The 0.1-100 keV view of NGC3998: An AGN origin for the LINER activity

We present a BeppoSAX observation of the LINER nucleus of the S0 galaxy NGC 3998. A long standing issue concerning LINER galaxies is the origin of thei r optical emission lines that could be due to either starburst or AGN-like activity. The broad band BeppoSAX spectral data of NGC3498 allow one for th e first time to single out a power law component extending from 0.1 to 100 keV. This provides a strong indication for an AGN origin of the LINER activ ity in this galaxy. The value of the photon index (Gamma similar to 1.9), t he small amount of intrinsic absorption required by the X-ray spectrum (N-H < 3 X 10(20) (-2)) and the presence of a broad H alpha component all sugge st cm that the nucleus of NGC3998 might be an example of a low luminosity t ype I AGN. This hypothesis is also supported by the (2-10)keV and H alpha l uminosity values placed on the extension to low luminosities of the correla tion observed for Seyfert galaxies and quasars, and by the possible presenc e of an OVII absorption edge in the spectral data. At variance with the pro perties commonly shown by more luminous type 1 AGNs is however the absence of rapid and strong Aux variability, the lack of signs of reflection from c old material and the low ratio (similar to 10(-1)) of the observed nuclear luminosity to the Eddington luminosity. So, accretion could be advection do minated in NGC3998, as already suggested for other low luminosity AGNs. The broad band spectral energy distribution of the nucleus, though, shows high UV emission that makes it more similar to that of high luminosity AGNs. A similar to 9.2 keV absorption feature has also been marginally detected in the X-ray spectrum; this could be produced by transmission through highly i onized material as recently suggested for a similar feature revealed by Bep poSAX with much higher significance in M81.