The nature of the low-luminosity active galactic nuclei in NGC 3147

A recent optical study has identified an active nucleus (AGN) in the spiral galaxy NGC 3147 based on the discovery of broad (FWHM similar to 400 km s(-1)) [N II] line emission. This object was previously misclassified as a normal galaxy. Here we present the first X-ray spectrum obtained by ASCA in the 0.4-10 keV bandpass of this source. The X-ray spectrum is typical of Seyfert 1 galaxies, namely, it has little intrinsic absorption, a power-law continuum with a photon index of 1.80 +/- 0.09, and Fe K alpha emission at around 6.4 keV (rest frame) with an equivalent width greater than 130 eV. However, if it really is a Seyfert 1 galaxy, at a 2-10 keV luminosity of similar to 5 x 10(41) ergs s(-1), it should be highly variable, in keeping with the established trend that the lowest luminosity AGNs exhibit the greatest variability. However, no variability is observed within the 50 ks ASCA observation or between the ASCA observation and a ROSAT observation 2 weeks later. Alternatively, NGC 3147 may harbor an obscured AGN in which the observed X-ray spectrum is seen only in scattered light and the intrinsic X-ray luminosity may be an order of magnitude or two higher than observed. This interpretation would be consistent with a Seyfert 2 galaxy in the unified Seyfert model. We discuss the physical and geometrical constraints on the obscuring matter and the scattering zone in such a scenario. We point out the contrast with the X-ray spectrum of other Seyfert 2 galaxies which contain a soft, possibly thermal, component not present in NGC 3147.