A Spitzer IRAC Measure of the Zodiacal Light
  The dominant non-instrumental background source for space-based infrared
observatories is the zo- diacal light. We present Spitzer Infrared Array Camera
(IRAC) measurements of the zodiacal light at 3.6, 4.5, 5.8, and 8.0 {\mu}m,
taken as part of the instrument calibrations. We measure the changing surface
brightness levels in approximately weekly IRAC observations near the north
ecliptic pole (NEP) over the period of roughly 8.5 years. This long time
baseline is crucial for measuring the annual sinusoidal variation in the signal
levels due to the tilt of the dust disk with respect to the ecliptic, which is
the true signal of the zodiacal light. This is compared to both Cosmic
Background Explorer Diffuse Infrared Background Experiment (COBE DIRBE) data
and a zodiacal light model based thereon. Our data show a few percent
discrepancy from the Kelsall et al. (1998) model including a potential warping
of the interplanetary dust disk and a previously detected overdensity in the
dust cloud directly behind the Earth in its orbit. Accurate knowledge of the
zodiacal light is important for both extragalactic and Galactic astronomy
including measurements of the cosmic infrared background, absolute measures of
extended sources, and comparison to extrasolar interplanetary dust models. IRAC
data can be used to further inform and test future zodiacal light models.
