Central black hole masses in active galactic nuclei inferred from X-ray variability

Central black hole masses in eight active galactic nuclei (AGNs) are evaluated by a method based on the X-ray flux variability of these AGNs and Cyg X-l observed with Ginga. We introduce a new definition for the variability timescale by employing a normalized power spectral density function. Assuming a linear proportionality between the variability timescale and the mass of the central black hale, and using the relation between these two for Cyg X-l as a reference point, we estimate the masses of eight AGNs (six Seyferts, one quasar, and one BL Lacertae type object). The masses evaluated in this way are lower than those in earlier estimates (or upper limits) using different methods by a factor of 1 or 2 orders of magnitude, for some sources. We thus obtain a logarithmically averaged Eddington ratio of 1.4 for eight sources, or 0.47 for six Seyferts excluding a quasar and a BL Lacertae-type object. We discuss significance of this high Eddington ratio for the emission mechanism and the cosmological evolution of AGNs.