The Kepler Light Curves of AGN: A Detailed Analysis

Abstract

We present a comprehensive analysis of 21 light curves of Type 1 AGN from the Kepler spacecraft. First, we describe the necessity and development of a customized pipeline for treating Kepler data of stochastically variable sources like AGN. We then present the light curves, power spectral density functions (PSDs), and flux histograms. The light curves display an astonishing variety of behaviors, many of which would not be detected in ground-based studies, including switching between distinct flux levels. Six objects exhibit PSD flattening at characteristic timescales which roughly correlate with black hole mass. These timescales are consistent with orbital timescales or freefall accretion timescales. We check for correlations of variability and high-frequency PSD slope with accretion rate, black hole mass, redshift and luminosity. We find that bolometric luminosity is anticorrelated with both variability and steepness of the PSD slope. We do not find evidence of the linear rmsflux relationships or lognormal flux distributions found in X-ray AGN light curves, indicating that reprocessing is not a significant contributor to optical variability at the 0.1−10% level.

Corona-heated Accretion-disk Reprocessing: A Physical Model to Decipher the Melody of AGN UV/Optical Twinkling

“…We obtain mock light curves for two cases, i.e., α = 0.01 and α = 0.2. The former case (i.e., α = 0.01) corresponds to the results of some recent radiation MHD shearing box simulations (Blaes 2014); the latter case (i.e., Barth et al (2011); for others, see Smith et al (2018a)). (3) The dimensionless accretion rateṁ =Ṁ /Ṁ Edd , whereṀ Edd = 10L Edd /c 2 .…”

“…A damped random walk (DRW) process (whose power spectral density, PSD, P (f ) ∝ 1/(f 2 0 + f 2 ), where f 0 = 1/τ is the damping frequency) seems to be able to describe AGN UV/optical variability on timescales of months to years (Kelly et al 2009;MacLeod et al 2010;Zu et al 2013). On very short timescales (e.g., days), the observed variability amplitude is lower than the prediction of the DRW model (Mushotzky et al 2011;Kasliwal et al 2015;Smith et al 2018a). On very long timescales (e.g., several decades), the DRW model seems to under-predict the actual variability amplitude (MacLeod et al 2012;Guo et al 2017).…”

“…We apply our CHAR model to explain two different sets of real observations. First, we consider the long-duration (∼ 3 years), high-cadence (∼ 30 min) Kepler light curves of three AGNs (namely Zw 229-15, kplr 12158940 and kplr 2694186) which have reliable black-hole mass measurements (Barth et al 2011;Smith et al 2018a). Second, we focus on the multi-wavelength light curves of NGC 5548.…”

“…We select Kepler AGNs with M BH estimates (via the reverberation-mapping or the single-epoch virial black-hole mass estimators) from Barth et al (2011) and Table 1 of Smith et al (2018a). The ∼ 3-year Kepler light curves were broken into multiple segments due to instrumental effects.…”

“…For kplr 2694186, the observed structure function has a dip feature around ∆t = 400 days which indicates periodicity in this source. However, this might be caused by instrumental effects (Smith et al 2018a). Note that when generating the model light curves, our simulations include the same time-sampling issues and photometric errors as the structure functions calculated from the real data.…”