  Besides the narrow peaks originating from the periodic signal, the power
spectra of accreting X-ray pulsars display continuum components usually
increasing towards low frequencies; these arise from the source aperiodic
variability. Most studies so far adopted the view that the periodic and
aperiodic variations are independent. However any aperiodic variability in the
emission from the accretion column(s) towards the magnetic neutron star should
be modulated at the X-ray pulsar period, by virtue of the same rotation-induced
geometric effects which give rise to the periodic signal. We develop here a
simple shot noise model to test the presence of a coupling between the periodic
and aperiodic variability of X-ray pulsars. The model power spectrum is fit to
the power spectra of three X-ray pulsars, Vela X-1, 4U 1145-62 and Cen X-3
observed with EXOSAT. In the first two cases we find that a highly significant
coupling is present, as testified by a substantialbroadening in the wings of
the power spectrum peaks due to the periodic modulation. We find also that
these wings can mimick the presence of a knee in the continuum power spectrum
components around the fundamental of the periodic modulation, therefore
questioning the correlation reported by Takeshima (1992) between the X-ray
pulsar frequency and the knee frequency, beyond which the continuum power
spectral component steepens.
