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Astron. Astrophys. 355, 485-498 (2000)


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The variability of the Seyfert galaxy NGC 2992: the case for a revived AGN

R. Gilli  *  1, R. Maiolino 2, A. Marconi 2, G. Risaliti 1, M. Dadina 3, K.A. Weaver 4 and E.J.M. Colbert 4

1 Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy (gilli, risaliti@arcetri.astro.it)
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy (maiolino, marconi@arcetri.astro.it)
3 BeppoSAX SDC, ASI, Via Corcolle 19, 00131 Roma, Italy (dadina@gavi.sdc.asi.it)
4 Laboratory for High Energy Astrophysics, Code 662, NASA/GSFC, Greenbelt, MD 20771, USA (kweaver, colbert@gsfc.nasa.gov)

Received 13 September 1999 / Accepted 22 December 1999

Abstract

We report the transition to an active state of the nucleus in the Seyfert 1.9 galaxy NGC 2992, discovered by means of new hard X-ray data. While the 2-10 keV flux declined by a factor of [FORMULA] 20 from 1978 to 1994, two recent BeppoSAX observations in 1997 and in 1998 caught the nuclear emission raising back to the same level of activity observed in 1978.

In both BeppoSAX observations the X-ray spectrum of the source is well represented by a power law with spectral index [FORMULA], absorbed by a column density of [FORMULA] cm-2 and characterized by a prominent iron K[FORMULA] line. While in the second BeppoSAX data set the line properties appear to be consistent with those expected from accretion disc models, in the first BeppoSAX data set the iron feature is rather peculiar. The broadening is not significant and the line energy is [FORMULA] keV, indicating emission from highly ionized iron. The line has too high equivalent width ([FORMULA] eV) to be produced by a hot scattering medium. By comparing these data with data previously in the literature, we interpret the spectral and flux changes in terms of different phases of rebuilding an accretion disc. The timescale for the disc rebuilding is estimated to range between 1 and 5 years.

The X-ray data are complemented with optical and near-infrared followup spectra taken 1.5 months after the discovery of the X-ray burst. The spectra are characterized by prominent broad emission lines. There is also evidence for hot dust emission in the H and K bands that, however, is probably still in the process of increasing.

Key words: X-rays: galaxies – galaxies: Seyfert – galaxies: individual: NGC 2992

* Present address : Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Send offprint requests to: R. Gilli

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000

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