  We study fractional variability as a function of energy from black-hole X-ray
binaries on timescales from milliseconds to hundreds of seconds. We build a
theoretical model of energy-dependent variability in which the X-ray energy
spectrum varies in response to a changing physical parameter. We compare these
models to rms spectra obtained from RXTE PCA observations of black-hole
binaries XTE J1550-564 and XTE J1650-500. We show that two main variability
models are consistent with the data: variable seed photon input in the hard
state and variable power in the Comptonized component in the soft and very high
states. The lack of clear reflection features in the rms spectra implies that
the reflection and the X-ray continuum, when integrated over Fourier
frequencies, are correlated and vary with similar fractional amplitudes. Our
models predict two important features of rms spectra, not possible to be
clearly seen by the PCA due sensitivity limits. At soft X-rays, >~3 keV, we
predict the presence of a break in the rms spectrum at energy directly related
to the seed photon temperature. At higher energies, ~20--30 keV, we predict a
peak in the rms spectrum originating from the variability of the spectrum
produced by a hybrid thermal/non-thermal electron distribution. If these
features are confirmed by broad-band observations, they will impose important
constraints on the origin of the seed photons for Comptonization and the
electron distribution in the hot plasma.
