Extracting Information From AGN Variability: an LSST AGN
Collaboration Proposal
Vishal Kasliwal, University of Pennsylvania and Princeton University
AGN exhibit rapid, high amplitude stochastic flux variability across the
entire electromagnetic spectrum on timescales ranging from hours to years.
The cause of this variability is poorly understood. We present a new
method for using variability to (1) measure the time-scales on which flux
perturbations evolve and (2) characterize the driving flux perturbations.
We model the observed light curve of an AGN as a linear differential
equation driven by stochastic impulses. Physically, the impulses could be
local `hot- and cold-spots' in the accretion disk—the linear differential
equation then governs how the spots evolve and dissipate. The impulse
response function of the accretion disk material is given by the Green’s
function of the linear differential equation. The timescales on which the
spots radiate energy is characterized by the power spectral density (PSD)
of the driving stochastic impulses. We analyze the light curve of the
Kepler AGN Zw 229-15 and find that the observed variability behavior can
be modeled as a damped harmonic oscillator perturbed by a colored noise
process. The model PSD turns over on time-scale 385 days. On shorter
time-scales, the log-PSD slope varies between 2 and 4, explaining the
behavior noted by previous studies. We recover and identify both the 5.6
day and 67 day timescales reported by previous work. These timescales
represent the time-scale on which flux perturbations grow, and the
time-scale on which flux perturbations decay back to the steady-state flux
level respectively. We present the software package used to study light
curves using this method, KALI, which is freely available to the
community. We conclude by discussing future prospects for the study of AGN
variability.