Extreme AGN variability: evidence of magnetically elevated accretion?

TL;DR

This paper suggests that magnetically supported, geometrically thick accretion discs can explain rapid, large amplitude variability in active galactic nuclei, challenging traditional thin disc models.

Contribution

It introduces magnetically elevated accretion discs as a plausible explanation for extreme AGN variability, with shorter inflow times than standard models.

Findings

Magnetically supported discs are geometrically thick at all luminosities.

Inflow times in these discs can be as short as a few years.

Future observations can distinguish between proposed models.

Abstract

Rapid, large amplitude variability at optical to X-ray wavelengths is now seen in an increasing number of Seyfert galaxies and luminous quasars. The variations imply a global change in accretion power, but are too rapid to be communicated by inflow through a standard thin accretion disc. Such discs are long known to have difficulty explaining the observed optical/UV emission from active galactic nuclei. Here we show that alternative models developed to explain these observations have larger scale heights and shorter inflow times. Accretion discs supported by magnetic pressure in particular are geometrically thick at all luminosities, with inflow times as short as the observed few year timescales in extreme variability events to date. Future time-resolved, multi-wavelength observations can distinguish between inflow through a geometrically thick disc as proposed here, and alternative scenarios of extreme reprocessing of a central source or instability-driven limit cycles.