Mid-IR Variability and Dust Reverberation Mapping of Low-$z$ Quasars. I.
  Data, Methods and Basic Results
  The continued operation of the Wide-field Infrared Survey Explorer (WISE)
combined with several ground-based optical transient surveys (e.g., CRTS,
ASAS-SN and PTF) offer an unprecedented opportunity to explore the dust
structures in luminous AGNs. We use these data for a mid-IR dust reverberation
mapping (RM) study of 87 archetypal Palomar-Green quasars at $z\lesssim0.5$. To
cope with various contaminations of the photometry data and the sparse time
sampling of the light curves, procedures to combine these datasets and retrieve
the dust RM signals have been developed. We find $\sim$70% of the sample (with
a completeness correction, up to 95%) have convincing mid-IR time-lags in the
WISE W1 ($\sim3.4~\mu m$) and W2 ($\sim4.5~\mu m$) bands and they are
proportional to the square root of the AGN luminosity. Combined with previous
K-band ($\sim2.2~\mu m$) RM results in the literature, the inferred dust
emission size ratios are $R_{K}:R_{W1}:R_{W2}=0.6:1:1.2$. Under simple
assumptions, we put preliminary constraints on the projected dust surface
density at these bands and reveal the possibly different torus structures among
hot-dust-deficient, warm-dust-deficient and normal quasars from the
reverberation signals. With multi-epoch Spitzer data and later WISE photometry,
we also explore AGN IR variability at 10--24 $\mu m$ over a 5-year time-scale.
Except for blazars and flat-spectrum radio sources, the majority of AGNs have
typical variation amplitudes at 24 $\mu m$ no more than 10% of that in the W1
band, indicating the dust reverberation signals damp out quickly at longer
wavelengths. In particular, steep-spectrum radio quasars also lack strong 24
$\mu m$ variability, consistent with the unification picture of radio-loud AGN.
