Massive black holes (BH) are now believed to power active galactic nuclei (e.g. quasars and Seyfert galaxies). BH detected in the centers of many nearby galaxies are linearly correlated with the luminosity of the host bulge (spheroid), the black hole mass being about 0.1% of the stellar mass. In active galaxies, the BH mass measured by reverberation mapping follows the same relation with the luminosity of the host galaxy as in ordinary (inactive) galaxies, with the exception of narrow line AGN which apparently have significantly lower BH/host mass/luminosity ratios. Then we review the empirical L-R ("Kaspi") relation between the luminosity and the size of the Broad-Line Region in AGN and derive a toy-model explanation. We also present a new method of estimating BH masses from the uctuations in their X-ray spectrum, which suggests a common mechanism for the X-ray radiation from accretion onto stellar and super massive BHs, over 8 orders of magnitude in BH mass.
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