On the efficient acceleration of clouds in active galactic nuclei

TL;DR

This paper demonstrates that time-varying radiation flux can significantly enhance cloud acceleration in active galactic nuclei, potentially explaining observed line widths, through hydrodynamical simulations.

Contribution

It introduces the effect of flux variability on cloud acceleration in AGN and quantifies its impact using hydrodynamical simulations, showing increased efficiency over steady flux models.

Findings

A 20% flux modulation doubles cloud acceleration.

Time-varying flux increases line widths consistent with observations.

Hydrodynamic turbulence occurs but does not negate acceleration enhancement.

Abstract

In the broad line region of AGN, acceleration occurs naturally when a cloud condenses out of the hot confining medium due to the increase in line opacity as the cloud cools. However, acceleration by radiation pressure is not very efficient when the flux is time-independent, unless the flow is one-dimensional. Here we explore how acceleration is affected by a time-varying flux, as AGN are known to be highly variable. If the period of flux oscillations is longer than the thermal timescale, we expect the gas to cool during the low flux state, and therefore line opacity should quickly increase. The cloud will receive a small kick due to the increased radiation force. We perform hydrodynamical simulations using Athena to confirm this effect and quantify its importance. We find that despite the flow becoming turbulent in 2D due to hydrodynamic instabilities, a 20% modulation of the flux leads to a net increase in acceleration --- by more than a factor of 2 --- in both 1D and 2D. We show that this is sufficient to produce the observed line widths, although we only consider optically thin clouds. We discuss the implications of our results for photoionization modeling and reverberation mapping.