  We present the results of a detailed study of the X-ray power spectra density
(PSD) functions of twelve X-ray bright AGN, using almost all the archival
XMM-Newton data. The total net exposure of the EPIC-pn light curves is larger
than 350 ks in all cases (and exceeds 1 Ms in the case of 1H 0707-497). In a
physical scenario in which X-ray reflection occurs in the inner part of the
accretion disc of AGN, the X-ray reflection component should be a filtered echo
of the X-ray continuum signal and should be equal to the convolution of the
primary emission with the response function of the disc. Our primary objective
is to search for these reflection features in the 5-7 keV (iron line) and 0.5-1
keV (soft) bands, where the X-ray reflection fraction is expected to be
dominant. We fit to the observed periodograms two models: a simple bending
power law model (BPL) and a BPL model convolved with the transfer function of
the accretion disc assuming the lamp-post geometry and X-ray reflection from a
homogeneous disc. We do not find any significant features in the best-fitting
BPL model residuals either in individual PSDs in the iron band, soft and full
band (0.3-10 keV) or in the average PSD residuals of the brightest and more
variable sources (with similar black hole mass estimates). The typical
amplitude of the soft and full-band residuals is around 3-5 per cent. It is
possible that the expected general relativistic effects are not detected
because they are intrinsically lower than the uncertainty of the current PSDs,
even in the strong relativistic case in which X-ray reflection occurs on a disc
around a fast rotating black hole having an X-ray source very close above it.
However, we could place strong constrains to the X-ray reflection geometry with
the current data sets if we knew in advance the intrinsic shape of the X-ray
PSDs, particularly its high frequency slope.
