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

Abstract

We utilize mid-infrared multiepoch data from the Wide-field Infrared Survey Explorer over a ∼10 yr period in the W1 (3.4 μ m) and W2 (4.6 μ m) bands to investigate the structure of dusty torus in low-redshift (0.15 < z ≤ 0.4) active galactic nuclei (AGNs). We calculate a Spearman correlation coefficient ( r 12 ) between the 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 a 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 the 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 the covering factor of the hot dust component decreases as the AGN luminosity increases.