CAIXA: a catalogue of AGN in the XMM-Newton archive III. Excess Variance Analysis

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.