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Astron. Astrophys. 325, L13-L16 (1997)

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1. Introduction

NGC 1068 is the archetypal object for both the class of Seyfert 2 galaxies as a whole and the Compton-thick subclass (see Matt 1997 for a brief review), i.e. those objects for which the column density of the line-of-sight absorbing matter (hereinafter identified with the torus, see e.g. Ward 1996) exceeds [FORMULA] cm-2 and is therefore optically thick to Compton scattering. A column of a few times this critical density is sufficient to strongly depress the transmitted intensity also in hard X-rays, since after a few scatterings photons are redshifted into the photoelectric dominated regime. However, even for very high column densities, nuclear radiation can still be observed in scattered light, as both the visible part of the inner surface of the torus and the warm medium responsible for scattering and polarizing the optical broad lines (Antonucci & Miller 1985) can act as mirrors (e.g. Ghisellini et al. 1994; Matt et al. 1996). The two reflectors produce rather different spectra: the torus is supposed to be essentially neutral, then giving rise to the so-called Compton reflection continuum (Lightman & White 1988); the warm mirror should be highly ionized, and its spectral shape basically the same as the nuclear one, apart from a high energy cutoff owing to Compton downscattering. Superimposed on both continua, emission lines are also expected, iron lines being the most prominent: a 6.4 keV fluorescent line from the torus, highly ionized resonant scattering and fluorescence/recombination lines from the warm mirror (such as reported by Matt et al. 1996).

This two-reflectors picture is confirmed (and it has been partly motivated) by ASCA observations of NGC 1068 (Ueno et al. 1994; Iwasawa et al. 1997), which clearly distinguished three different iron lines: one at 6.4 keV, the other two consistent with He- and H-like iron, respectively (the lines being possibly redshifted by a few thousands km/s, Iwasawa et al. 1997) with equivalent widths of the order of 1 keV each, as expected in the reflection model (Matt et al. 1996). The continuum is not well constrained by ASCA due to the limited energy range available for its evaluation (below 4 keV a thermal-like component dominates, Ueno et al. 1994), but it looks very flat, as expected if the cold reflector were providing a significant contribution.

To confirm this scenario, hard X-rays observations are necessary. We then proposed to observe NGC 1068 with BeppoSAX, in order to take full advantage of the good sensitivity of the PDS, the Phoswich Detector System working between 15 and 300 keV. In fact, a cold reflection-dominated spectrum would be hard enough to be observable with the PDS with a [FORMULA] 105 s pointing even at the relatively low flux level of NGC 1068 (i.e. about a quarter of a mCrab 1 between 2 and 10 keV). Here we report what is to our knowledge the first detection of NGC 1068 up to 100 keV.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998