Quasar Variability in the Mid-Infrared
  The Decadal IRAC Bootes Survey is a mid-IR variability survey of the ~9 sq.
deg. of the NDWFS Bootes Field and extends the time baseline of its
predecessor, the Spitzer Deep, Wide-Field Survey (SDWFS), from 4 to 10 years.
The Spitzer Space Telescope visited the field five times between 2004 and 2014
at 3.6 and 4.5 microns. We provide the difference image analysis photometry for
a half a million mostly extragalactic sources. In mid-IR color-color plane,
sources with quasar colors constitute the largest variability class (75%), 16%
of the variable objects have stellar colors and the remaining 9% have the
colors of galaxies. Adding the fifth epoch doubles the number of variable
active galactic nuclei (AGNs) for the same false positive rates as in SDWFS, or
increases the number of sources by 20% while decreasing the false positive
rates by factors of 2-3 for the same variability amplitude. We quantify the
ensemble mid-IR variability of ~1500 spectroscopically confirmed AGNs using
single power-law structure functions (SFs), which we find to be steeper (index
$\gamma=0.45$) than in the optical ($\gamma=0.3$), leading to much lower
amplitudes at short time-lags. This provides evidence for large emission
regions, smoothing out any fast UV/optical variations, as the origin of
infrared quasar variability. The mid-IR AGN SF slope $\gamma$ seems to be
uncorrelated with both the luminosity and rest-frame wavelength, while the
amplitude shows an anti-correlation with the luminosity and a correlation with
the rest-frame wavelength.
