# Long Term Photometric Variability in Kepler Full Frame Images: Magnetic Cycles of Sun-Like Stars

## TL;DR

This study introduces f3, an open-source tool for analyzing long-term stellar brightness variations in Kepler FFIs, revealing magnetic activity cycles and variability patterns in Sun-like stars over multi-year periods.

## Contribution

The paper presents f3, a new open-source photometry package for Kepler FFIs, enabling long-term variability studies of Sun-like stars, and uncovers magnetic cycle transitions and activity patterns.

## Key findings

- Approximately 10% of Sun-like stars show long-term flux variability.
- Most stars exhibit either correlated or anticorrelated brightness changes with starspot activity.
- A transition from starspot-dominated to facula-dominated variability occurs between 15 and 25 days rotation period.

## Abstract

Photometry from the Kepler mission is optimized to detect small, short duration signals like planet transits at the expense of long-term trends. This long-term variability can be recovered in photometry from the Full Frame Images (FFIs), a set of calibration data collected approximately monthly during the Kepler mission. Here, we present f3, an open-source package to perform photometry on the Kepler FFIs in order to detect changes in the brightness of stars in the Kepler field of view over long time baselines. We apply this package to a sample of 4,000 Sun-like stars with measured rotation periods. We find $\approx 10\%$ of these targets have long-term variability in their observed flux. For the majority of targets we identify the luminosity variations are either correlated or anticorrelated with the short-term variability due to starspots on the stellar surface. We find a transition between anticorrelated (starspot-dominated) variability and correlated (facula-dominated) variability between rotation periods of 15 and 25 days, suggesting the transition between the two modes is complete for stars at the age of the Sun. We also identify a sample of stars with apparently complete cycles, as well as a collection of short-period binaries with extreme photometric variation over the Kepler mission.