Unification models of Seyfert galaxies, at least in their simplest formulation, postulate the presence of a geometrically and optically thick gas and dust torus which, at a distance of several to several tens of parsecs from the nucleus (Krolik, Madau, and Zycki 1994), hides the primary source and the broad line forming region (BLR). The orientation of the torus is independent of the host galaxy, and the observed differences between Seyfert 1 and Seyfert 2 nuclei are thus to be ascribed simply to the angle formed by the line of sight direction and the axis of the torus. After the seminal work by Antonucci & Miller (1985) various pieces of evidence have been accumulating in favour of the unification model of Seyfert galaxies (see Antonucci 1993 for a review of the subject): detection of broadened emission lines in optical spectropolarimetry observations of several Seyfert 2 galaxies (Antonucci & Miller 1985; Miller & Goodrich 1990; Tran 1995; Young et al. 1996) interpreted as scattering of the BLR emission by warm material placed above the torus; biconical structures in the light of the forbidden [OIII] line at 5007 (e.g. Tadhunter and Tsvetanov 1989) which is expected if the ionizing radiation field from the nucleus is anisotropic; large ( cm-2 or greater) absorbing column densities observed in the hard X-ray spectra of Seyfert 2 galaxies (Awaki 1997).
In the optical and UV bands the nuclear emission is visible only if the line of sight does not intercept the torus. In the X-ray band, the interaction between electromagnetic radiation and matter is dominated by photoelectric absorption up to keV and by Compton scattering at higher energies. When the line of sight intercepts the torus (Seyfert 2) primary X-rays are able to leak through it (Compton thin source) if the column density is less than cm-2. When cm-2 the primary X-rays cannot escape and are either promptly absorbed via photoelectric interaction, or first Compton downscattered and eventually photoabsorbed making the nucleus invisible (Compton thick source) in direct emission.
Emission from Compton thick Seyfert 2 galaxies can be detected by means of the reflection caused by the visible inner surface of the torus and/or via scattering by the material responsible for producing the broad lines observed in polarized light. The detection of these sources in X-rays is hampered because of their faintness ( erg cm-2 s-1) and only a few of them are currently known (Matt 1997 and references therein). Their expected observable features in X-rays are: a 2-10 keV continuum flatter than the canonical slope observed for Seyfert 1, a strong (equivalent width (EW) 1 keV or higher when measured against the reflected continuum) Fe feature around 6.4 keV, and a reflection "hump" at energies keV (e.g. Ghisellini, Haardt, and Matt 1994).
NGC 7674 is a Seyfert 2 nucleus hosted in a face on spiral galaxy (Sab) with asymmetrical arms and a tidal connection to a nearby compact elliptical galaxy. Broad and components in polarized flux were first observed by Miller & Goodrich (1990), and then by Young et al. (1996). The spectropolarimetric observations suggest that NGC 7674 is the only known Seyfert 2 galaxy with hidden broad line region for which dust scattering is the dominant cause of the observed nuclear polarization (Tran 1995).
Preliminary analysis of the GINGA observation of NGC 7674 (Awaki et al. 1991) gave a flat () 2-10 keV spectrum with no indication for a Fe line (EW 110 eV). However, subsequent analysis indicated that the observed count rate was below the 3 rms fluctuation of the Cosmic X-Ray Background between 2 and 10 keV in the eight detectors of the Large Area Counter onboard GINGA (Smith and Done 1996). As a consequence, the 2-10 keV flux of erg cm-2 s-1 previously reported by Awaki et al. (1991) has to be considered only as a 2 upper limit (Smith, private communication). In the following, detailed measurement of the X-ray spectral characteristics of NGC 7674 obtained by BeppoSAX is reported.
© European Southern Observatory (ESO) 1998
Online publication: February 16, 1998