We analyse the scaling, of the X-ray power density spectra with the mass of
the black hole in the examples of Cyg X-1 and the Seyfert I galaxy NGC 554
8. We show that the high-frequency tail of the power density spectrum can b
e successfully used for the determination of the black hole mass. We determ
ine the masses of the black holes in six broad-line Seyfert 1 galaxies, fiv
e narrow-line Seyfert 1 galaxies and two quasi-stellar objects (QSOs) using
the available power density spectra. The proposed scaling is clearly appro
priate for other Seyfert,galaxies and QSOs. In all but one of the normal Se
yferts, the resulting luminosity to Eddington luminosity ratio is smaller t
han 0.15, with the source MCG -6-15-30 being an exception. The applicabilit
y of the same scaling to a narrow-line Seyfert 1 is less clear and there ma
y be a systematic shift between the power spectra of NLS1 and S1 galaxies o
f the same mass, leading to underestimation of the black hole mass. However
, both the method based on variability and the method based on spectral fit
ting show that those galaxies have relatively low masses and a high luminos
ity to Eddington luminosity ratio, supporting the view of those objects as
analogues of galactic sources in their high, soft or very high state, based
on the overall spectral shape. The bulge masses of their host galaxies are
similar to that of normal Seyfert galaxies, so they do not follow the blac
k hole mass-bulge mass relation for Seyfert galaxies, being evolutionarily
less advanced, as suggested by Mathur. The bulge mass-black hole mass relat
ion in our sample is consistent with being linear, with the black hole to b
ulge ratio similar to0.03 per cent, similar to Wandel and Laor for low-mass
objects, but significantly shifted from the relation of Magorrian et al. a
nd McLure & Dunlop.