Are the Variability Properties of the Kepler AGN Light Curves Consistent with a Damped Random Walk?

TL;DR

This study analyzes 20 Kepler AGN light curves to test if their variability aligns with the damped random walk model, revealing diverse behaviors and suggesting the need for more complex models.

Contribution

It provides the first detailed comparison of Kepler AGN variability with the DRW model, identifying limitations and proposing a more nuanced understanding of AGN variability.

Findings

Less than half of the AGN are consistent with the DRW model.

Observed short-timescale variability indicates complex behavior.

Presence of dips and slope changes in structure functions.

Abstract

We test the consistency of active galactic nuclei (AGN) optical flux variability with the $\textit{damped random walk}$ (DRW) model. Our sample consists of 20 multi-quarter $\textit{Kepler}$ AGN light curves including both Type 1 and 2 Seyferts, radio-loud and -quiet AGN, quasars, and blazars. $\textit{Kepler}$ observations of AGN light curves offer a unique insight into the variability properties of AGN light curves because of the very rapid ($11.6-28.6$ min) and highly uniform rest-frame sampling combined with a photometric precision of $1$ part in $10^{5}$ over a period of 3.5 yr. We categorize the light curves of all 20 objects based on visual similarities and find that the light curves fall into 5 broad categories. We measure the first order structure function of these light curves and model the observed light curve with a general broken power-law PSD characterized by a short-timescale power-law index $\gamma$ and turnover timescale $\tau$. We find that less than half the objects are consistent with a DRW and observe variability on short timescales ($\sim 2$ h). The turnover timescale $\tau$ ranges from $\sim 10-135$ d. Interesting structure function features include pronounced dips on rest-frame timescales ranging from $10-100$ d and varying slopes on different timescales. The range of observed short-timescale PSD slopes and the presence of dip and varying slope features suggests that the DRW model may not be appropriate for all AGN. We conclude that AGN variability is a complex phenomenon that requires a more sophisticated statistical treatment.