EDP Sciences
Free access
Volume 375, Number 3, September 2001
Page(s) 781 - 790
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010913

A&A 375, 781-790 (2001)
DOI: 10.1051/0004-6361:20010913

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

R. Della Ceca1, S. Pellegrini2, L. Bassani3, V. Beckmann4, M. Cappi3, G. G. C. Palumbo2, 5, G. Trinchieri1 and A. Wolter1

1  Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy
2  Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
3  Istituto TeSRE/CNR Via Gobetti 101, 40129 Bologna, Italy
4  INTEGRAL Science Data Centre, Chemin d'Ecogia 16, 1290 Geneva, Switzerland
5  Agenzia Spaziale Italiana (ASI), Viale Liegi 26, 00198 Rome, Italy

(Received 19 February 2001 / Accepted 5 June 2001 )

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 ( $L_{0.1-50 \rm keV} \sim 2.9 \times
10^{43}$ erg s-1) and variable source having a minimum observed doubling/halving time scale of ~10-20 ksec. A simple power law with photon index of $\Gamma \sim 1.75$ and small absorption ( $N_{\rm H} < 4\times 10^{20}$ 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 H$ \alpha$ imaging, 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.

Key words: galaxies: active -- galaxies: individual: NGC 7679 -- galaxies: Seyfert -- galaxies: starburst -- X-rays: galaxies

Offprint request: R. Della Ceca, rdc@brera.mi.astro.it

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