Kavli Affiliate: Luis C. Ho
| First 5 Authors: Suyeon Son, Minjin Kim, Luis C. Ho, ,
| Summary:
Using the multi-epoch mid-infrared (MIR) photometry from the Wide-field
Infrared Survey Explorer spanning a baseline of $sim10$ yr, we extensively
investigate the MIR variability of nearby active galactic nuclei (AGNs) at
$0.15 < z < 0.4$. We find that the ensemble structure function in the W1 band
($3.4 mu$m) can be modeled with a broken power law. Type 1 AGNs tend to
exhibit larger variability amplitudes than type 2 AGNs, possibly due to the
extinction by the torus. The variability amplitude is inversely correlated with
the AGN luminosity, consistent with a similar relation known in the optical.
Meanwhile, the slope of the power law increases with AGN luminosity. This trend
can be attributed to the fact that the inner radius of the torus is
proportional to the AGN luminosity, as expected from the size$-$luminosity
relation of the torus. Interestingly, low-luminosity type 2 AGNs, unlike
low-luminosity type 1 AGNs, tend to exhibit smaller variability amplitude than
do high-luminosity AGNs. We argue that either low-luminosity type 2 AGNs have
distinctive central structures due to their low luminosity or their MIR
brightness is contaminated by emission from the cold dust in the host galaxy.
Our findings suggest that the AGN unification scheme may need to be revised. We
find that the variability amplitude of dust-deficient AGNs is systematically
larger than that of normal AGNs, supporting the notion that the hot and warm
dust in dust-deficient AGNs may be destroyed and reformed according to the
strength of the ultraviolet radiation from the accretion disk.
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