  There is increasing evidence that supermassive black holes in active galactic
nuclei (AGN) are scaled-up versions of Galactic black holes. We show that the
amplitude of high-frequency X-ray variability in the hard spectral state is
inversely proportional to the black hole mass over eight orders of magnitude.
We have analyzed all available hard-state data from RXTE of seven Galactic
black holes. Their power density spectra change dramatically from observation
to observation, except for the high-frequency (>10 Hz) tail, which seems to
have a universal shape, roughly represented by a power law of index -2. The
amplitude of the tail, C_M (extrapolated to 1 Hz), remains approximately
constant for a given source, regardless of the luminosity, unlike the break or
QPO frequencies, which are usually strongly correlated with luminosity.
Comparison with a moderate-luminosity sample of AGN shows that the amplitude of
the tail is a simple function of black hole mass, C_M = C/M, where C = 1.25
M_Sol / Hz. This makes C_M a robust estimator of the black hole mass which is
easy to apply to low- to moderate-luminosity supermassive black holes. The
high-frequency tail with its universal shape is an invariant feature of a black
hole and, possibly, an imprint of the last stable orbit.
