Mid-infrared Variability of Low-redshift Active Galactic Nuclei : Constraints on a Hot Dust Component with a Variable Covering Factor

TL;DR

This study uses a decade of mid-infrared data from WISE to explore dusty torus structure in low-redshift AGN, revealing correlations between variability, dust covering factor, and luminosity, suggesting a decreasing hot dust covering factor with increasing AGN brightness.

Contribution

It provides new insights into the variability of hot dust in AGN and proposes a luminosity-dependent decrease in hot dust covering factor, based on long-term mid-infrared observations.

Findings

Correlation between W1 magnitude and W1-W2 color varies across objects.

Hot dust covering factor likely decreases as AGN luminosity increases.

Host galaxy contribution affects spectral energy distribution but does not alter main correlations.

Abstract

We utilize mid-infrared multi-epoch data from Wide-field Infrared Survey Explorer (WISE) over a $\sim10$-year period in W1 (3.4$\mu$m) and W2 (4.6$\mu$m) bands to investigate the structure of dusty torus in low-redshift ($0.15<z\le 0.4$) active galactic nuclei (AGN). We calculate a Spearman correlation coefficient ($r_{12}$) between W1 magnitude and W1$-$W2 color based on the light curve in individual objects. Interestingly, $r_{12}$ spans a broad range from $-1$ to $1$, and is detected to be correlated with mean W1$-$W2 color and AGN bolometric luminosity, in the sense that objects with blue W1$-$W2 color and low AGN luminosity tend to become redder (bluer) with increasing (decreasing) W1 brightness in the light curve (i.e. $r_{12} < 0$), although the correlation of $r_{12}$ with the bolometric luminosity is relatively weak. The fit for the spectral energy distribution reveals a significant contribution from the host galaxy in the W1 and W2 bands. However, the dependencies of $r_{12}$ on W1$-$W2 color and AGN luminosity still persist even after careful elimination of the host light contribution. We propose that this result can be explained if covering factor of the hot dust component decreases as AGN luminosity increases.