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7.6.4 Processing steps

The short-timescale processing starts with the variogram analysis, similarly to what
is described in Roelens et al. (2017). In short, the flagging of short timescale
candidates is based on the comparison of the variogram values of the considered source
with a magnitude-dependent detection threshold ${\gamma}_{\mathrm{det}}(\overline{m})$, completed with
an upper limit of $0.5$d on the detection timescale ${\tau}_{\mathrm{det}}$ (which is the
shortest lag for which the variogram value goes above the detection threshold).
However, a different formulation of the variogram is used here, based on the IQR and
not on the variance. For more details about the variogram approach in the Gaia context, see Roelens et al. (2018).

To define the appropriate detection threshold ${\gamma}_{\mathrm{det}}(\overline{m})$, the variograms
associated to the Gaia light-curves of known OGLE periodic variables (including short
timescale sources as well as long timescale ones), and constant sources, with more
than 20 FoV transits in $G$, are calculated. By comparing the maximum variogram
values of short timescale, longer timescale and constant sources, as it is done in
Roelens et al. (2017), it is possible to retrieve a relevant detection threshold,
enabling to separate constant sources from variable stars on the basis of their
variograms, and also eliminating a significant fraction of longer period variables. In
the end, the detection threshold used is simply a scaled version of the detection
threshold deduced from simulations in Roelens et al. (2017):
${\gamma}_{\mathrm{det}}=10{\gamma}_{\mathrm{det},\mathrm{simu}}$. At this point, the recovery rate of short
timescale variables is around 50%, contamination from false positives about 2%, and contamination from variable sources with period greater than $1$d around 20%.

For the candidates passing the variogram short timescale selection, a Least-Square
period search algorithm is run on the per-CCD time series, searching the frequency
range 10min –1d.

The short timescale analysis also relies on classical statistics calculated in the
corresponding statistics module, such as the Spearman correlation between the three
Gaia photometric bands or the Abbe value on those time series.
Additional statistics are defined, such as the ratio of IQRs between the different
photometric bands ($G$, ${G}_{\mathrm{BP}}$ and ${G}_{\mathrm{RP}}$), or the ratio between the median of variogram
values at CCD lags (i.e. up to 40s) and the median of variogram values at FoV lags
(i.e. above 40s). They are specific to short timescale analysis, and mostly not
published in the Gaia DR2 archive.

So as to both focus the analysis on short-timescale suspected periodic candidates and
reduce the contamination from false positives and long period variables, the short
timescale analysis (variogram analysis, period search, and complementary statistics
calculation) was performed on a few hundred known constant and variable (periodic and
non-periodic) sources, not only from the OGLE survey but also from other crossmatched
catalogues from the literature (LINEAR, Catalina, etc…). From this analysis,
additional cuts on the statistics mentioned above are defined to focus on short
period candidates. This series of selection criteria (variogram + cuts on statistics)
is refered to as the preliminary selection criteria, and will be refined afterwards
(see Section 7.6.5).