New structures of Power Density Spectra for four Kepler AGNs
  Many nearby AGNs display a significant short-term variability. In this work
we re-analyze photometric data of four active galactic nuclei observed by
Kepler in order to study the flickering activity, having as main goal that of
searching for multiple components in the power density spectra. We find that
all four objects have similar characteristics, with two break frequencies at
approximately log(f/Hz)=-5.2 and -4.7. We consider some physical phenomena
whose characteristic time-scales are consistent with those observed, in
particular mass accretion fluctuations in the inner geometrically thick disc
(hot X-ray corona) and unstable relativistic Rayleigh-Taylor modes. The former
is supported by detection of the same break frequencies in the Swift X-ray data
of ZW229-15. We also discuss rms-flux relations, and we detect a possible
typical linear trend at lower flux levels. Our findings support the hypothesis
of a multiplicative character of variability, in agreement with the propagating
accretion fluctuation model.
