Discovery of a ~5 day characteristic timescale in the Kepler power spectrum of Zw 229-15

TL;DR

This study analyzes Kepler data of Zw 229-15 to identify a ~5 day characteristic variability timescale in optical light, revealing a feature previously observed only in X-ray spectra of AGN.

Contribution

The paper presents the first detection of a ~5 day bend in the optical power spectrum of an AGN, using high-quality Kepler data and two independent analysis methods.

Findings

Identification of a ~5 day bend in optical variability spectrum

Evidence of instrumental effects at high frequencies

Indication of a second longer timescale related to viscous processes

Abstract

We present time series analyses of the full Kepler dataset of Zw 229-15. This Kepler light curve --- with a baseline greater than three years, composed of virtually continuous, evenly sampled 30-minute measurements --- is unprecedented in its quality and precision. We utilize two methods of power spectral analysis to investigate the optical variability and search for evidence of a bend frequency associated with a characteristic optical variability timescale. Each method yields similar results. The first interpolates across data gaps to use the standard Fourier periodogram. The second, using the CARMA-based time-domain modeling technique of Kelly et al. (2014), does not need evenly-sampled data. Both methods find excess power at high frequencies that may be due to Kepler instrumental effects. More importantly both also show strong bends ({\Delta}{\alpha} ~ 2) at timescales of ~5 days, a feature similar to those seen in the X-ray PSDs of AGN but never before in the optical. This observed ~5 day timescale may be associated with one of several physical processes potentially responsible for the variability. A plausible association could be made with light-crossing, dynamical or thermal timescales, depending on the assumed value of the accretion disk size and on unobserved disk parameters such as {\alpha} and H/R. This timescale is not consistent with the viscous timescale, which would be years in a ~10^7 Solar mass AGN such as Zw 229-15. However there must be a second bend on long (>~1 year) timescales, and that feature could be associated with the viscous timescale.