  The masses of supermassive black holes in active galactic nuclei (AGN) can be
derived spectroscopically via virial mass estimators based on selected broad
optical/ultraviolet emission lines. These estimates commonly use the line width
as a proxy for the gas speed and the monochromatic continuum luminosity as a
proxy for the radius of the broad line region. However, if the size of the
broad line region scales with bolometric rather than monochromatic AGN
luminosity, mass estimates based on different emission lines will show a
systematic discrepancy which is a function of the color of the AGN continuum.
This has actually been observed in mass estimates based on H-alpha / H-beta and
C IV lines, indicating that AGN broad line regions indeed scale with bolometric
luminosity. Given that this effect seems to have been overlooked as yet,
currently used single-epoch mass estimates are likely to be biased.
