Tests for Standard Accretion Disk Models by Variability in Active Galactic Nuclei

TL;DR

This study tests standard accretion disk models of AGNs using variability data from 26 objects, finding general consistency with model predictions, especially regarding variability time scales and their relation to black hole mass.

Contribution

It provides observational evidence supporting standard accretion disk models through variability analysis, including time scales and inter-band lags, with implications for disk viscosity and thermal processes.

Findings

Measured variability time scales align with thermal time scales of standard disks.

A linear relation exists between variability time scales and black hole mass.

Time lags between bands are consistent with continuum thermal reprocessing in some AGNs.

Abstract

In this paper, standard accretion disk models of AGNs are tested using light curves of 26 objects well observed for reverberation mapping. Time scales of variations are estimated by the most common definition of the variability time scale and the zero-crossing time of the autocorrelation function of the optical light curves for each source. The measured time scales of variations by the two methods are consistent with each other. If the typical value of the viscosity parameter $\alpha \sim 0.1$ is adopted, the measured optical variability time scales are most close to the thermal time scales of the standard disks. If $\alpha$ is allowed to range from $\sim 0.03$ to $\sim 0.2$, the measured time scales are consistent with the thermal time scales of the standard disks. There is a linear relation between the measured variability time scales and black hole masses, and this linear relation is qualitatively consistent with expectation of the standard accretion disk models. The time lags measured by the ZDCF between different bands are on the order of days. The measured time lags of NGC 4151 and NGC 7469 are marginally consistent with the time lags estimated in the case of continuum thermal reprocessing for the standard accretion disk models. However, the measured time lags of NGC 5548 and Fairall 9 are unlikely to be the case of continuum thermal reprocessing. Our results are unlikely to be inconsistent with or are likely to be conditionally in favor of the standard accretion disk models of AGNs.