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 We identified variables using the following methods.21
1. the Stetson variability index (P. Stetson 1996);22
2. reduced chi-squared (χ2
ν );23
3. the Lomb-Scargle periodogram (N. R. Lomb 1976; J. D. Scargle 1982; J. T. VanderPlas 2018).24
Objects were identified as variable if they met any of the following three criteria:25
1. the ”automatic Stetson” criterion: its light curve was high signal-to-noise, free from major photometric processing26
error flags, and showed a Stetson index exceeding the 99.7th percentile (a ”three-sigma” equivalent) for its27
magnitude range;28
2. the ”periodic” criterion: the object was identified as a periodic variable;29
3. the ”subjective” criterion: the object was in neither of the above two groups, but had a χ2
ν in at least one30
band consistent with variability, and upon manual inspection, displayed a light curve which showed unambigu-31
ous correlated variability across multiple bands that was not purely stochastic and could not be produced by32
photometric processing errors or artifacts.