  Scaling relations are the most powerful astrophysical tools to set
constraints to the physical mechanisms of astro- nomical sources and to infer
properties that cannot be accessed directly. We re-investigate here one of
these scaling relations in active galactic nuclei (AGN); the so-called X-ray
variability plane (or mass-luminosity-timescale relation, McHardy et al. 2006).
This relation links the power-spectral density (PSD) break frequency with the
super-massive black hole (SMBH) mass and the bolometric luminosity. We used
available XMM -Newton observations of a sample of 22 AGN to study the PSD and
spectra in short segments within each observation. This allows us to report for
the first time that the PSD break frequency varies for each object, showing
variations in 19 out of the 22 AGN analyzed. Our analysis of the variability
plane confirms the relation between the break frequency and the SMBH mass and
finds that the obscuration along the line of sight NH (or the variations on the
obscuration using its standard deviation, $\rm{\Delta}$(NH)) is also a required
parameter, at least for the range of frequencies analyzed here
(3x10E-5-5x10E-2Hz). We constrain a new variability plane of the form:
log($\nu_{Break}$) = (-0.589$\rm{\pm}$0.005) log(MBH ) + (0.10$\rm{\pm}$0.01)
log(NH ) - (1.5$\rm{\pm}$0.3) (or
log($\nu_{Break}$)=(-0.549$\rm{\pm}$0.009)log(MBH)+(0.56$\rm{\pm}$0.06)$\rm{\Delta}$(NH)+(0.19$\rm{\pm}$0.08)).
The X-ray variability plane found by McHardy et al. (2006) is roughly recovered
when we use unobscured segments. We speculate that this behavior is well
explained if most of the reported frequencies are related to inner clouds
(within 1pc), following Kepler orbits under the gravitational field of the
SMBH.
