crests/troughs as seen in the light curve of Ark 120; and the fourth criterion aims to reduce possible false detections in the LS periodogram (@Graham2013).

The obtained false alarm probabilities are roughly $(5-10)\times10^{-4}$ (equivalent to a significance of $\sim3.3\sigma$) for the above three PSD models. Combining with the Bayes factors calculated in Section 4.2, the values of the false alarm probabilities suggest that the significance of the periodicity in Ark 120 is marginal. Continued monitoring of Ark 120 is therefore [necessary]{} to confirm the periodicity.

![Dependence of the estimated false alarm probability on the monitoring time length. [The]{} vertical dashed line represents the baseline of the observed data. Beyond the data baseline, the false alarm probability is calculated using [artificial]{} light curves for Ark 120. Note that the false alarm probabilities depend on the configurations of [artificial]{} data, which are described in Section 4.4.[]{data-label="fig_fap"}](fig_fap.pdf){width="47.00000%"}

Requirement of Temporal Baseline for the Periodicity Detection
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The preceding analysis shows that the [evidence for the periodicity is inconclusive]{} for the current data. A question then arises: how long do we need to monitor Ark 120 to ascertain that the periodicity is significant at a given confidence level? To address this question, we make 