We use the multi-epoch, mid-infrared Spitzer Deep, Wide-Field Survey toinvestigate the variability of 474,179 objects in 8.1 deg^2 of the NDWFS Bootesfield. We perform a Difference Image Analysis of the four available epochsbetween 2004 and 2008, focusing on the deeper 3.6 and 4.5 micron bands. We findthat 1.1% of the studied sample meet our standard selection criteria for beingclassed as a variable source. We require that the 3.6 and 4.5 micronlight-curves are strongly correlated (r>0.8) and that their joint varianceexceeds that for all sources with the same magnitude by 2 sigma. We thenexamine the mid-IR colors of the variable sources and match them with X-raysources from the XBootes survey, radio catalogs, 24 micron-selected AGNcandidates, and spectroscopically identified AGNs from the AGN and GalaxyEvolution Survey (AGES). Based on their mid-IR colors, most of the variablesources are AGNs (76%), with smaller contributions from stars (11%), galaxies(6%), and unclassified objects. Most of the stellar, galaxy and unclassifiedsources are false positives. For our standard selection criteria, 11-12% of themid-IR counterparts to X-ray sources, 24 micron-selected AGN candidates andspectroscopically identified AGNs show variability. Mid-IR AGN variability canbe well described by a single power-law structure function with a power-lawindex of 0.5 at both 3.6 and 4.5 microns, and an amplitude of 0.1 mag onrest-frame time scales of 2 years. The variability amplitude is higher forshorter rest-frame wavelengths and lower luminosities. (Abridged)
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