  The relation between the 2-10 keV, long term, excess variance and AGN black
hole mass is considered in this work. A significant anti-correlation is found
between these two quantities in the sense that the excess variance decreases
with increasing black hole mass. This anti-correlation is consistent with the
hypothesis that the 2-10 keV power spectrum in AGN follows a power law of slope
-2 at high frequencies. It then flattens to a slope of -1 below a break
frequency until a second break frequency below which it flattens to a slope of
zero. The ratio of the two break frequencies is equal to 10-30, similar to the
ratio of the respective frequencies in Cyg X-1. The power spectrum amplitude in
the "frequency x power" space does not depend on black hole mass. Instead it is
roughly equal to 0.02 in all objects. The high frequency break decreases with
increasing black hole mass according to the relation 1.5x(10^-6)/(BHmass/(10^7)
solar masses) Hz, in the case of "classical" Seyfert 1 galaxies. The excess
variance of NGC4051, a Narrow Line Seyfert 1 object, is larger than what is
expected for its black hole mass and X-ray luminosity. This can be explained if
its high frequency break is 20 times larger than the value expected in the case
of a "classical" Seyfert 1 with the same black hole mass. Finally, the excess
variance vs X-ray luminosity correlation is a byproduct of the excess variance
vs black hole mass correlation, with AGN accreting at ~ 0.1-0.15 the Eddington
limit. These results are consistent with recent results from the power spectral
analysis of AGN.
