We present a study of galaxies showing mid-infrared variability in data taken
in the deepest Spitzer/MIPS 24 μm surveys in the GOODS-South field. We
divide the dataset in epochs and subepochs to study the long-term
(months-years) and the short-term (days) variability. We use a
χ2-statistics method to select AGN candidates with a probability ≤
1% that the observed variability is due to statistical errors alone. We find 39
(1.7% of the parent sample) sources that show long-term variability and 55
(2.2% of the parent sample) showing short-term variability. That is, 0.03
sources × arcmin−2 for both, long-term and short-term variable
sources. After removing the expected number of false positives inherent to the
method, the estimated percentages are 1.0% and 1.4% of the parent sample for
the long-term and short-term respectively. We compare our candidates with AGN
selected in the X-ray and radio bands, and AGN candidates selected by their IR
emission. Approximately, 50% of the MIPS 24 μm variable sources would be
identified as AGN with these other methods. Therefore, MIPS 24 μm
variability is a new method to identify AGN candidates, possibly dust obscured
and low luminosity AGN, that might be missed by other methods. However, the
contribution of the MIPS 24 μm variable identified AGN to the general AGN
population is small (≤ 13%) in GOODS-South.Comment: Accepted for publication in MNRA