Properties of a Previously Unidentified Instrumental Signature in $\textit{Kepler/K2}$ that was Confused for AGN Variability

TL;DR

This paper identifies and characterizes a previously unknown instrumental noise in Kepler/K2 data affecting faint targets like AGN, which is linked to Sun-spacecraft orientation and temperature-dependent focus changes, impacting data analysis.

Contribution

The study uncovers a new instrumental signature in Kepler/K2 data related to Sun-spacecraft orientation and focus changes, aiding in data correction for AGN variability studies.

Findings

Instrumental signature correlates with Sun-spacecraft orientation.

Signature varies with detector radius and channel boundaries.

Temperature-dependent focus changes likely cause the systematics.

Abstract

The $\textit{Kepler}$ satellite potentially provides the highest precision photometry of active galactic nuclei (AGN) 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 $\textit{Kepler/K2}$ 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 AGN and other faint-target, guest-observer science proposals. Weakly illuminated pixels are dominated by low frequency trends that are both non-astrophysical and correlated from object to object. A critical clue to understanding this instrumental noise is that different targets observed in the same channels of Campaign 8 (rear facing) and Campaign 16 (forward facing) had nearly identical light curves after time reversal of one of the campaigns. This observation strongly suggests that the underlying problem relates to the relative Sun-spacecraft-field orientation, which was approximately the same on day 1 of Campaign 8 as the last day of Campaign 16. Furthermore, we measure that the instrumental signature lags in time as a function of radius from the center of the detector, crossing channel boundaries. Systematics documented in this investigation are unlikely to be due to Moir\'{e} noise, rolling band, or pointing jitter. Instead this work strongly suggests temperature-dependent focus changes that are further subject to channel variations. Further characterization of this signature is crucial for rehabilitating K2 data for use in investigations of AGN light curves.