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