标题： 塞弗特星系 NGC 2992 的变化性：复兴的活动星系核案例 显示英文标题

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

摘要： 我们报道了塞弗特1.9型星系NGC 2992核活动状态的转变，这一发现是通过新的硬X射线数据实现的。 虽然从1978年到1994年，2-10千电子伏特（keV）的通量下降了约20倍，但两次最近的BeppoSAX观测（分别于1997年和1998年）捕捉到了核发射回到1978年的活跃水平。 在两次BeppoSAX观测中，源的X射线光谱可以用幂律很好地表示，光谱指数Gamma约为1.7，被约10^{22}cm^{-2}的柱密度吸收，并表现出明显的铁K\alpha 线。 虽然在第二个BeppoSAX数据集中，线的特性似乎与吸积盘模型预期的一致，在第一个BeppoSAX数据集中，铁特征相当特殊。 展宽并不显著，线能量为E_{K\alpha }=6.62±0.07 keV，表明高度电离铁的发射。 这条线的等效宽度（~700 eV）太高，不能由热散射介质产生。 通过将这些数据与文献中的先前数据进行比较，我们将光谱和通量的变化解释为重建吸积盘的不同阶段。 盘重建的时间尺度估计在1到5年之间。 X射线数据得到了光学和近红外后随光谱的补充，这些光谱是在X射线爆发发现后的1.5个月内获得的。 光谱以突出的宽发射线为特征。 H和K波段也有证据表明热尘埃发射，但这可能仍处于增加过程中。 显示英文摘要

摘要： 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 Gamma ~ 1.7, absorbed by a column density of N_H ~ 10^{22} cm^{-2} and characterized by a prominent iron K\alpha 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 E_{K\alpha}=6.62 +/- 0.07 keV, indicating emission from highly ionized iron. The line has too high equivalent width (~ 700 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.