2019MNRAS.483...38D

We analysed the light curves of four active galactic nuclei (AGNs) 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 reanalysis of these Swift data and test simulations, we now distinguish the instrumental artefact 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 time-scales of the Kepler artefact and Swift features is coincidental.