# 7.6.3 Calibration models

The short-timescale candidate selection criteria are based on the variogram analysis (see Section 7.6.4) with crossmatched catalogues of known variables (including both short and longer timescale sources) and known constant / standard stars, from OGLE catalogues. The idea here is to define a relevant detection threshold $\gamma_{\mathrm{det}}$ that can be compared to the variogram values of each investigated source. This threshold corresponds to the level of variability above which the observed variability is considered as not spurious. A magnitude-dependent detection threshold is defined based on the variogram analysis of crossmatched sources and on the simulation work done previously to assess the power of the variogram method for short timescale variability detection with Gaia (see Roelens et al. 2017).

As mentioned previously, for Gaia Data Release 2, the aim is to focus only on periodic variability with periods below 0.5–1 day. Thus, CU7 also uses the crossmatched catalogues of known constant and variable sources, to define additional criteria to select suspected periodic short-timescale candidates, taking advantage of the period search performed on sources flagged as short timescale candidates from the variogram analysis (see Section 7.6.4). Those additional criteria are basically ‘boxes’ on various metrics, be it classical statistics or specific parameters calculated in the short timescale framework.

Additional criteria are verified by running ‘blindly’ on a subsample of the sources to be investigated, and then are refined to remove some spurious candidates and focus on bona fide on short-timescale suspected periodic candidates, as detailed in Section 7.6.4 and Section 7.6.5.