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 NGC3998. A long standing issue concerning LINER galaxies is the origin of their optical emission lines that could be due to either starburst or AGN-like activity. The broad band BeppoSAX spectral data of NGC3998 allow one for the 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 activity in this galaxy. The value of the photon index (Γ ∼ 1.9), the small amount of intrinsic absorption required by the X-ray spectrum (NH < 3 × 1020 cm-2) and the presence of a broad Ha component all suggest that the nucleus of NGC3998 might be an example of a low luminosity type 1 AGN. This hypothesis is also supported by the (2-10) keV and Ha luminosity values placed on the extension to low luminosities of the correlation observed for Seyfert galaxies and quasars, and by the possible presence of an OVII absorption edge in the spectral data. At variance with the properties commonly shown by more luminous type 1 AGNs is however the absence of rapid and strong flux variability, the lack of signs of reflection from cold material and the low ratio (∼ 10-4) of the observed nuclear luminosity to the Eddington luminosity. So, accretion could be advection dominated 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 ∼ 9.2 keV absorption feature has also been marginally detected in the X-ray spectrum; this could be produced by transmission through highly ionized material as recently suggested for a similar feature revealed by BeppoSAX with much higher significance in M81.