Astron. Astrophys. 331, 519-523 (1998)

4. Discussion

The 2-10 keV flat spectrum together with the high equivalent width of the Fe line strongly suggest that the observed spectrum of the Seyfert 2 galaxy NGC 7674 is reflection dominated. The observed luminosity, erg s^-1 is, therefore, expected to be much lower than the intrinsic one. Assuming a scattering model (model #4), the best fit luminosity of the soft component extrapolated in the 2-10 keV band is erg s^-1. If the material that electron-scatters the nuclear radiation is confined in a cone-shaped region above the torus, then is linked to the nuclear intrinsic luminosity by the relation , where and are the mirror optical depth and subtended solid angle respectively. Assuming NGC 1068 values of and 0.25 for and respectively (Iwasawa, Fabian, and Matt 1997), then is estimated to be erg s^-1. It is interesting to note that this value is very close to the intrinsic [OIII] luminosity of erg s^-1 evaluated from the observed flux corrected for the reddening in the narrow line region (Maiolino, private communication). The fact that also the far infrared integrated 8-120 µm luminosity erg s^-1 (Spinoglio and Malkan 1989) is of the same order of magnitude suggests that both and and can be good indicators of the intrinsic X-ray luminosity.

If the reflection is caused by cold materials at the inner surface of the torus (Ghisellini, Haardt, and Matt 1994; Krolik, Madau, and Zycki 1994), then only a fraction of the reflecting surface is visible. This fraction can be estimated from the intrinsic luminosity. Using the notation of Iwasawa, Fabian, and Matt (1997), the observed luminosity can be written as where is the albedo for a isotropic illumination. Using the above values for and and assuming , becomes equal to 0.1, which is consistent within a factor of 2 of what found for NGC 1068.

Pure reflection spectra have been observed in X-rays by the ASCA satellite in at least few other Seyfert 2 galaxies such as NGC 6552 (Fukazawa et al. 1994; Reynolds et al. 1994), NGC 6240 (Kii et al. 1997), the Circinus galaxy (Matt et al. 1996), and NGC 1068 which has also been confirmed by BeppoSAX (Matt et al. 1997, and references therein). In NGC 7674, even if some contribution from a warm reflection component cannot in principle be excluded, as demonstrated by the acceptable fit obtained with a blend of ionized lines (model #5), the observed X-ray emission is certainly dominated by reflection from cold matter. In fact, on statistical basis (better , see Table 1) and self-consistency arguments, the most convincing explanation for the possible higher energy line is emission from neutral iron (model #6). This is contrary to what observed in NGC 1068 and NGC 6240 where the ionized reflection component is much stronger, but similar to what observed in the Circinus galaxy and NGC 6552. Such differences may have different physical interpretations. They could be ascribed to an orientation effect (more face-on than NGC 1068 and NGC 6240) with respect to the line of sight of the torus, or to the condition (e.g. low scattering efficiency) of the warm mirror itself. In conclusion, the case of NGC 7674 adds to the increasing number of Compton-thick Seyfert 2 galaxies, thus suggesting that a dedicated study of a complete sample of optically selected Seyfert 2 galaxies may allow the discovery of a significant number of Compton-thick objects as indicated by recent BeppoSAX results (Salvati et al. 1997; Maiolino et al. 1997).

© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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