Asymmetric torus variability in active galactic nuclei driven by global brightening and dimming

TL;DR

This study investigates the asymmetry in mid-infrared variability of active galactic nuclei, revealing that hot-dust-rich and hot-dust-poor AGNs exhibit opposite temporal asymmetries linked to their dust emission evolution.

Contribution

It demonstrates that MIR variability asymmetry correlates with optical-to-MIR color, indicating intrinsic torus processes dominate MIR variability rather than accretion disk reflection.

Findings

Bluer AGNs show positive MIR asymmetry with larger brightening variability.

Redder AGNs exhibit negative MIR asymmetry with larger decay variability.

No significant asymmetry found in optical g-band variability.

Abstract

Temporal asymmetry in the flux variability of active galactic nuclei (AGNs) offers key insights into the physical mechanisms driving AGN variability. In this study, we investigated the variability of the torus by analyzing temporal asymmetry in the mid-infrared (MIR) continuum. We compared ensemble structure functions between the brightening and dimming phases for AGNs at $0.15<z<0.4$, using monitoring data in the optical from the Zwicky Transient Facility and in the MIR from the Near-Earth Object Wide-field Infrared Survey Explorer. We found that AGNs with bluer optical-to-MIR colors exhibit positive temporal asymmetry in the MIR, indicating that their variability amplitude is larger when brightening. Conversely, those with redder colors show negative asymmetry, exhibiting larger variability amplitude when decaying. However, there is no significant temporal asymmetry in the $g$-band variability driven by the accretion disk, suggesting that the temporal asymmetry in the MIR continuum primarily originates from intrinsic processes in the torus instead of the reflection of the ultraviolet-optical variability from the accretion disk. Analysis of the composite light curves revealed that AGNs with bluer optical-to-MIR colors tend to brighten gradually in the MIR, leading to the observed temporal asymmetry. This finding suggests that hot-dust-rich AGNs evolve with a gradual decline in hot dust emission, while hot-dust-poor AGNs are associated with a steady increase.