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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
Received 13 September 1999 / Accepted 22 December 1999
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 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 , absorbed by a column density of cm-2 and characterized by a prominent iron K 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 keV, indicating emission from highly ionized iron. The line has too high equivalent width ( 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
Send offprint requests to: R. Gilli
Online publication: March 9, 2000