  We report on the results of the first XMM systematic "excess variance" study
of all the radio quiet, X-ray un-obscured AGN. The entire sample consist of 161
sources observed by XMM for more than 10 ks in pointed observations which is
the largest sample used so far to study AGN X-ray variability on time scales
less than a day. We compute the excess variance for all AGN, on different
time-scales (10, 20, 40 and 80 ks) and in different energy bands (0.3-0.7,
0.7-2 and 2-10 keV). We observe a highly significant and tight (~0.7 dex)
correlation between excess variance and MBH. The subsample of reverberation
mapped AGN shows an even smaller scatter (~0.45 dex) comparable to the one
induced by the MBH uncertainties. This implies that X-ray variability can be
used as an accurate tool to measure MBH and this method is more accurate than
the ones based on single epoch optical spectra. The excess variance vs.
accretion rate dependence is weaker than expected based on the PSD break
frequency scaling, suggesting that both the PSD high frequency break and the
normalisation depend on accretion rate in such a way that they almost
completely counterbalance each other. A highly significant correlation between
excess variance and 2-10 keV spectral index is observed. Both the variability
vs. LBol and FWHM_Hbeta correlations are consistent with being just by-products
of the correlation with MBH. The soft and medium variability is very well
correlated with the hard variability, suggesting that the additional soft
components (i.e. soft excess, warm absorber) add a minor contribution to the
total variability. Once the variability is rescaled for MBH and mdot, no
significant difference between narrow-line and broad-line Seyfert 1 is
observed. The results are in agreement with a picture where, to first
approximation, all local AGN have the same variability properties once rescaled
for MBH and accretion rate.
