  We study power density spectra (PDS) of X-ray flux variability in binary
systems where the accretion flow is truncated by the magnetosphere. PDS of
accreting X-ray pulsars where the neutron star is close to the corotation with
the accretion disk at the magnetospheric boundary, have a distinct break/cutoff
at the neutron star spin frequency. This break can naturally be explained in
the "perturbation propagation" model, which assumes that at any given radius in
the accretion disk stochastic perturbations are introduced to the flow with
frequencies characteristic for this radius. These perturbations are then
advected to the region of main energy release leading to a self-similar
variability of X-ray flux P~f^{-1...-1.5}. The break in the PDS is then a
natural manifestation of the transition from the disk to magnetospheric flow at
the frequency characteristic for the accretion disk truncation radius
(magnetospheric radius). The proximity of the PDS break frequency to the spin
frequency in corotating pulsars strongly suggests that the typical variability
time scale in accretion disks is close to the Keplerian one. In transient
accreting X-ray pulsars characterized by large variations of the mass accretion
rate during outbursts, the PDS break frequency follows the variations of the
X-ray flux, reflecting the change of the magnetosphere size with the accretion
rate. Above the break frequency the PDS steepens to ~f^{-2} law which holds
over a broad frequency range. These results suggest that strong f^{-1...-1.5}
aperiodic variability which is ubiquitous in accretion disks is not
characteristic for magnetospheric flows.
