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

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 similar to 20 from 1978 to 1994, two r ecent 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 repr esented by a power law with spectral index Gamma similar or equal to 1.7, a bsorbed by a column density of N-H similar or equal to 10(22) cm(-2) and ch aracterized by a prominent iron Ka line. While in the second BeppoSAX data set the line properties appear to be consistent with those expected from ac cretion disc models, in the first BeppoSAX data set the iron feature is rat her peculiar. The broadening is not significant and the line energy is E-K alpha = 6.62 +/- 0.07 keV, indicating emission from highly ionized iron. Th e line has too high equivalent width (similar to 700 eV) to be produced by a hot scattering medium. By comparing these data with data previously in th e literature, we interpret the spectral and flux changes in terms of differ ent phases of rebuilding an accretion disc. The timescale for the disc rebu ilding is estimated to range between I and 5 years. The X-ray data are complemented with optical and near-infrared followup spe ctra taken 1.5 months after the discovery of the X-ray burst. The spectra a re 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.