THE DROP-OFF FREQUENCY IN THE X-RAY VARIABILITY POWER SPECTRUM OF ACTIVE GALACTIC NUCLEI - A POSSIBLE CHARACTERISTIC TIMESCALE IN X-RAY-EMISSION OF NGC-4051

The study of characteristic timescales is of great importance in confronting the theory of active galactic nuclei involving accretion onto a supermassive black hole, and it puts physical constraints on the emission mechanism of the source. The observed chaotic X-ray light curves of active galactic nuclei imply that the innermost region, from which most of the X-ray originates, is nonaxisymmetric and nonuniform, that is, it may be understood as a collection of ''hot spots'' that rotate relativistically around the central black hole. In this Letter we discuss the properties of the variability power spectrum derived from the ''hot spot'' model, pointing out that the ''drop-off '' at the high-frequency end in the theoretical spectrum might be an indication of a characteristic timescale related to the central black hole of active galactic nuclei, provided that the current AGN paradigm is correct. We contend that this feature has appeared in the X-ray power spectrum of NGC 4051 and should therefore indicate a value for the mass of the central black hole approximately 4 x 10(6) M.; if so, the source radiates at a few percent of its Eddington luminosity.