risaliti et al. 1999; Bassani et al. 1999).

However, not all Seyfert 2 galaxies have a Broad Line Region in polarized light and not all Seyfert 2 galaxies have column densities higher than 10²² cm^-2. Polarimetric surveys of complete samples of Seyfert 2s indicate that a large fraction of these objects (up to 50%) do not show a hidden BLR typical of an obscured Seyfert 1 nucleus. Furthermore there have been some recent examples of Seyfert 2 galaxies, such as NGC 3147 (Ptak et al. 1996), NGC 4698 (Pappa et al. 2001) and NGC 7590 (Bassani et al. 1999), which have no or low absorption measured from the X-ray spectrum. It can be argued that these are Compton thick objects i.e. in which the medium is thick to Compton scattering such that the transmitted component is completely suppressed below 10 keV and the 2-10 keV spectrum is dominated by reprocessed components. In this case the hard X-ray spectrum is characterized by a flat Compton reflection component from the inner surface of the torus and/or a steeper component ascribed to an ionized, warm scattering medium. When the absorbing medium has column density $N_{\rm H}> 10^{24}$ cm^-2, then the transmitted component can be observed above 10 keV. Therefore, in these sources the true column density can only be estimated by higher energy data for $N_{\rm H}> 10^{24}$ cm^-2or measured indirectly by comparing the X-ray luminosity with the Far-Infrared or [OIII] luminosities for even higher column densities. In the above mentioned sources these absorption indicators suggest that they are actually Compton thin objects. At the moment at least $\sim$4% of Seyfert 2s have $N_{\rm H}<10^{22}$ cm^-2 (Risaliti et al. 1999). The exact nature of these peculiar Seyfert 2s is still unclear, as it is not obvious what obscures their Broad Line Region: they may be intrinsically different objects than those explained by the unified theory or in other words they may be the "true'' Seyfert 2 galaxies which are sometimes discussed in the literature (Tran 2001).

In this paper we have collected a sample of Seyfert 2 galaxies characterized by low X-ray absorption in order to study their properties, estimate their abundance and understand better their nature.