  We analysed the light curves of four active galactic nuclei (AGN) from the
Kepler field, and find multicomponent power density spectra with characteristic
frequencies that are surprisingly similar to other Kepler AGNs (including
ZW229-15). An identical time series analysis of randomly selected planet
candidate stars revealed the same features, suggesting an instrumental origin
for the variability. This result is enigmatic, as these signals have been
confirmed for ZW229-15 using independent observations from Swift. Based on our
re-analysis of these Swift data and test simulations, we now distinguish the
instrumental artifact in Kepler data from the real pattern in Swift
observations. It appears that some other AGNs observed with instruments such as
XMM-Newton show similar frequency components. This supports the conclusion that
the similarity between the variability timescales of the Kepler artifact and
real Swift features is coincidental.
