Variability of Active Galactic Nuclei from the Optical to X-ray Regions

TL;DR

This paper reviews AGN variability across optical and X-ray regions, discussing mechanisms, geometrical effects, and challenging standard accretion disk models, with implications for understanding diverse AGN behaviors.

Contribution

It proposes that large-scale organization and geometry changes drive AGN variability, challenging the standard viscous timescale model and suggesting similar mechanisms for radio-loud and radio-quiet AGNs.

Findings

Optical variability can exceed X-ray variability in some AGNs.

Reprocessing of X-ray radiation is unlikely to explain optical variability.

Variability occurs on light-crossing timescales, not viscous timescales.

Abstract

Some progress in understanding AGN variability is reviewed. Reprocessing of X-ray radiation to produce significant amounts of longer-wavelength continua seems to be ruled out. In some objects where there has been correlated X-ray and optical variability, the amplitude of the optical variability has exceeded the amplitude of X-ray variability. We suggest that accelerated particles striking material could be linking X-ray and optical variability (as in activity in the solar chromosphere). Beaming effects could be significant in all types of AGN. The diversity in optical/X-ray relationships at different times in the same object, and between different objects, might be explained by changes in geometry and directions of motion relative to our line of sight. Linear shot-noise models of the variability are ruled out; instead there must be large-scale organization of variability. Variability occurs on light-crossing timescales rather than viscous timescales and this probably rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and radio-quiet AGNs have similar continuum shapes and similar variability properties. This suggests similar continuum origins and variability mechanisms. Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not show extreme optical variability.