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We present and analyze the variability of three Seyfert galaxies on intra-daytimescales. We have analyzed in a uniform manner the 38 longest ( $>30$ ks)observations made for NGC 4051, MCG$-06-30-15$ and NGC 4151 by XMM-Newtonbetween 2000 and 2015. The nuclei were quite active during essentially all ofthese observations and the overall X-ray fluxes (0.3-10 keV) varied by an orderof magnitude. Most of the observations do appear to show characteristictimescales, estimated through their auto-correlation functions, ranging between$\sim 2.9$ ks and $\sim 45.3$ ks. The hard (2-10 keV) and soft (0.3-02 keV)bands are very well correlated but consideration of their hardness ratios showthat the sources typically soften during flares. We also provide new estimatesof the central black hole masses for these three AGN that support thehypothesis that Narrow Line Seyfert 1 galaxies have relatively small centralblack holes.

Intra-day variability of three Seyfert Galaxies measured with XMM-Newton