Accretion Disc Time Lag Distributions: Applying CREAM to Simulated AGN Light Curves

TL;DR

This paper introduces CREAM, a new MCMC-based method to model and analyze time delays in AGN light curves caused by accretion disc reprocessing, enabling estimation of disc parameters from synthetic data.

Contribution

CREAM is a novel approach that models AGN continuum variations using a lamp post model and infers disc parameters and driving light curves from simulated observations.

Findings

CREAM accurately recovers disc inclination within 5 degrees at high SNR.

CREAM estimates the product of black hole mass and accretion rate with 0.04 dex precision.

The method performs well on synthetic light curves with realistic observational noise.

Abstract

Active Galactic Nuclei (AGN) vary in their brightness across all wavelengths. Moreover, longer wavelength ultraviolet - optical continuum light curves appear to be delayed with respect to shorter wavelength light curves. A simple way to model these delays is by assuming thermal reprocessing of a variable point source (a lamp post) by a blackbody accretion disc. We introduce a new method, CREAM (\textbf{C}ontinuum \textbf{RE}processed \textbf{A}GN \textbf{M}arkov Chain Monte Carlo), that models continuum variations using this lamp post model. The disc light curves lag the lamp post emission with a time delay distribution sensitive to the disc temperature-radius profile and inclination. We test CREAM's ability to recover both inclination and product of black hole mass and accretion rate $\mmdot$, and show that the code is also able to infer the shape of the driving light curve. CREAM is applied to synthetic light curves expected from 1000 second exposures of a 17th magnitude AGN with a 2m telescope in Sloan g and i bands with signal to noise of 500 - 900 depending on the filter and lunar phase. We also tests CREAM on poorer quality g and i light curves with SNR = 100. We find in the high SNR case that CREAM can recover the accretion disc inclination to within an uncertainty of 5 degrees and an $\mmdot$ to within 0.04 dex.