  We develop a formalism to calculate energy-dependent fractional variability
(rms) in accretion flows. We consider rms spectra resulting from radial
dependencies of the level of local variability (as expected from propagation of
disturbances in accretion flows) assuming the constant shape of the spectrum
emitted at a given radius. We consider the cases when the variability of the
flow is either coherent or incoherent between different radial zones. As
example local emission, we consider blackbody, Wien and thermal Comptonization
spectra. In addition to numerical results, we present a number of analytical
formulae for the resulting rms. We also find an analytical formula for the disc
Wien spectrum, which we find to be a very good approximation to the disc
blackbody. We compare our results to the rms spectrum observed in an ultrasoft
state of GRS 1915+105.

Astrophysics