Properties of a Previously Unidentified Instrumental Signature in Kepler/K2 That was Confused for AGN Variability

The Kepler satellite potentially provides the highest precision photometry of active galactic nuclei (AGNs) available to investigate short-timescale optical variability. We targeted quasars from the Sloan Digital Sky Survey that lie in the fields of view of the Kepler/K2 campaigns. Based on those observations, we report the discovery and properties of a previously unidentified instrumental signature in K2. Systematic errors in K2, beyond those due to the motion of the detector, plague our AGNs and other faint-target, guest observer science proposals. Weakly illuminated pixels are dominated by low-frequency trends that are both nonastrophysical and correlated from object to object. The instrumental signature lags in time as a function of radius from the center of the detector, crossing channel boundaries. Thus, systematics documented in this investigation are unlikely to be due to Moire noise, rolling band, or pointing jitter. A critical clue to understanding this instrumental systematic is that different targets observed in the same channels of Campaign 8 (rear facing) and Campaign 16 (forward facing) have nearly identical light curves after time reversal of one of the campaigns. We find evidence of temperature trends that also reverse according to the Sun-spacecraft field orientation and that may dominate the systematics. These temperature variations are larger in K2 than in the nominal Kepler mission and strongly support our hypothesis of temperature-driven focus changes. Further characterization of this signature is crucial for rehabilitating K2 data for use in investigations of AGN light curves.