Unveiling the AGN powering the "Composite" Seyfert/Star-forming galaxy NGC 7679: BeppoSAX and ASCA results

Abstract

We discuss BeppoSAX observations and archive ASCA data of NGC 7679, a nearby, nearly face-on SB0 galaxy in which starburst and AGN activities coexist. The X-ray observations reveal a bright (L_{0.1-50 keV} \sim 2.9 \times 10^{43} erg s^{-1}) and variable source having a minimum observed doubling/halving time scale of \sim 10 - 20 ksec. A simple power law with photon index of Γ\sim 1.75 and small absorption (N_H < 4\times 10^{20} cm^{-2}) can reproduce the NGC 7679 spectrum from 0.1 up to 50 keV. These X-ray properties are unambiguous signs of Seyfert 1 activity in the nucleus of NGC 7679. The starburst activity, revealed by the IR emission, optical spectroscopy and Hαimaging, and dominating in the optical and IR bands, is clearly overwhelmed by the AGN in the X-ray band. Although, at first glance, this is similar to what is observed in other starburst-AGN galaxies (e.g. NGC 6240, NGC 4945), most strikingly here and at odds with the above examples, the X-ray spectrum of NGC 7679 does not appear to be highly absorbed. The main peculiarity of objects like NGC 7679 is not the strength of their starburst but the apparent optical weakness of the Seyfert 1 nucleus when compared with its X-ray luminosity. To date NGC 7679 is one of the few Seyfert 1/ Starburst composites for which the broad-band X-ray properties have been investigated in detail. The results presented here imply that optical and infrared spectroscopy could be highly inefficient in revealing the presence of an AGN in these kinds of objects, which instead is clearly revealed from X-ray spectroscopic and variability investigations.

10 pages, 4 figures, Latex manuscript, Accepted for publication in the Astronomy and Astrophysics - Main Journal

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