Black hole masses from power density spectra: determinations and consequences

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 1 galaxy NGC 5548. We show that the high‐frequency tail of the power density spectrum can be successfully used for the determination of the black hole mass. We determine the masses of the black holes in six broad‐line Seyfert 1 galaxies, five narrow‐line Seyfert 1 galaxies and two quasi‐stellar objects (QSOs) using the available power density spectra. The proposed scaling is clearly appropriate for other Seyfert galaxies and QSOs. In all but one of the normal Seyferts, the resulting luminosity to Eddington luminosity ratio is smaller than 0.15, with the source MCG ‐6‐15‐30 being an exception. The applicability of the same scaling to a narrow‐line Seyfert 1 is less clear and there may be a systematic shift between the power spectra of NLS1 and S1 galaxies of the same mass, leading to underestimation of the black hole mass. However, both the method based on variability and the method based on spectral fitting show that those galaxies have relatively low masses and a high luminosity 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 black hole mass–bulge mass relation for Seyfert galaxies, being evolutionarily less advanced, as suggested by Mathur. The bulge mass–black hole mass relation in our sample is consistent with being linear, with the black hole to bulge ratio ∼0.03 per cent, similar to Wandel and Laor for low‐mass objects, but significantly shifted from the relation of Magorrian et al. and McLure & Dunlop.