WISE view of changing-look AGNs: evidence for a transitional stage of AGNs

TL;DR

This study investigates changing-look AGNs using WISE mid-infrared data, revealing their transitional accretion states and correlating IR variability with optical changes, challenging traditional unification models.

Contribution

It provides new mid-IR observational evidence for the transitional accretion states of changing-look AGNs, linking IR variability to optical changes and accretion flow models.

Findings

Mid-IR luminosities of CLAGNs are between LLAGNs and QSOs.

Eddington-scaled mid-IR luminosities indicate a transition between RIAF and SSD.

Time lags in IR variability correlate with bolometric luminosity, following QSO patterns.

Abstract

The discovery of changing-look active galactic nuclei (CLAGNs) with the significant change of optical broad emission lines (optical CLAGNs) and/or strong variation of line-of-sight column densities (X-ray CLAGNs) challenges the orientation-based AGN unification model. We explore mid-infrared (mid-IR) properties for a sample of 57 optical CLAGNs and 11 X-ray CLAGNs based on the {\it Wide-field Infrared Survey Explorer} ({\it WISE}) archive data. We find that Eddington-scaled mid-IR luminosities of both optical and X-ray CLAGNs stay just between low-luminosity AGNs (LLAGNs) and luminous QSOs. The average Eddington-scaled mid-IR luminosities for optical and X-ray CLAGNs are $\sim 0.4$\% and $\sim 0.5$\%, respectively, which roughly correspond the bolometric luminosity of transition between a radiatively inefficient accretion flow (RIAF) and Shakura-Sunyaev disk (SSD). We estimate the time lags of the variation in the mid-IR behind that in the optical band for 13 CLAGNs with strong mid-IR variability, where the tight correlation between the time lag and the bolometric luminosity ($\tau - L$) for CLAGNs roughly follows that found in the luminous QSOs.